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According to Al Gore, Global warming in general refers to any unequivocal current and projected rise in time in the average temperature of the Earth's atmosphere and oceans in recent decades and in the troposphere, which can contribute to changes in global climate patterns. The scientific consensus is that global warming is occurring and was initiated by human activities, especially those that increase concentrations of greenhouse gases in the atmosphere, such as deforestation and burning of fossil fuels. This finding is recognized by the national science academies of all the major industrialized countries and is not rejected by any scientific body of national or international standing. The global average temperature peaked in the mid 1990s. Global warming can occur from a variety of causes, both natural and human induced. Climate observations indicate the Earth may have entered a period of global warming, due to global industrialization, its downline carbon-dioxide increase, in its recent decades, with some climate models projecting its approximately continuation in coming years in atmosphere that is large enough to change the global climate. Much as Newsweek calculated that the average annual temperatures had been decreasing since 1880 in its famous 1975 article, The Cooling World, some advocates of this theory believe that global warming, a trend since the industrial revolution, must be stopped. Human caused global warming has actually been proven to not exist, as environmentalists have failed actually link human emmisions with global temperature beyond a simple example of correlation. In fact, it has been proven that Global Warming is caused by changes in solar activity. Graphs of solar activity, CO2, and temperatures are all very similar, and since neither temperatures nor CO2 can be the causes of solar activity, solar activity has been proven to cause both CO2 levels and temperature changes, including Global Warming. Although there have been periods of both global warming and cooling in the past, it is currently a topic of hightened international attention due to observations of increased average temperatures over the last ten decades and projections of continued increases in the next century. This recent increase is believed to be caused by the greenhouse gas effect. The phrase "global warming" came into use during the late 1970's. In 2009 the 4 Degrees and Beyond International Climate Conference explored the likelihood and consequences of a significant global average temperature in that higher range in the decades ahead, and possible ways to avoid it. Models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures are likely to increase by 1.1 to 6.4°C (2.0 to 11.5°F) between 1990 and 2100. This exceeds the 2°C threshold for dangerous climate change recognized by the United Nations Framework Convention on Climate Change, and warming from this century's emissions will extend well beyond 2100 due to the longevity of some greenhouse gases in the atmosphere. The uncertainty in this range results from both the difficulty of predicting the volume of future greenhouse gas emissions and uncertainty about climate sensitivity. Climate scientists agree that the earth's temperature has fluctuated throughout history by about 3 degrees Celsius (5 degrees Fahrenheit). The term is also used for the more specific scientific theory of anthropogenic global warming, which states that much of the recent observed (and projected) global warming is human-induced and the result of a strengthened greenhouse effect caused primarily by man-made increases in carbon dioxide (through the burning of fossil fuels and deforestation) and other greenhouse gases (GHGs) are the primary sources of the human-induced component of warming. They are released by the burning of fossil fuels, land clearing and agriculture, etc. Adding simulation of the carbon cycle to the models generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback. Most of the increase is due to human activity causing an amplified greenhouse effect. On this theory, the increased volumes of carbon dioxide and other greenhouse gases released mainly by the burning of fossil fuels, and, to a lesser extent, land clearing and agriculture, are the primary sources of warming. The natural greenhouse effect keeps the Earth 30°C warmer than it otherwise would be; adding carbon dioxide to a planet's atmosphere, with no other changes, will make a planet's surface warmer. Current research is attempting to find out more details about the processes and factors that would affect a temperature increase especially about positive and negative feedback mechanisms, to allow a more precise quantification of the effects of global warming. The National Academy of Science reported in 2001 that, "Because of the large and still uncertain level of natural variability inherent in the climate record and the uncertainties in the time histories of the various forcing agents...a causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the 20th century cannot be unequivocally established". There is much debate in the scientific community as to how much man-emitted carbon dioxide has increased the earth's temperature. The term may be used to describe theories explaining this increase (see global warming hypothesis) or the crisis that many advocates say will arise if no one does anything about it (see global warming controversy). It is generally used to describe the temperature rise over the past century or so, and the effects of humans on the temperature. The term may be used to describe theories explaining such an increase or the crisis that many say will arise if no one does anything to prevent such an increase from occurring. Global warming theories attempt to account for the documented rise in average global temperatures since the late 19th century. The Earth's average near-surface atmospheric temperature rose (0.6 ± 0.2°CCelcius (1° Fahrenheit) and assess the extent to which the effects are due to human causes, principally emissions of carbon dioxide (CO2) increasing the "greenhouse effect". All climate models currently predict that if the only variable considered is human emission of greenhouse gases, then this will cause temperatures to increase in the future; however the precise magnitude of these increases is still uncertain. The most common global warming theories attribute temperature increases to increases in the greenhouse effect caused primarily by anthropogenic (human-generated) carbon dioxide and to possible increases in solar activity. Temperature change is just one aspect of the broader subject of (human-induced) climate change. The scientific opinion on climate change, as expressed by the UN Intergovernmental Panel on Climate Change (IPCC) indicate that the climate sensitivity to a doubling of CO2 in the atmosphere is in the range 1.5-4.5°C (2.7-8.1°F); models and endorsed by the national science academies of the G8 nations, is that the average global temperature has risen since the late 19th century, and that it is likely that "most of the warming observed over the last 50 years is attributable to human activities", most prominently the emission of greenhouse gases such as carbon dioxide (CO2) and methane (CH4). However, other scientific phenomenon such as solar variation may explain part of the global warming in terms of increasing energy output of the Sun. For example in 2003 NASA reported an increasing trend in solar irradiance during the last few decades. Since the late 1970s, the amount of solar radiation the sun emits, during times of quiet sunspot activity, has increased by nearly .05 percent per decade, according to a NASA funded study. However, another 2006 study has shown that humans have little to do with the effects with the warming of the planet, and it has been part of a 1,500 year cycle of gradual warming and cooling of the planet's mean temperature. This has been documented by the means from researchers around the world using tree rings, ice caps, stalagmites, and dust plumes, prehistoric villages and collapsed cultures, fossilized pollen, and other sources.

According to the IPCC the temperature in the northern hemisphere and at the poles is increasing at an increasing rate along with carbon emmissions so in those regions the relative global warming is above average. Ocean temperatures may be different from air temperatures. Sea level rise may also vary regionally due to the hotter regions having a higher coefficient of expansion and some areas being subject to subsidence or isostatic rebound as the ice at the poles and glaciated areas melts. Average global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the past century, but northern hemisphere temperatures are projected to increase by 6°F) before the end of the century with recent projections tending to go higher and sooner. Modern geologists and geophysicists consider the age of the Earth to be around 4.55 billion years. 100 years is 0.0000022% of earth's history. The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for "most of the observed temperature increase since the middle of the 20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations", which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect, although disagreement still persists over the base causes of the current interglacial period. Other phenomena such as solar variation have had smaller, but non-negligible effects on global temperature trends from pre-industrial times to 1950, but a small cooling effect since 1950. The Royal Society states that the overwhelming majority of scientists working on climate change agree with the IPCC's main conclusions. There is also a danger of saying 'there is a scientific consensus' on something. Science relies heavily on skepticism. Different views, as long as they are based on facts, should not ignored by saying that there is a consensus. However, it is important to note that one must study the data and the facts themselves because an appeal to the majority is a logical fallacy. During the 20th century, global surface temperature increased by about 0.74°C (1.33°F) Using computer models of the climate system based on six greenhouse-gas emission scenarios, the 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) projected that global surface temperature is likely to rise 1.1 to 6.4&°C (2.0 to 11.5°F) by 2100, and the upper limit of that range does not include any warming from the potential release of certain carbon cycle feedbacks and the upper limit of that range does not include any warming from the potential release of certain carbon-cycle feedbacks. Actual emissions since 2000 have equaled or exceeded the "A2 scenario", except for small dips during two global recessions. Research published by NCAR in November 2012 suggests that the models which show greater warming are likely to be the most accurate. The "best estimate" of global average warming by 2100 for the A2 scenario given in AR4 is 3.4°C (6.1°F). This exceeds the 2°C threshold for dangerous climate change recognized by the United Nations Framework Convention on Climate Change. By 2010, more recent observations of emissions made the A1FI scenario the "business as usual" case, and confirmed that "the worst-case IPCC scenario trajectories or even worse are being realised". Recent research including some carbon cycle feedbacks predicts a global average temperature rise of 4°C by the 2070's.{{POV-statement|date=June 2011} In a 4°C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved. For these reasons, virtually all climatologists are now convinced that global warming poses a clear and present danger to civilization. 2011 analyses by the United Nations Environment Programme and International Energy Agency suggest that current efforts to reduce emissions may be inadequately stringent to meet the UNFCCC's 2°C target.

These basic conclusions have been endorsed by at least 30-40 scientists disagree with regard to the nature of the observed warming, the American Association of Petroleum Geologists is the only scientific society that officially rejects these conclusions who publicly disagree with these conclusions, the IPCC does represent the general scientific consensus, especially among active climate scientists and researchers, the overwhelming majority of scientists working on climate change are in agreement with them even though the studies failed to include affecting anthropogenic direct heating of the atmosphere of which amounted to 460 quadrillion BTUs for the year of 2005 according to the Department of Energy. The serious scientific criticism of the IPCC findings has mainly been limited to its failure to consider positive feedback effects which may lead to runaway climate change, especially Arctic methane release from permafrost and the clathrate gun effect. In recent years, there is growing evidence that IPCC climate models have atmoshpheric implications that are contradicted by empirical evidence. This has led an increasing number of scientists to doubt the accuracy of current climate change models, especially those emphasizing human influence. There is concern, however that scientists have been manipulated by organizations such as Greenpeace to support the hypothesis that global warming is due to human activity, despite a lack of scientific data to support that claim. It should be noted, though, that no increasing trend in the solar irradiance has been observed during the last decades, in contradiction with the temperatures trend to increase. The increased amounts of carbon dioxide (CO2 and other greenhouse gases (GHGs) are the primary causes of the human-induced component of warming. They are released by the burning of fossil fuels, land clearing and agriculture, etc. and lead to an increase in the greenhouse effect. The first speculation that a greenhouse effect might occur was by the Swedish chemist Svante Arrhenius in 1897, although it did not become a topic of popular debate until some 90 years later. While this conclusion represents the consensus of a vast majority of the scientific community, a few scientists disagree about the primary causes of the observed warming with parts of this conclusion as does the American Association of Petroleum Geologists.

The measure of the climate response to increased GHGs, and other andthropogenic and natural climate forcings is climate sensitivity. Another uncertainty is how warming and related changes will vary from region to region around the globe. It is found by observational studies and climate models. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere. The current literature estimates a likelihood between 66% and 90% for a climate sensitivity in the range 1.5-4.5°C (2.7-8.1°F). This should not be confused with the expected temperature change by a given date, which also includes a dependence on the future GHG emissions and a delayed response due to thermal lag, principally from the oceans. Models referenced by the [[Intergovernmental Panel on Climate Change (IPCC), using a range of SRES scenarios, predict that global temperatures may increase by between 1.4 and 5.8°C (2.5 to 10.5°F) between 1990 and 2100. The NCPA also supports the idea of the warming of the Earth as well as it is part of the 1,500 cycle that has started since 1850 and the government contributions to climate change could cause some economic problems along with the weakening the ability for humans to adapt to climate change.

The uncertainty in this range results from both the difficulty of predicting the volume of future greenhouse gas emissions and uncertainty about climate sensitivity. Anthropogenic global warming is also a heavily politicized issue]]s. Commonly proposed human agents of climate change include: the byproducts of the combustion of coal and petroleum; the manufacture, use, and disposal of plastics and petrochemical derivatives; along with deforestation and "slash-and-burn" farming. These agents are intertwined with the industrial, technological, and economic developments since the Second Industrial Revolution. Any public policies that would effectively curtail or reverse anthropogenic climate change would incur widespread economic effects and global wealth redistribution. With the large stakes, the motivation and potential bias of scientists expressing opinions on global warming are heavily scrutinized. A small minority of qualified scientists still contest the view that humanity's actions have played a significant role in increasing recent temperatures. However, uncertainties do exist regarding how much climate change should be expected in the future, and a hotly contested political and public debate exists over implementation of policies that deal with predicted consequences, what actions, if anything, should be taken in light of global warming.

Use of the term "global warming" usually implies a human influence. The more neutral term climate change is usually used to describe previous natural variations and no characterization of the kind of change involved; these are the senses in which the terms are used in Wikipedia. Projections based on basic science, observational sensitivity studies, and the Climate models, forced by estimates of increasing CO2 and to a lesser extent, by generally decreasing sulfate aerosols, predict that temperatures will increase (with a range of 1.4°C to 5.8°C for change between 1990 and 2100). However, although warming is expected to affect the number frequency and magnitude of these events, it is very difficult to connect casually any particular event to global warming. Models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures are likely to increase by 1.1 to 6.4°C (2.0 to 11.5°F) between 1990 and 2100. The range of values reflects the use of differing scenarios of future greenhouse gas emissions as well as uncertainties regarding climate sensitivity. Although most studies focus on the period up to 2100, even if no further greenhouse gases were released after this date, warming (and sea level rise due to thermal expansion) would be expected to continue to rise for more than a millenium, since CO2 has a long average atmospheric lifetime  Only a small minority of climate scientists continue to disagree that humanity's actions have played a major role in recent warming. There is more significant uncertainty regarding how much climate change should be expected in the future. Much of the evidence is statistical; a significant increase in certain events which is correlated with warming. This article describes the current understanding of global warming as accepted by a wide consensus of the scientific community. For a discussion of opposing views, see global warming controversy. For a discussion of public policy issues concerning responses to global warming, see Kyoto Protocol, Politics of global warming, Economics of global warming, Mitigation of global warming, and Adaptation to global warming. Although much of the response to global warming is undertaken at the government and intergovernmental level, there is also an important role for businesses and individuals. Climate commitment studies predict that even if levels of greenhouse gases and solar activity were to remain constant, the global climate is committed to 0.5°C of warming over the next one hundred years due to the lag in warming caused by the oceans. Even if man-made emissions stopped immediately, temperatures would still continue to increase, and would remain unnaturally high for centuries. The models show that 10 cm of sea level rise is also committed for the next century, but that while the temperature lag will have been caught up after 100 years, there are several more centuries of sea level rise already committed even without accounting for any glacier or ice cap melting. Much of this uncertainty results from not knowing future CO2 emissions, but there is also uncertainty about the accuracy of climate models, especially the representation of clouds and aerosols and the failure to correctly model the vertical temperature profile over the tropical oceans. New research suggests that ocean temperature and associated sea level increases between 1961 and 2003 were 50 percent larger than estimated in the 2007 Intergovernmental Panel on Climate Change report. Although current temperature rise is too weak to be fatal for the planet earth itself, the increase in temperature and its additional consequences (sea level rise, altered precipitation decreasing the agricultural yield, extreme weather events, ...) will be devestating for our current society and way of life. This, as the ecosystem services on which we depend will quickly become too affected to sustain our current population. The Kyoto Protocol proposes binding greenhouse gas limits for developed countries, but there are considerable disagreements about the extent to which the Kyoto Protocol will be able to address the issue of greenhouse gases and global warming even if it is successfully implemented. Current estimates predict that even if successfully and completely implemented, the Kyoto Accord will reduce global temperature by between 0.02°C and 0.28°C by the year 2050. According to the contribution of Working Group III to the IPCC Fourth Assessment Report, to limit temperature rise to 2 Degrees Centigrade, "developed countries as a group would need to reduce their emissions to below 1990 levels in 2020 (on the order of –10% to 40% below 1990 levels for most of the considered regimes) and to still lower levels by 2050 (40% (Sic. 80% in Box 13.7, p776) to 95% below 1990 levels), even if developing countries make substantial reductions". Global temperatures have not increased in 11 years.

There is no scholarly debate amongst the scientific community as to whether or not human-made global warming is real. The recent climatology literature does not contain any disagreement that human actions have contributed to global warming. However, the expert credibility in climate change is doubted. Nevertheless, political and public debate continues for a variety of reasons; one major reason being that some oil companies have funded public relations campaigns and faulty contrary research to discredit the global scientific consensus. Almost identical public relations and faulty research strategies were used leading up to the tobacco/cancer lawsuits in the United States and around the world. Some oil companies have spent more than $70 million dollars funding public relations campaigns and deeply flawed research studies intended to discredit the global scientific consensus. The Kyoto Protocol is aimed at stabilizing greenhouse gas concentration to prevent a "dangerous anthropogenic interference". As of November 2009, 187 states have signed and ratified the protocol. Proposed responses to climate change include mitigation to reduce emissions, adaptation to the effects of global warming, and geoengineering to remove greenhouse gases from the atmosphere or block incoming sunlight.

In the 1970s it was unclear whether global warming or global cooling were more likely in the near future (next 100 years). By 1980, most opinion was in the warming camp, though uncertainties remained large. Since 1990, however, the prospect that the earth's surface might become dangerously overheated -- because of heat trapped by carbon dioxide and other "greenhouse gases" -- took over and it has been a hotly debated topic ever since. Since 1990, the prospect that the earth's atmosphere might heat up too much -- because of carbon dioxide and other "greenhouse gases" -- has been a hotly debate topic. Some advocates maintain that temperatures have been roughly stable for the last 1,000 years but have sharply risen in the last century, due to human activity. They urge quick and radical action to save the environment. Others say that the historical temperature record have shown patterns of warming and cooling, which correlate with sunspots and other aspects of solar activity; for example, they argue that during the Medieval Warm Period the earth became so much warmer that Icelandic and European colonists could thrive in the colonization of Greenland. These advocates reject calls for quick action (see global warming controversy, Maunder Minimum, Little Ice Age).

In the twentieth century, both marine and land-based thermometers have recorded such a warming from the 1880s to about 1940, followed by a lesser cooling from 1940 to 1975, and another period of warming from 1975 to present. The fluctuation in the surface temperature record is generally acknowledged, although the causes remain controversial and some researchers dispute the accuracy of the readings. John Christy and Roy Spencer at at the University of Alabama-Huntsville (UAH) maintain a record of satellite-based temperature measurements that begins in January 1979 and is updated monthly. "According to a just-published paper (Christy et al, 2003), the global temperature trend in the lower atmosphere from earth’s surface to about 25,000 feet is 0.06ºC (± 0.05ºC per decade) from January 1979 through April 2002"; others question why the US and Europe show much less of a warming trend than non-industrialized countries (presumably conveniently forgetting about the marine record). -- see 'Historical temperature record' for more discussion; and anthropogenic climate change for attribution of the change, 'Urban heat islands' and 'Evidence against a warming period' below. On average, human activities produce in just three to five days, the equivalent amount of carbon dioxide that volcanoes produce globally each year.

Many are afraid that this will lead to a worldwide harm to the environment and damage to agriculture (see global warming hypothesis), so the issue has become a matter of public policy. Not all global warming is attributed to the action of humans; global warming that is attributed to such causes is called anthropogenic global warming (see also greenhouse gases).

Researchers disagree on whether the effects of global warming will be beneficial or detrimental. Some researchers feel that up to 1.5 degrees Centrigade of warming would increase crop yields and stabilize weather. Many researchers, especially those associated with the UN's Intergovernmental Panel on Climate Change (IPCC) predict disastrous consequences for such a warming; moreover, they predict warming of 2 to 5 times that amount.

There is also some disagreement on the historical temperature record. Depending on what direct measurements and proxies are accepted, researchers have presented various scenarios: stable temperature followed by a sudden, steady rise in the 20th century vs. fluctuations of 1 or 2 degrees Centigrade, with near-stable temperature since 1940.

It is the observed increase over time of the average temperature of Earth's atmosphere and oceans. It is somewhat disputed as a phenomenon by a small minority of scientists and by a considerably larger political faction in the US - see 'Historical temperature record' and 'evidence against a warming period' below. The theory that the human release of various "Greenhouse gases" is connected with the observed heating of the Earth's atmosphere in the 20th century - while politically contentious - has steadily gained adherents in the scientific community within the past 15 years, to the extent that the US Academy of Sciences (and many other national scientific bodies around the world) has strongly endorsed it. Public controversy continues to surround the hypothesis that human activities are contributing to significant global warming. A small number of scientists with backgrounds in climate research -- notably S. Fred Singer, Patrick Michaels, Robert Balling, Sherwood Idso, and Richard S. Lindzen -- dispute the theory (see global warming skepticism). Also, a number of industry-backed organizations (including the Global Climate Coalition, the Greening Earth Society, and Singer's Science and Environmental Policy Project have claimed that the theory is fraudulent or unproven. Considerable uncertainty remains about the magnitude of future global warming and its environmental impacts, but the potential impacts are profound. The potential impacts of Global Warming are profound:

If warming continues at the present rate, it is very widely believed to result in changes in ocean circulation, catastrophic global climate change, loss of biodiversity and irreversible damage to agriculture in those ecoregions most affected. In some regions, e.g. Western Europe, Bangladesh, damage is projected to be extreme, due to loss of Gulf Stream warming and global sea level rise respectively. More frequent bouts of destructive weather are also anticipated, and risk experts in the insurance industry have expressed very strong concerns, advocating a proactive approach based on the precautionary principle. Estimates accepted by the IPCC and by some insurance industry bodies estimate up to 3.5 billion people could be affected by rising disease, loss of fresh water supply, and other impacts. In opposition stand the oil industry and its advocates, who have taken a strong stand in opposing most theories of human-caused global warming as well as action to mitigate Global Warming. They argue that crippling the energy industry to prevent an ecological catastrophe does not make so-called "economic sense" - that healthy economies are required to fund technologically innovative "solutions". President G.W. Bush, made this argument in rejecting the Kyoto Protocol. Bush did not reject the science outright, and argued that the greenhouse gas control was a matter of voluntary restraint by industry. Many U.S. states have nonetheless put strong controls on greenhouse gases, rejecting this simple ideology. This stark standoff between advocates of existing industries have made the scientific questions difficult to distinguish from political ones. While the scientific questions are clear, positions on them and appropriate tests of the theses advanced tend to be hopelessly mired in starting assumptions: The proportion of scientists who support or oppose any of the various global warming theories is a matter of controversy in its own right.

Environmentalists and their allies claim virtually unanimous support for the global warming theory from the scientific community. Opponents maintain that it's the other way around, claiming that the overwhelming majority of scientists either dismiss global warming altogether or merely consider it "unproven" (see global warming scepticism).

The hypothesis that a man-made increase in greenhouse gas concentration would lead to a higher global mean temperature was postulated in the late 19th century due to human causes, principally emissions of CO2 increasing the "greenhouse effect". The theory further predicts that temperatures will continue to increase in the future, if human emissions of greenhouse gases continue by Swedish chemist and 1903 Nobel Laureate Svante Arrhenius (see global warming hypothesis), although, his peers largely rejected that theory. The period of time over which the change has been observed may vary according to the focus of the user of the term: sometime since the Industrial Revolution, or since the beginning of an approximately global historical temperature record in about 1860; or over the past century; or the most recent 50 years.

If the only variable considered is the emission of greenhouse gases related to human activity, then climate models predict that temperatures will increase in the future, and how warming and related changes will vary from region to region around the globe; however, the precise magnitude of these increases is still uncertain, with a range of +1.4°C to +5.8°C for the temperature change between 1990 and 2100. The range of uncertainty is primarily the result of the difficulty of predicting the volume of future carbon dioxide emissions, but there is also some uncertainty about the climate sensitivity. There is a hotly contested political and public debate on a world scale over what, if anything, should be done to reduce or reverse future warming, and how to deal with the predicted consequences. Much of this uncertainty results from not knowing future CO2 emissions, but there is also uncertainty about the accuracy of climate models and it is not clear if they under- or overpredict future climate change. It is also uncertain how much significance human emission of greenhouse gases has to past and future climate change. The next significant international agreement may come in December at the COP15 talks. Recent published research has challenged the notion of a single global temperature. Note that although the discussion often focuses on temperature, global warming or any climate change implies changes in other variables: overall precipitation and its patterns, cloud cover, weather, and all the other elements of our atmospheric system will be impacted by the increase in "radiative forcing" due to human changes in greenhouse gas concentrations in the atmosphere.

Some scientists point out that global warming correlates closely with natural factors, especially solar activity. The balance is attributed to the action of humans (see anthropogenic global warming). How much warming is natural or man-made has been hotly debated since the 1990s, by scientists, politicians and advocacy organizations (see global warming controversy). Some scientists dispute the consensus view.

There is also disagreement on whether the effects of global warming will be beneficial or detrimental. Many researchers predict disastrous consequences for a warming of 1.5 to 7 degrees celsius. The UN's Intergovernmental Panel on Climate Change (IPCC) predicts such a warming is likely within the 21st century, unless severe measures are taken (see Kyoto Protocol. Other researchers (a very small minority, it should be noted), feel that up to 1.5 degrees Centrigade of warming would increase crop yields and stabilize weather; many of these doubt a large warming is likely.

To add to the controversy, new findings -within the last 15 years- have indicated the the Earth's climate system is inherently unstable, and that Global Warming could, counter-intuitively, precipitate such non-linear, sudden climate shifts as have been discovered to have occurred within the Earth's recent past. Ocean circulation, believed to be the key to such climate shifts, has been observed to be slowing, causing alarm among oceanographers. The National Academy Press of the US National Academy of Science issued a report on this phenomenon in 2002, titled "Abrupt Climate Change - Inevitable Surprises".

There is also some disagreement on the historical temperature record. Depending on what direct measurements and proxies are accepted, researchers have presented various scenarios: stable temperature followed by a sudden, steady rise in the 20th century vs. fluctuations of 1 or 2 degrees Centigrade, with near-stable temperature since 1940. However, the US National Academy of Science, both in its 2002 report to President George W. Bush, and in its latest publications, has strongly endorsed evidence of an average global temperature increase in the 20th century and stated that human activity is heavily implicated in causing this increase. Advocates of the global warming hypothesis who predict adverse consequences from as little as 1.5 degrees Centigrade of warming nearly all support the Kyoto Protocol as a countermeasure. Some researchers, politicians and businesses oppose it.

When this warming is attributed to man-made causes, it is called anthopogenic global warming. In public policy discussions, global warming usually means excessive and harmful warming. This article uses global warming in the latter sense and explains its scientific basis and steps proposed to combat it. Global warming is a world-wide climatic phenomenon--the average global surface temperature increased over the last 150 years. Whether this increase is significant or not is subject to debate. Natural and anthropogenic (man-made) causes were proposed to explain the phenomenon. Scientists generally believe that increased concentrations of anthropogenic greenhouse gases in the atmosphere, which causes more of the energy radiated from the Sun to be absorbed by the Earth (through the greenhouse effect), play an important role in global warming. However, the number of scientists expressing skepticism on the global warming issue continues to grow (see Heidelberg Appeal, Leipzig Declaration).

The main evidence for global warming comes from thermometer measurements from land stations all over the world since 1860. According to one source, the data, corrected for the urban heat island effect, shows that the average surface temperature has increased by 0.6+-0.2 C during the 20th century. Most of the warming occurred during two periods: 1910 to 1945 and 1976 to 2000. Some scientists dispute the arguments of the IPCC report, on the grounds that the corrected data as well as satellite and weather balloon data disprove the IPCC thesis (see SEPP below). People who favor the global warming view claim SEPP to be politically biased.

However, data from weather satellites and balloon instruments show no warming whatsoever, which calls into question how well land station data has been corrected for the urban heat island effect. Another source says disagrees, saying that well-controlled surface thermometer data for the United States show no warming trend since about 1940, once "urban heat-island" effects are removed. Furthermore, proxy temperatures, as deduced from tree rings, ice cores, etc., show no perceptible warming trend in the past 60 years. To do: explain how the data was corrected for the urban heat island effect.

Secondary evidence comes from observed variations on the snow cover and ice extent, global average sea level, precipitation, cloud cover, El Nino, and extreme weather events during the 20th century. For example, satellite data shows a 10% decrease of snow cover since the late 1960s. The Northern Hemisphere spring and summer sea-ice extent has decreased by about 10% to 15% since the 1950s and there has been a widespread retreat of mountain glaciers in non-polar regions during all the 20th century. Though earlier geological periods also experienced episodes of warming, the term commonly refers to the observed and continuing increase in average air and ocean temperatures since 1900 caused mainly by emissions of greenhouse gases (GHGs) in the modern industrial economy. In the modern context the terms global warming and climate change are commonly used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region. Many of the observed changes in climate since the 1950s are unprecedented in the instrumental temperature record, and in historical and paleoclimate proxy records of climate change over thousands to millions of years.

In 2013, the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded, "It is that human influence has been the dominant cause of the observed warming since the mid-20th century". The largest human influence has been the emission of greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Climate model projections summarized in the report indicated that during the 21st century, the global surface temperature is likely to rise a further 0.3 to 1.7 C-change in a moderate scenario, or as much as 2.6 to 4.8 C-change in an extreme scenario, depending on the rate of future greenhouse gas emissions and on climate feedback effects. These findings have been recognized by the national science academies of the major industrialized nations The 2001 joint statement was signed by the national academies of science of Australia, Belgium, Brazil, Canada, the Caribbean, the People's Republic of China, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, New Zealand, Sweden, and the UK. Professional scientific societies include American Astronomical Society, American Chemical Society, American Geophysical Union, American Institute of Physics, American Meteorological Society, American Physical Society, American Quaternary Association, Australian Meteorological and Oceanographic Society, Canadian Foundation for Climate and Atmospheric Sciences, Canadian Meteorological and Oceanographic Society, European Academy of Sciences and Arts, European Geosciences Union, European Science Foundation, Geological Society of America, Geological Society of Australia, Geological Society of London-Stratigraphy Commission, InterAcademy Council, International Union of Geodesy and Geophysics, International Union for Quaternary Research, National Association of Geoscience Teachers, National Research Council (US), Royal Meteorological Society, and World Meteorological Organization.}} and are not disputed by any scientific body of national or international standing.

Effects of global warming include rising sea levels, regional changes in precipitation, more frequent extreme weather events such as heat waves, and expansion of deserts. Surface temperature increases are greatest in the Arctic, with the continuing retreat of glaciers, permafrost, and sea ice. Climate change impacts humans by, amongst other things, threatening food security from decreasing crop yields, and the abandonment of populated areas and damage to infrastructure due to rising sea levels. Environmental impacts include the extinction or relocation of ecosystems as they adapt to climate change, with coral reefs, mountain ecosystems, and Arctic ecosystems most immediately threatened. Because the climate system has a large "inertia" and greenhouse gases will remain in the atmosphere for a long time, climatic changes and their effects will continue to become more pronounced for many centuries even if further increases to greenhouse gases stop.

Globally, a majority of people consider global warming a serious or very serious issue. Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future climate engineering. Every country in the world is a party to the United Nations Framework Convention on Climate Change (UNFCCC), 12 parties have not ratified the convention. Non-ratification means they are not legally bound by it whose ultimate objective is to prevent dangerous anthropogenic climate change. Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2 C-change compared to pre-industrial levels,{{efn|Earth has already experienced almost 1/2 of the {{convert|2.0|C-change|F-change|1}} described in the Cancún Agreement. In the last 100 years, Earth's average surface temperature increased by about {{convert|0.8|C-change|F-change|1}} with about two-thirds of the increase occurring over just the last three decades with efforts made to limit warming to {{convert|1.5|C-change|1}}. Some scientists call into question climate adaptation feasibility, with higher emissions scenarios, or the two degree temperature target.

Twentieth Century temperature trends
A 1998 article in Natural Science sums up the generally accepted observation of climatologists:
 * Mean temperatures rose steeply in the decades before 1940 and dropped from 1940 to about 1975.

The global temperature record of the last three or four decades is disputed, however, with often bitter wrangles between two groups: those touting records from weather balloons and satellites, and those promoting land-based thermometer readings.
 * Dr. Richard Courtney, an IPCC "expert reviewer" who is with the European Science and Environment Forum (UK), passionately argued a lack of measured "global" warming. He demonstrated that nearly all measured increases in temperatures have occurred in regions, for example Siberia, where data are sparse and not continuous, and are therefore doubtful. Dr. Singer speculated that the urban heat island effect (large cities holding on to heat) is likely responsible for the differential in the less rural measurements.

Theories and criticisms
Leaving the realm of scientific journals, the debate has spilled out into the public arena, with some politicians making the issue a component of their campaigns for high office. One example of this is 2000 U.S. presidential candidate Al Gore, author of Earth in the Balance. Global warming is a more central and sustained issue, however, for the EU. Nearly everything about global warming theories is controversial, not the least of which is whether there exists a scientific consensus sufficient to justify radical action to ameliorate its effects (see Kyoto Protocol). Proponents of global warming theory (GWT) express a wide range of opinions. Some believe that the environmental damage will have such severe impact that immediate steps must be taken to reduce CO2 emissions, regardless of the economic costs to advanced nations such as the United States (the United States has the largest emissions of greenhouse gases of any country in absolute terms, and the second largest per capita emissions after Australia. There are no known climatologists supporting this viewpoint. Others merely "believe in" the observed increase in temperature. Somewhere in between are those who support measures such as the Kyoto Protocol, intended to have minor climate effects. Critics of the global warming theory similarly offer a wide spectrum of opinions. Some, such as Patrick Michaels, propose that human influence has warmed the atmosphere yet dispute the conclusion of the IPCC, which claims recent warming is anthropogenic ("There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities"). Other scientists conclude observations of global temperatures over much larger time spans, spans of thousands of years rather than decades – epochs rather than centuries, show global temperatures fluctuated wildly in the past long before the introduction of human industrial activity such as the industrial revolution. The additional assertion of many of these scientists is that it cannot be possible to assertain any definitive global temperature trend with such a limited amount of data -- the Earth is much older than that, they claim. Other scientists theorize Global temperature change may in fact be induced by erratic solar events such as sun spots, solar spicules, or changes in the earth's magnetic field (see below). Controversial subjects are discussed further in the article Global warming controversy, such as volcanism and solar activity. The above paragraphs might give the impression that belief in the course of past climate change correlates strongly with advocacy for future actions: this is not necessarily so. It is possible, perhaps common, to study the past record and give no counsel on the future.

Causes of global warming
The climate system varies both through natural, "internal" processes as well as in response to variations in external "forcing" from both human and non-human causes, including changes in the Earth's orbit around the Sun (Milankovitch cycles), solar activity, and volcanic emissions variations in the earth's orbit ("orbital forcing") as well as greenhouse gases. See Climate change for further discussion of these forcing processes. Climatologists accept that the earth has warmed recently. Somewhat more controversial is what may have caused this change. See attribution of recent climate change for further discussion. These forcing factors include solar activity, changes in solar luminosity, volcanic emissions, variations in the earth's orbit (orbital forcing) and greenhouse gases. Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing) volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus identifies increased levels of greenhouse gases due to human activity as the main influence. The livestock industry is at the forefront of these activities. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. In contrast to the scientific consensus that recent warming is mainly attributable to elevated levels of greenhouse gases, other hypotheses have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of increased solar radiation associated with greater numbers of sunspots. Atmospheric scientists know that adding carbon dioxide (CO2) or methane to an atmosphere, with no other changes, will tend to make a planet's surface warmer (this is know as "climate forcing"). Indeed, greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be estimated 30°C lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the theory that adding CO2 or CH4 to the Earth's atmosphere will result in warmer surface temperatures on Earth, on average, absent indirect mitigating effects. Rather, the debate is about what the net effect of the addition of CO2 and CH4 will be, and whether changes in water vapor, clouds, the biosphere and various other climate factors will cancel out its warming effect, when allowing for associated compounding or mitigating factors. The concentration of carbon dioxide in the atmosphere, currently 380 ppm, might be naively taken to be too low to have much effect. But the importance of carbon dioxide arises from a feedback effect: a little of the long-term carbon dioxide injected into the atmosphere causes a little warming, which causes a little more of the potent short-term water vapor to be evaporated into the atmosphere, which causes still more warming, which causes more of the potent water vapor to be evaporated, and so forth, until a new dynamic equilibrium concentration of water vapor is reached at a slightly higher humidity and with a much larger greenhouse effect than that due to carbon dioxide alone. This feedback effect is reversed only as the carbon dioxide is slowly removed from the atmosphere.

Another important feedback process is ice-albedo feedback. The increased CO2 in the atmosphere warms the Earth's surface and leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. Feedback effects due to clouds are an area of ongoing research and debate. Seen from below, water aerosol clouds absorb infrared radiation and so exert a positive greenhouse effect. Seen from above, the same clouds reflect sunlight and so exert a negative greenhouse effect. Increased global water vapor concentration may or may not cause an increase in global average cloud cover. The net effect of clouds thus has not been well modeled. Positive feedback due to release of carbon dioxide and methane from thawing permafrost is an additional mechanism contributing to warming. Possible positive feedback due to methane release from melting seabed ices is a further mechanism to be considered.

None of the effects of greenhouse gases are instantaneous. Due to the thermal inertia of the Earth's oceans and slow responses of other indirect effects, the Earth's current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilized at present day levels, a further warming of about 0.5°C (0.9°F) would still occur.

Contrasting with this consensus view, other hypotheses have been proposed to explain all or most of the observed increase in global temperatures, including: The warming is within the range of natural variation; the warming is a consequence of coming out of a prior cool period, namely the Little Ice Age; and the warming is primarily a result of variances in solar radiation; or the warming is primarily the result of increased activity of the solar magnetic field, which increases shielding of the Earth from cosmic rays which would otherwise cause raindrop nucleation in clouds, which would remove greenhouse-gas water vapor from the atmosphere.

One example of an important feedback process is ice-albedo feedback. The increased CO2 in the atmosphere warms the Earth's surface and leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and so thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues.

Due to the thermal inertia of the earth's oceans and slow responses of other indirect effects, the Earth's current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if GHG levels were stabilised at present day levels, there is a further warming of perhaps 0.5°C to 1.0°C (0.9; 1.8°F) already "committed", but not yet realised. The observed warming of the Earth over the past 50 years appears to be at odds with the skeptics' theory that climate feedbacks will cancel out the warming, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain most of the temperature increase. One such hypothesis proposes that warming may be the result of variations in solar activity. The decade 1999-2009 marked what the Met Office called "very small global temperature rises", but this is in line with expectations of climate scientists during warming trends. Although evidence suggests that temperatures have been relatively stable since 1999, climate scientists generally agree that global temperatures are likely to continue to increase in the long-term. Climate changes occur due to internal and external factors. Internal factors are factors associated with the complexity of the climate systems which are chaotic non-linear systems. External factors can be natural factors and anthropogenic factors. The main natural external factor is the solar radiation variability. The solar radiation can change because of the solar cycles and because the Sun is getting hotter. Anthropogenic factors are related to changes in the environment influenced by Man. The main anthropogenic factors are the emission of greenhouse gases, the depletion of stratospheric ozone, the land use and the emission of aerosols such as sulphates. To do: describe correlation between sun-spots (or solar radiation generally), and global temperature.

Scientists agree that internal factors and natural external factors can cause significant climate changes. In the last millennium, two important periods occurred: a warm period known as the Medieval Warming Period and a cold period known as the Little Ice Age. These periods have a magnitude similar to the current warming and it's agreed that they were cause by internal factors and natural external factors only. The Little Ice Age is usually attributed to the reduction of solar activity. Some scientists have claimed in the past that the observed warming since 1860 is a natural climate recovery from the Little Ice Age.

However, large amounts of anthropogenic greenhouse gases were emitted to the atmosphere since the beginning of the industrial revolution. Since 1750, the carbon dioxide concentration has increased by 31%, methane has increased 151%, nitrous oxide has increased 17% and tropospheric ozone has increased 36%. To do: show correlation, if any, between release of greenhouse gases since 1750, and average global temperature.

The majority of these gases are produced by the combustion of fossil fuels. It is thought that the reduction in tropical forested area has also played a role, as old forests store large amounts of carbon. However, growing forests in North America and Russia contribute to absorb carbon dioxide (they act as CO2 sinks) and since 1990, the amount of carbon absorbed is larger than the amount released by deforestation. Not all the CO2 emitted to the atmosphere accumulates there. Half of it is absorbed by sea and forests.

The real importance of each of the proposed causes can only be established through the exact quantification of every factor involved. Internal and external factors can be quantified by the analysis of climate simulations based on the best climate models.

The influence of external factors can be compared using the concept of radiative forcing. A positive radiative forcing warms the planet and negative radiative forcing cools the planet. Anthropogenic emissions of greenhouse gases, stratospheric ozone depletion, and solar radiation have positive radiative forcing and aerosols and land use have negative radiative forcing.

Proponents and Opponents
Since 1990, the prospect that the earth's atmosphere might heat up too much -- because of carbon dioxide and other "greenhouse gases" -- has been a hotly debate topic. Some advocates maintain that temperatures have been roughly stable for the last 1,000 years but have sharply risen in the last century, due to human activity. They urge quick and radical action to save the environment.

To be a "proponent" of this theory encompasses a wide range of opinions. Some believe that the environmental damage will have such severe impact that immediate steps must be taken to reduce CO2 emissions, regardless of the economic costs to advanced nations such as the United States (according to the EPA, the United States is the second greatest producer of greenhouse gases in the world). Others merely "believe in" the observed increase in temperature. Somewhere in between are those who support relatively mild measures such as the Kyoto Protocol.

Others say that the historical temperature record has shown patterns of warming and cooling, which correlate with sunspots and other aspects of solar activity; for example, they argue that the Medieval Warm Period enabled the colonization of Greenland. These advocates reject calls for quick action (see global warming controversy, Maunder Minimum, Little Ice Age).

"Opponent" similarly covers a wide range. Some dismiss the theory as completely lacking any basis in fact; however, there are no known climate scientists adopting this position. Others, such as the well-known skeptic Patrick Michaels are milder, accepting that human influence has warmed the atmosphere while disputing the current and future warming given by the IPCC.

The Intergovernmental Panel on Climate Change
Since it is such an important issue, governments need predictions of future trends in global change so they can take political decisions to avoid undesired impacts.

Global warming is being studied by the Intergovernmental Panel on Climate Change (IPCC). The IPCC does not commission or carry out research it's self, but rather disseminates the body of published research. The reports are intended to reflect the consensus of the published science. In its last report, the IPCC made some predictions about future climate change. These predictions are the basis for current political and scientific discussion. IPCC predictions are based on the same models used to establish the importance of the different factors in global warming. These models need data about anthropogenic emissions of greenhouse gases and aerosols. These data are predicted from economic models based on 35 different scenarios. Scenarios go from pessimistic to optimistic, and predictions of global warming depend on the kind of scenario considered. None of these scenarios consider any kind of measures to avoid global warming. The models used are primarily based on surface temperature measurements but attempt to predict the temperature of the entire troposphere, not just surface temperatures. However, a NASA report challenges the success of these models:

"The IPCC's 1995 estimate of average global warming at the surface until the year 2100 is +0.18;C/decade. Climate models suggest that the deep layer measured by the satellite and weather balloons should be warming about 30% faster than the surface (+0.23; C/decade). None of the satellite or weather balloon estimates are near this value". Note that "deep layer" refers to the entire troposphere, rather than just the surface.

Nevertheless, proponents of the IPCC assessment say that the current climate models are good in predicting surface temperatures and that this is significant. They furthermore argue that it is surface temperatures that will have the greatest and most direct effect on the environment, agriculture and the stability of polar ice. The IPCC says that it has corrected the land station data to account for the urban heat island effect. Most temperature measuring stations are in rural areas, and the urban measurements are simply corrected against the rural ones. The change resulted in a 0.05 degree Celsius lowering of temperature estimates according to the National Academy of Science's book, Reconciling Observations of Global Temperature Change (2000). This is far too small to account for the nearly 0.4 degree rise in temperatures observed during the late 20th century. The difference is small because most urban measuring stations have had a heat island effect ever since measurements were first taken. In some cases, the effect has decreased due to decreasing population density and increasing energy efficiency.

The work of the Intergovernmental Panel on Climate Change (IPCC)
Since it is such an important issue, governments need predictions of future trends in global change so they can take political decisions to avoid undesired impacts. Global warming is being studied by the Intergovernmental Panel on Climate Change (IPCC). In its last report, IPCC made some predictions about future climate change. These predictions are the basis for current political and scientific discussion.

IPCC predictions are based on the same models used to establish the importance of the different factors in global warming. These models have the need for data of anthropogenic emissions of greenhouse gases and aerosols. This data is predicted from economical models based on 35 different scenarios. Scenarios go from pessimistic to optimistic and predictions of global warming depend on the kind of scenario considered. No scenario considers any kind of measures to avoid global warming. In its last report, IPCC stated that average surface temperature is projected to increase by 1.4 to 5.8 C over the period 1990 to 2100, and the sea level is projected to rise by 0.1 to 0.9 meters over the same period. The wide range in predictions is based upon several different scenarios that assume different levels of future CO2 emissions. Each scenario then has a range of possible outcomes associated with it. The most optimistic outcome assumes an aggressive campaign to reduce CO2 emissions, while the most pessimistic is a "business as usual" scenario. The more realistic scenarios fall in between.

IPCC prediction are the best predictions available, but are under strong scientific controversy. The IPCC recognizes that there is a need for better models and better scientific understanding of some climate phenomena, as well as the uncertainties involved. Critics point out that the available data is not sufficient to determine the real importance of greenhouse gases in climate change. Sensibility of climate to greenhouse gases may be over-estimated or under-estimated because of some flaws in the models and because the importance of some external factors may be underestimated. To do: point out differences between the IPCC report as initially submitted and the final report which was changed to conform to the policy-makers summary.

On the other hand, predictions are based on scenarios and the IPCC did not assign any probability to the 35 scenarios used. Critics charge and point out that some of the scenarios that predict the largest impacts are not realistic because they contradict "basic economic reasoning".

The United Nations Framework Convention on Climate Change and the Kyoto Protocol
Even if there are some doubts about its importance, global warming is perceived by the general public, by many scientists, and by some political leaders as a potential threat to human health and economic prosperity. Reductions of the emissions of greenhouse gases by developed countries was proposed, with developing countries such as India and China being exempted from reductions.

Since this problem is like the tragedy of the commons-like problem, only an international agreement could reduce the emissions of greenhouse gases since voluntary reductions would be avoided by individual countries. In light of this expectation, the global warming treaty will not take effect unless enough countries ratify it.

The United Nations Framework Convention on Climate Change (UNFCCC) establishes a process for developing an international response to the perceived global warming problem. Most of the countries in the world are parties to the UNFCCC, including all the major industrial nations. The UNFCCC, however, does not provide any binding emission targets.

The Kyoto protocol to the United Nations Framework Convention on Climate Change was signed by all industrialized countries who agreed to reduce emissions below the 1990 level, although a few are permitted to limit their emissions to a level higher than what they emitted in 1990 (e.g. Iceland). It was agreed that developing countries would be out of the agreement. It has been ratified by 104 countries, representing 43.9% of emissions. Developed countries were required to limit their emissions to, on average, 5.2% below 1990 levels: 29% below pre-Kyoto estimates for 2010.

Most of the parties to the UNFCCC, including all the major industrial nations, are party to the Kyoto Protocol to the UNFCCC, which provides binding greenhouse gas limits for developed countries (developing countries are not subject to limits under the Protocol). The limits are based on the level of their emissions in 1990; most developed countries are required to limit their emissions to a level below the 1990 level, although a few are permitted to limit their emissions to a level higher than what they emitted in 1990 (e.g. Iceland).

The United States, responsible for one-third (1/3) of the world-wide emissions of greenhouse gases, left the Kyoto protocol before its final ratification by Congress. President Bush's new administration (2001) took the decision of leaving the United States out. During the Presidential administration of Bill Clinton, the United States Senate voted on a resolution against ratifying the protocol, with not a single Democratic Senator voting in favor of it.

George W. Bush, has indicated that he does not intend to submit the treaty for ratification, not because he doesn't support the general idea, but because he is not happy with the language used in the treaty. For example, the Kyoto treaty does not apply to every country. "The world's second-largest emitter of greenhouse gases is China. Yet, China was entirely exempted from the requirements of the Kyoto Protocol", Bush said of the treaty, "This is a challenge that requires a 100 percent effort; ours, and the rest of the worlds. America's unwillingness to embrace a flawed treaty should not be read by our friends and allies as any abdication of responsibility. To the contrary, my administration is committed to a leadership role on the issue of climate change. Our approach must be consistent with the long-term goal of stabilizing greenhouse gas concentrations in the atmosphere".

This decision resulted in internal and international controversy with major political and ideological ramifications. ''To do: list other countries responsible for the emissions of greenhouse gases, such as China and India, and the percentage for which they are responsible. Also, list countries which have (A) signed and/or (B) ratified the treaty (I think only two countries have ratified it so far)''.

To evaluate the effectiveness of the Kyoto protocol, it is necessary to compare global warming with and without the agreement. Several independent authors agree that the impact of the Kyoto protocol in global warming is very small (a reduction of 0.15 in 2 C warming by 2100). Even some defenders of the Kyoto protocol agree that the impact of it is small, but they view it as a first step, with more political than practical importance, for future reductions, perhaps of up to 70%. At the moment, an analysis made by the IPCC is needed to clarify this issue.

The Kyoto Protocol can also be evaluated by comparing costs and gains. Several economical analyses were made that show that the Kyoto Protocol is more expensive than the global warming that it avoids. Defenders of the Kyoto Protocol argue however that while the initial greenhouse gas cuts may have little effect, they set the political precedent for bigger (and more effective) cuts in the future. Also, they demonstrate commitment to the precautionary principle and a respect for life in developing nations - albeit at a 15 to 1 price ratio (embedded in the logic of the Protocol is the assumption that developed nations can pay 15x more to save a citizen's life than a developing one).

Human Factors
Anthropogenic factors are acts by humans (Homo sapiens) that change the environment and influence the climate. The main anthropogenic factors are the emission of greenhouse gases which are thought to contribute to the greenhouse effect, the depletion of stratospheric ozone, changes in land use and the emission of aerosols such as sulphates. Large amounts of anthropogenic greenhouse gases have been emitted to the atmosphere since the beginning of the industrial revolution. Since 1750, the carbon dioxide concentration has increased by 31%, methane has increased 151%, nitrous oxide has increased 17% and tropospheric ozone has increased 36%. The majority of the anthropogenic carbon dioxide is produced by the combustion of fossil fuels. Methane is mainly produced by cattle and by emissions of landfills. It is thought that the reduction in tropical forested area has also played a role, as old forests store large amounts of carbon. However, growing forests in North America and Russia contribute to absorbing carbon dioxide (they act as CO2 sinks), and since 1990, the amount of carbon absorbed is larger than the amount released by deforestation. Not all the CO2 emitted to the atmosphere accumulates there; half of it is absorbed by oceans and forests. The real importance of each of the proposed causes can only be established through the exact quantification of every factor involved. Internal and external factors can be quantified by the analysis of climate simulations based on the best climate models. The influence of external factors can be compared using the concept of radiative forcing. A positive radiative forcing warms the planet, and negative radiative forcing cools the planet. Anthropogenic emissions of greenhouse gases, stratospheric ozone depletion, and solar radiation have positive radiative forcing, whereas aerosols and land use changes have negative radiative forcing.

Natural Factors/Evidence for a current warming period
The main natural external factor is the variability in the amount of solar radiation that reaches Earth. The solar radiation can change because of solar cycles, because of cyclic changes in Earth's orbit, and because the Sun is getting hotter. Scientists agree that internal factors and natural external factors can cause significant climate changes. These periods have a magnitude similar to the current warming and it's agreed that they were cause by internal factors and natural external factors only. The Medieval Warm Period mentioned above had a magnitude similar to the warming in the 20th century, and it is agreed that these changes were caused by internal factors and natural external factors only. The subsequent Little Ice Age is usually attributed to the reduction of solar activity. Some scientists have claimed in the past that the observed warming since 1860 is a natural climate recovery from the Little Ice Age. Climate changes occur due to internal and external factors. Internal factors are factors associated with the complexity of the climate systems which are chaotic non-linear systems. External factors can be natural factors and anthropogenic factors. The main natural external factor is the solar radiation variability. The solar radiation can change because of the solar cycles and because the Sun is getting hotter. Anthropogenic factors are related to changes in the environment influenced by Man. The main anthropogenic factors are the emission of greenhouse gases, the depletion of stratospheric ozone, the land use and the emission of aerosols such as sulphates. According to the IPCC, Earth has seen a significant increase in average global surface temperature over the last 150 years. The main evidence for global warming comes from thermometer measurements from land stations since 1860. The data from these stations, particularly those located outside of the United States, show an average surface temperature increase of 0.4 to 0.8 Celsius degrees during the 20th century. Most of the warming occurred during two periods: 1910 to 1945 and 1976 to 2000.

The increase since before 1900 is less relevant than that since 1975, as "...about 80 percent of the carbon dioxide from human activities was added to the air after 1940. Thus increased carbon dioxide in the air cannot account for the pre-1940 warming trend. That trend had to be largely natural. Then, as the air's carbon dioxide content increased most rapidly, temperatures dropped for nearly 40 years. And it seems that human effects amount at most to about 0.1 degree Celsius per decade -- the maximum increase in warming seen after the 1970s". (Sally Baliunas)

Secondary evidence comes from observed variations in the snow cover and ice extent, global average sea level, precipitation, cloud cover, El Nino, and extreme weather events during the 20th century. For example, satellite data shows a 10% decrease of snow cover since the late 1960s, and the Northern Hemisphere spring and summer sea-ice extent has decreased by about 10% to 15% since the 1950s and there has been a widespread retreat of mountain glaciers in non-polar regions throughout the 20th century. Evidence of global warming is uneven with certain places, especially the north shore of Alaska, showing dramatic rises temperature far above the average for the globe as a whole.

Evidence against a current warming period
In January 2002, scientists released data showing that the Antarctic ice cap had grown about 25%. Some editorial writers claimed that this contradicts the expectation that rising temperatures should cause the ice cap to shrink. However, the scientists studying the situation in the Antarctic who released this data point out that local cooling in some areas is consistent with an overall trend of global warming and insist that "the ice-sheet growth that we have documented in our study area has absolutely nothing to do with any recent climate trends." Weather balloons and satellites measuring the temperature of the atmosphere above the earth's surface show no warming trend. Weather balloons and satellites measuring the temperature of the atmosphere above the earth's surface since 1979 show a warming trend in the troposhere of 0.074°C/decade up to August 2003; see satellite temperature measurements for details. A National Academy of Sciences panel that reviewed the apparent discrepancy concluded that "the warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century. The disparity between surface and upper air trends in no way invalidates the conclusion that surface temperature has been rising".

The satellite data are controversial and incorporate a number of corrections for, amongst other effects, orbital drift of the satellites. Other analyses of the same data produce different (higher) trends.

Climate change factors
Climate changes occur due to internal and external factors. Internal factors are factors associated with the complexity of the climate systems which are chaotic non-linear dynamic systems. External factors can be natural factors (such as increased solar radiation) and anthropogenic factors (due to human actions).

Scientific opinion
The current scientific consensus on global warming is that the Earth is warming, and that humanity's greenhouse gas emissions are making a significant contribution. This consensus is summarized by the findings of the Intergovernmental Panel on Climate Change (IPCC). In the Third Assessment Report, the IPCC concluded that most of the warming observed over the last 50 years is attributable to human activities (see attributable to human activities) and the extent of this consensus is discussed at scientific opinion on climate change. Since it is such an important issue, governments need predictions of future trends in global change so they can take political decisions to avoid undesired impacts. This position was recently supported by an international group of science academies from various G8 countries including Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, the United Kingdom, and the United States. In 2004, Naomi Oreskes published a survey of 928 peer-reviewed scientific abstracts on climate change, concluding that there is a scientific consensus on the reality of anthropogenic climate change. It was also pointed out that "authors evaluating impacts, developing methods, or studying paleoclimatic change might believe that current climate change is natural. However, none of these papers (sic) argued that point".

Over the past century or so the global (land and sea) temperature has increased by approximately 0.6 ± 0.2°C. Temperatures in the lower troposphere have increased at somewhere between 0.12 and 0.22; °C per decade since 1979. Over the past 1-2 thousand years before 1850 the temperature is believed to have been relatively stable, with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age. Over the past 20,000 years the dominant temperature signal has been the end of the last ice age, approximately 12,000 years ago. Since then the temperature has been relatively stable, though with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age. The graph, based on Mann et al. 1999, entitled '10 centuries of NH temperature' does not show either of these anomalies. For further discussion see Temperature record of the past 1000 years. For details over the last century see the article historical temperature record; for the longer term see Temperature record of the past 1000 years; for attribution see anthropogenic climate change. The effects of global warming are increasingly visible. At the same time, atmospheric carbon dioxide has increased from around 280 parts per million (by volume) in 1800 to around 315 in 1958 and 367 in 2000, a 31% increase over 200 years, and about 380 in 2006. Other greenhouse gas emissions have also increased. Future CO2 levels cannot be predicted with any precision, since they depend on uncertain economic, sociological, technological, and natural developments. The IPCC SRES gives a wide range of future CO2 scenarios, ranging from about 400 to 1000 ppmv by 2100.

Climate models, driven by estimates of increasing carbon dioxide and to a lesser extent by generally decreasing sulphate aerosols, predict that temperatures will increase (with a range of 1.4° C to 5.8°C for change between 1990 and 2100. Much of this uncertainty results from not knowing future CO2 emissions, but there is also uncertainty about the accuracy of climate models. Note that although most studies focus on the period up to 2100, warming would be expected to continue past then, since CO2 has a long average atmospheric lifetime. Climate commitment studies predict that even if levels of greenhouse gases and solar activity were to remain constant, the global climate is committed to 0.5°C of warming (some model results are as high as 1.0°C) over the next one hundred years due to the lag in warming caused by the oceans.

Past 200 years/From the present to the dawn of human settlemen
In the past 200 years human industrial activity has injected carbon dioxide and other greenhouse gases into the atmosphere, and recently global average temperatures have been increasing. The scientific consensus is that these greenhouse gases have been responsible for most of the present warming trend. That consensus is not unanimous.

Global temperatures on both land and sea have increased by 0.75°C (1.4°F) relative to 1860-1900 the global (land and sea) temperature has increased by 0.75°C (1.4°F), according to the instrumental temperature record; the urban heat island is not believed to be significant. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25°C/decade against 0.13°C/decade). Temperatures in the lower troposphere have increased between 0.12 and 0.22°C per decade since 1979. Over the past one or two thousand years before 1850, world temperature is believed to have been relatively stable, with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age. Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable wide-spread instrumental measurements became available in the late 1800s, beating the previous record set in 1998 by a few hundredths of a degree Celsius. Similar estimates prepared by the World Meterological Organization and the UK's Climatic Research Unit concluded that 2005 was still only the second warmest year behind 1998.

Depending on the time frame, different temperature records are available. These are based on different data sets, with different degrees of precision and reliability. An approximately global instrumental temperature record begins in about 1860; contamination from the urban heat island effect is believed to be small and well controlled for. A longer-term perspective is available from various proxy records for recent millenia; see Temperature record of the past 1000 years for a discussion of these records and their differences.

A number of temperature records are available based on different data sets with different time frames. The longest perspective is available from various proxy records for recent millennia. An approximately global instrumental record of temperature near the earth's surface begins in about 1860. Global observations of the atmosphere well above the Earth's surface using data from radiosondes began shortly after World War II, while satellite temperature measurements of the tropospheric temperature date from 1979. An earlier suspicion that the urban heat island effect was inflating surface measurements and hence responsible for a discrepancy between satellite and surface records could not be confirmed. As of 2007, all temperature records are in good general agreement. The attribution of recent climate change is clearest for the most recent period of the last 50 years, for which the most detailed data are available. Note that the anthropogenic emissions of other pollutants exert a cooling effect by increasing the reflection of incoming sunlight; this partially accounts for the plateau/cooling seen in the temperature record in the middle of the twentieth century, though this may also be due to intervening natural cycles.

Global warming is being studied by the Intergovernmental Panel on Climate Change (IPCC). The IPCC does not commission or carry out research itself, but rather disseminates the body of published research. The reports reflect the consensus of the published science. The 1995 IPCC report concluded that "The balance of evidence suggests a discernible human influence on global climate"; this was strengthened in the 2001 report to "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities". The Fourth Assessment Report of the IPCC was released in November 2007.

A survey by Dennis Bray and Hans von Storch in 1996 showed a tendency of scientists in this field to agree that it is "certain that, without change in human behavior, global warming will definitely occur sometime in the future" - with the survey showing these scientists giving this statement an average score of 2.6 on a scale from 1 to 7, where 1 indicated complete agreement and 7 indicated complete disagreement. The proportion of scientists who support or oppose any of the various global warming theories is a matter of controversy in its own right. Environmentalists and their allies claim virtually unanimous support for the global warming theory from the scientific community. Opponents maintain that it's the other way around, claiming that the overwhelming majority of scientists either dismiss global warming altogether or merely consider it "unproven" (see global warming scepticism). See scientific opinion of global warming for discussion of an opinion survey of scientists done on this topic. A survey in 1996 by Dennis Bray and Hans von Storch of the Meteorologisches Institut der Universität Hamburg generated responses from over 400 German, American and Canadian climate researchers and was reported in the United Nations Climate Change Bulletin. See scientific opinion of global warming for further discussion of this and other opinion surveys of scientists.

Overview/Temperature records/Historical warming of the Earth since mid-1800s


According to the Goddard Institute for Space Studies (12/2005) temperature rose in 100 years 0.8°C. There is a 25-30 years lag in the warming effect of carbon dioxide, before the complete influence in the climate. In 2006 the 0.8°C temperature raise was the consequence of the carbon emissions in the atmosphere until the end of year 1980. Since then the annual carbon emissions have increased. According to the best estimates the emissions during the past 30 years have raised the temperature an additional 0.7°C. Thus, humans have created already 1.5°C temperature increase in total. ''The scientist may still argue what caused the extinction of dinosaurs. But there is no doubt, who is responsible for extinction now – humans. We may have only 15 years of time to prevent the irreversible climate change''. Multiple independently produced datasets confirm that between 1880 and 2012, the global average (land and ocean) surface temperature increased by 0.85 [0.65 to 1.06]°C. Currently, surface temperature rise with about 0.2°C degrees per decade. Since 1950, the number of cold days and nights have decreased, and the number of warm days and night have increased.These trends can be compared to historical temperature trends: patterns of warming and cooling like the Medieval Climate Anomaly and the Little Ice Age were not as synchronous as current warming, but did reach temperatures as high as late-20th century regionally.

Although the increase of the average near-surface atmospheric temperature is commonly used to track global warming, over 90% of the additional energy stored in the climate system over the last 50 years is in warmer ocean water. The rest has melted ice and warmed the continents and the atmosphere. The warming evident in the instrumental temperature record is consistent with a wide range of observations, documented by many independent scientific groups, for example in most continental regions the frequency and intensity of heavy precipitation has increased. Further examples include sea level rise, widespread melting of snow and land ice, increased heat content of the oceans, increased humidity, and the earlier timing of spring events, such as the flowering of plants.



Global temperatures on both land and sea have increased by 0.75°C (1.35°F) relative to the period 1860–1900, according to the instrumental temperature record. According to many, this measured temperature increase is not significantly affected by the urban heat island effect, though a current physical review of USHCN stations is finding that well over half of stations surveyed (survey 34.5% complete) have errors greater than or equal to 2°C. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25°C per decade against 0.13°C per decade). Evidence for warming of the climate system includes observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. The most common measure of global warming is the trend in globally averaged temperature near the Earth's surface. The Earth's average surface temperature, expressed as a linear trend, rose by 0.74 ± 0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13 ± 0.03 °C per decade, versus 0.07 °C ± 0.02 °C per decade). The urban heat island effect is estimated to account for about 0.002°C of warming per decade since 1900. Temperatures in the lower troposphere have increased between 0.12 and 0.22°C (0.22 and 0.4°F) per decade since 1979, according to satellite temperature measurements. Depending on what data one chooses to emphasize, different conclusions are possible (see historical temperature record). Over the past 20,000 years the dominant temperature signal has been the end of the last ice age, approximately 12,000 years ago. Since then the temperature has been quite stable, though with regionally various fluctuations, e.g. Medieval Warm Period or Little Ice Age. The validity of a single "global temperature for the Earth" has recently been questioned, however, opening the possibility of interpreting a given temperature field "as both 'warming' and 'cooling'". Over the past century or so the global temperature has increased by approximately 0.4 - 0.8 oC. For details see the article historical temperature record.

The measured temperature increase is not significantly affected by the urban heat island effect according to the IPCC. Statistical evidence suggests that this data has been exaggerated by changing patterns of human activity in the vicinity of weather stations. Global warming skeptics McKitrick and Michaels disagree, estimating that more than half the measured global average land temperature increase from 1980 to 2002 is actually the result of changing socioeconomic activity in the regions where the temperature data was gathered. The Fourth Assessment Report from the IPCC disagrees, concluding that "any urban-related trend is an order of magnitude smaller than decadal and longer time-scale trends". The IPCC authors acknowledge that warming is geographically correlated with socioeconomic factors, but attribute this correlation to coincidence: "The locations of socioeconomic development happen to have coincided with maximum warming, not for the reason given by McKitrick and Mihaels [sic] (2004), but because of the strengthening of the Arctic Oscillation and the greater sensitivity of land than ocean to greenhouse forcing owing to the smaller thermal capacity of land". According to McKitrick and Michaels, this explanation is inconsistent with existing peer reviewed literature.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land. Since the northern hemisphere has more land mass than the southern it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the northern than southern hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center]], since reliable 2005 and 2010 tied for the planet's the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree. 2007 also tied 1998 as being the second warmest year in recorded history, which is noteworthy because there was a stong La Nina present during the second half of 2007 and not an El Nino which occured in 1998. 2008 was the ninth warmest year according to NASA because of a strong La Nina pattern and reduced solar irradiance. NASA's Goddard Institute states that because of increasing human made greenhouse gases, a record warm year exceeding 2005 can be expected by at least 2012. Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998. Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year. Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 1999 to 2009 is consistent with such an episode. More recently, GISS has estimated that a new record 12-month global temperature record will be set in 2010, and that 2010 calandar year surface temperature may also set record global temperatures.

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century, though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion -- CO2 and aerosols -- have, for the short term, largely offset one another, so that net warming in recent decades has been driven mainly by non-CO2 greenhouse gases.

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation. Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed. Dr. Richard S. Lindzen, the Alfred P. Sloan Professor of Meteorology at MIT, indicates that the "current climate models exaggerate the impact of carbon dioxide on temperature because of a poor understanding and representation of the feedback effects due to clouds and water vapor". Professor Lindzen further states "Attributing global warming to the rise in greenhouse gases has been reduced to an issue of religious faith modulated by policy relevance". All four major global temperature tracking outlets (Hadley, NASA's GISS, UAH, RSS) reported global cooling in 2007, ranging from cooling of 0.65C to 0.75C.



All the quantitative reconstructions favored by groups promoting the GW theory show temperatures as having been roughly stable for the last 1,000 years, but sharply rising in the last century (see Temperature record of the past 1000 years). These records, combined with attribution analysis, indicate that it is likely this is due to human activity (see anthropogenic climate change). Environmentalists have been quick to believe this and generally urge quick and radical action to save the environment (see Kyoto Protocol and UNFCCC).

Qualitative historical evidence show warming and cooling, and there is some evidence that these correlate with sunspots and other aspects of solar activity; for example, some historians argue that the Medieval Warm Period enabled the colonization of Greenland. Recent research suggests that episodes of warming and cooling such as the Medieval Warm Period or the Little Ice Age may have been regional, not global.

The difference between the interpretations of the historical record affects how the most recent warming trend is viewed: the quantitative records show the recent warming trend, and the current warmth, as unusual; from the qualitative record, many "skeptics" believe that the recent trend is not unusual, and reject calls for actions such as the Kyoto Protocol (see global warming controversy, historical temperature record, Temperature record of the past 1000 years).

The current conflict results in professional and personal disagreements as well as pressuring political forces. Presently this is particularly visible in various interpretations in topics such as the UNFCCC Kyoto Protocol (see global warming controversy). The above paragraphs might give the impression that belief in the course of past climate change correlates strongly with advocacy for future actions: this is not necessarily so. It is possible, perhaps common, to study the past record and give no counsel on the future.

The picture is actually more complicated than this. Atmospheric scientists know that adding CO2 to a dry atmosphere, with no other changes, will tend to make a planet's surface warmer. The issue is that we do not live on a planet with a dry atmosphere; there is an important amount of water vapor (humidity, clouds) in the Earth's atmosphere, and water vapor is a strong greenhouse gas. If adding CO2 to the atmosphere changes processes that regulate the amount of water vapor in the Earth's atmosphere, that could have a profound effect on the climate.

The effect of clouds is also critical. Clouds have competing effects on the climate; everyone has noticed that surface temperature drops when a cloud passes overhead on an otherwise hot, sunny summer day. So clouds cool the surface by reflecting sunlight back into space. But many people have also noticed that clear winter nights tend to be colder than cloudy winter nights. That's because clouds also radiate heat back to the surface of the Earth. Bottom line, clouds have competing effects on the climate. If CO2 changes the amount of distribution of clouds, it could have various complex effects on the climate.

Given this, it is not correct to imagine that there is a debate is between those who "believe in" and "oppose" the theory that adding CO2 to the Earth's atmosphere will result in warmer surface temperatures on Earth, on average. Rather, since it is known that adding CO2 to a dry atmosphere warms a planet, on one side of the debate are those who believe that adding CO2 to the Earth's atmosphere (which is not dry) still has a net warming effect. On the other side of the debate are those who theorize that changes in water vapor, clouds, and so on will act such as to exactly cancel out the warming effect that would be seen in a dry atmosphere. Unfortunately, the observed warming of the Earth over the past 50 years appears to be at odds with the skeptic's theory that climate feedbacks will exactly cancel out the CO2 warming.

Scientists have also studied this issue with computer models of the climate (see below). These models are accepted by the scientific community as being valid only after it has been shown that they do a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Nino. It is universally found that climate models that pass these tests always predict that the net effect of adding CO2 will be a warmer climate in the future, when all the water vapor and cloud changes are taken into account. The amount of predicted warming varies by model, however, which probably reflects the way different models depict clouds differently. Skeptics of "global warming" say that all the models are incorrect, but have been unable to produce a model of the climate that correctly simulates known natural variability but does not predict temperatures will increase in the future. Thus, the skeptics' theory that climate feedbacks will eliminate any CO2 warming effect is not supported by either the observations or any credible model. The period of time over which the change has been observed may vary according to the focus of the user of the term and the datasets available for investigation: sometime since the Industrial Revolution, or since the beginning of an approximately global historical temperature record in about 1860; or over the past century; or the most recent 50 years. Temperature record holds a discussion of the various records.

For details of changes during various periods:


 * General discussion of Temperature records see: Temperature record


 * Record of past century see: Instrumental temperature record


 * Record of recent millenia see: Temperature record of the past 1000 years


 * For attribution see: Anthropogenic climate change

The temperature increase has not been uniform over the globe or over time. Recent research (Peterson 2003; Parker 2004) indicates that estimates of temperature trends may not be much influenced by the urban heat island effect. While the accuracy of collected station data is not in dispute, the records suffer from incomplete coverage, geographically and historically, making the conclusions drawn from the data subject to disagreement.

While some cities may show warming, others show cooling relative to rural trends. Hansen et al. adjusted trends in urban stations around the world to match rural stations in their regions, in an effort to homogenise the temperature record. Of these adjustments, 42% warmed the urban trends: which is to say that in 42% of cases, the cities were getting cooler relative to their surroundings rather than warmer. The individual surface temperature records may be influenced by the urban heat island effect.

Temperatures in the lower troposphere have increased at somewhere between 0.08 and 0.22°C per decade since 1979 (see Satellite temperature measurements). Just like the surface record, the average temperature rise is not linear, but has rises and falls superimposed on it due to natural variability, most notably El Niño's. Over the same period the surface record shows a warming of approximately 0.15°C/decade.

A new reconstruction by Moberg, et al, published in February 10, 2005 shows both the Medieval Warm Period and Little Ice Age anomalies (although not by name) and concludes that the temperatures around 1000 and 1100 AD were comparable to those of the 20th century before 1990. "Moberg's reconstruction will help to put the record straight in one of the most contested issues in palaeoclimatology", says Hans von Storch. "But it does not weaken in any way the hypothesis that recent observed warming is a result mainly of human activity". Moberg's results are consistent with those of Von Storch, et al, who conducted a modeling analysis that showed the variability to be about twice as great as previously published.

Mann concedes that past climate variations may be larger than thought, but that "The contrarians would have us believe that the entire argument of anthropogenic climate change rests on our hockey-stick construction. But in fact some of the most compelling evidence has absolutely nothing to do with it, and has been around much longer than our curve".

Theories to explain temperature change
The climate system varies both through natural, "internal" processes as well as in response to variations in "external forcing" from both human and non-human causes, including changes in solar activity and volcanic emissions as well as greenhouse gases. Most climatologists accept that the earth has warmed recently, but the cause or causes of this change is somewhat more controversial, especially outside the scientific community. Somewhat more controversial is what may have caused this change. See Anthropogenic Climate Change for further discussion of "attribution" of change. See Climate change for further discussion of these forcing processes.

Regional trends
Global warming refers to global averages, with the amount of warming varying by region. Since 1979, global average land temperatures have increased about twice as fast as global average ocean temperatures. This is due to the larger heat capacity of the oceans and because oceans lose more heat by evaporation. Where greenhouse gas emissions occur does not impact the location of warming because the major greenhouse gases persist long enough to diffuse across the planet, although localized black carbon deposits on snow and ice do contribute to Arctic warming.

The Northern Hemisphere and North Pole have heated much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, its arrangement around the Arctic Ocean has resulted in the maximum surface area flipping from reflective snow and ice cover to ocean and land surfaces that absorb more sunlight. There are no known climatologists supporting this viewpoint. Arctic temperatures have increased and are predicted to continue to increase during this century at over twice the rate of the rest of the world. As the temperature difference between the Arctic and the equator decreases ocean currents that are driven by that temperature difference, like the Gulf Stream, are weakening.

Short-term slowdowns and surges
Because the climate system has large thermal inertia, it can take centuries for the climate to fully adjust. While record-breaking years attract considerable public interest, individual years are less significant than the overall trend. Global surface temperature is subject to short-term fluctuations that overlay long-term trends, and can temporarily mask or magnify them.

An example of such an episode is the slower rate of surface temperature increase from 1998 to 2012, which was dubbed the global warming hiatus by the media and some scientists. Throughout this period ocean heat storage continued to progress steadily upwards, and in subsequent years surface temperatures have spiked upwards. The slower pace of warming can be attributed to heating and cooling in the Pacific Ocean from natural variability such as El Niño and La Nina events, reduced solar activity, and a number of volcanic eruptions that inserted sunlight-reflecting particles into the atmosphere.

Physical drivers of climate change


By itself, the climate system may generate changes in global temperatures for years (such as the El Niño–Southern Oscillation) to decades at a time. Other changes emanate from so-called external forcings. These forcings are "external" to the climate system, but not necessarily external to Earth. Examples of external forcings include changes in the composition of the atmosphere (e.g., increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.

Attributing detected temperature changes and extreme events to man-made increases in greenhouse gases requires scientists to rule out known internal climate variability and natural external forcings. So a key approach is to use physical models of the climate system to determine unique fingerprints for all potential external forcings. By comparing these fingerprints with the observed pattern and evolution of the climate change, and the observed evolution of the forcings, the causes of the observed changes can be determined.

The Greenhouse gas theory


The Earth's climate changes in response to external forcings. External forcing is a term used in climate science for processes external to the climate system (though not necessarily external to Earth). Climate responds to several types of external forcing, such as changes in greenhouse gas concentrations, changes in solar luminosity, and volcanic eruptions. Climate also changes in response to variations in Earth's orbit around the Sun, but as these orbital cycles vary over thousands of years they are too gradual to have affected temperature changes observed in the past century. Attribution of recent climate change focuses on the first three types of forcing. Greenhouse gases trap heat radiating from Earth to space. This heat, in the form of infrared radiation, gets absorbed and emitted by these gases in the planet's atmosphere thus warming the lower atmosphere and the surface. On Earth, an atmosphere containing naturally occurring amounts of greenhouse gases causes air temperature near the surface to be warmer by about 33 C-change than it would be in their absence. This is much colder than the conditions that actually exist at the Earth's surface (the global mean surface temperature is about 14 °C). Instead, the necessary −19 °C is found at an altitude about 5 km above the surface". The greenhouse effect produces an average worldwide temperature increase of about 33 C-change compared to black body predictions without the greenhouse effect, not an average surface temperature of 33 °C. The average worldwide surface temperature is about 14 °C.}} Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing temperature of water. The measure of the temperature response to increased greenhouse gas concentrations and other anthropogenic and natural climate forcings is climate sensitivity. It is found by observational and model studies. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere. The current literature estimates sensitivity in the range of 1.5 to 4.5°C (2.7 to 8.1°F). The major greenhouse gases are water vapour, which causes about 36–70% of the greenhouse effect; carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone (O3), which causes 3–7%.

The hypothesis that increases or decreases greenhouse gas concentration would lead to a higher or lower global mean temperature was first postulated in the late 19th century by Swedish chemist and 1903 Nobel Laureate Svante Arrhenius, largely as an attempt to explain ice ages. At the time his peers largely rejected his theory. Svante Arrhenius used the measured infrared absorption of carbon dioxide to calculate that increases in greenhouse gas concentration would lead to higher global mean temperatures while decreases would lead to colder global mean temperatures. The idea arose largely as Arrhenius' attempt to explain ice ages. At the time his peers largely rejected this theory. Arrhenius' colleague Arvid Högbom was one of the first to study the carbon cycle. Through him Arrhenius was aware that in 1890 emission and absorption of CO2 were roughly in balance. Their best estimates were that burning of fossil fuels would not be a future problem, however, this was based on coal consumption at the end of the 19th century. The longest sustained measurement of CO2 in the atmosphere is reflected in the Keeling Curve. Measurements conducted by Dr. Charles David Keeling atop Mauna Loa in Hawaii show an increase of more than 50 parts per million by volume from 1958 to the present.

The theory that human greenhouse gas emissions have contributed to the warming of the Earth's atmosphere in the 20th century, has gained both adherents and opponents in the scientific community within the past 25 years. Climate researcher Stephen Schneider, who thought in the 1970s that aerosols might lead to global cooling, has become a strong proponent of the global warming hypothesis. Atmospheric physicist Richard Lindzen and Harvard astronomer Sallie Balunias oppose the theory. The IPCC endorsed the theory, as did the United States National Academy of Sciences.



The picture is actually more complicated than this. There are many subtle aspects to the question. Atmospheric scientists know that adding carbon dioxide (CO2) to a dry atmosphere, with no other changes, will tend to make a planet's surface warmer. The issue is that we do not live on a planet with a dry atmosphere; but there is an important amount of water vapor (humidity, clouds) in the Earth's atmosphere, and water vapor is a strong greenhouse gas. If adding CO2 to the atmosphere changes processes that regulate the amount of water vapor in the Earth's atmosphere, that could have a profound effect on the climate: more water vapor means more warmth.

The effect of clouds is also critical. Clouds have competing effects on the climate; everyone has noticed that surface temperature drops when a cloud passes overhead on an otherwise hot, sunny summer day. So hence, one of the roles that clouds play in climate is in cooling the surface by reflecting sunlight back into space. But, however, many people have also noticed that clear winter nights tend to be colder than cloudy winter nights. Yet, seemingly opposite phenomena have occurred, such as when clear winter nights become colder, rather than warmer, in contrast to cloudy winter nights. The general concept therefore, is that because clouds also block the radiation of heat away from the surface (and eventually into space), and radiate it back to the surface of the Earth, moderating otherwise more extreme temperatures. Bottom line, clouds have competing effects on the climate. If CO2 changes the amount of distribution of clouds, it could have various complex effects on the climate. In the 2001 IPCC report on climate change, the possible changes in cloud cover were highlighted as one of the dominant uncertainties in predicting future climate change.

Given this, it is not correct to imagine that there is a debate is between those who "believe in" and "oppose" the theory that adding CO2 to the Earth's atmosphere will result in warmer surface temperatures on Earth, on average. Rather, the debate is about what the net effect of the addition of CO2 will be, and whether changes in water vapor, clouds, and so on will cancel out its warming effect. The observed warming of the Earth over the past 50 years appears to be at odds with the skeptics' theory that climate feedbacks will cancel out the CO2 warming, unless other environmental factors have contributed to that warming.

Scientists have also studied this issue with computer models of the climate (see below). These models are accepted by the scientific community as being valid only after it has been shown that they do a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Nino. It is universally found that climate models that pass these tests always predict that the net effect of adding CO2 will be a warmer climate in the future, when all the water vapor and cloud changes are taken into account. The amount of predicted warming varies by model, however, which probably reflects the way different models depict clouds differently. Skeptics point to the growing evidence that variation in cosmic ray flux represents an indirect effect of changes in solar activity that increases the warming response to increases in solar activity. Climate models that pass the above tests while only modeling the direct effects of increases in solar activity will have attributed too much of the historical warming to greenhouse gas forcing, and will predict larger increases in temperature in the future. Skeptics of "global warming" (point to potential feedbacks which current models poorly understand, such as changes in vegetation and cloud cover, and suggest that these processes reduce the sensitivity of the climate to greenhouse gas forcing; although the uncertainty could just as easily be in the other direction) say that all the models are incorrect, but have been unable to produce a model of the climate that correctly simulates known natural variability, but does not predict temperatures will increase in the future. Thus, the skeptics' theory that climate feedbacks will eliminate any CO2 warming effect is not supported by either the observations or any credible model. Proponents of established global warming theory argue that the uncertainty could just as easily extend to increases in warming as reductions in it. For example, it has been hypothesized that an increase in average temperature would cause volatilization of methane clathrates, which would increase the amount of methane in the atmosphere, causing further warming and hence further volatilization.

The combustion of fossil fuels, including the automobile exhausts, coal-burning power plants, factory smokestacks, and other waste vents of the industrial age now pump about 22 billion tons of carbon dioxide (corresponding to 6 billion tons of pure carbon) and other greenhouse gases into the earth's atmosphere each year. About half of human emissions have remained in the atmosphere. Animal agriculture, manure, natural gas, rice paddies, landfills, coal, and other anthropogenic sources contribute about 450 million tons of methane each year according to TAR. Greenhouse gases are transparent to shortwave radiation from the sun, the main source of heat on the Earth. However, they absorb some of the longer infra-red radiation from the earth, making it more difficult for the Earth to cool and thus reducing radiational cooling. How much they warm the world by is shown in their global warming potential.

The measure of the response to increased GHGs, and other anthropogenic and natural climate forcings is climate sensitivity. It is found by observational and model studies. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere. The current literature estimates sensitivity in the range 1.5-4.5°C (2.7-8.1°F).



The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere are purported to warm a planet's lower atmosphere and surface. It was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. The existence of the greenhouse effect is a subject of controversy; for example, Khabibullo Abdusamatov, head of the space research laboratory at the Saint Petersburg-based Pulkovo Observatory has stated that "ascribing 'greenhouse' effect properties to the Earth's atmosphere is not scientifically substantiated". It is the process by which absorption and emission of infrared radiation by atmospheric gases warms a planet's lower atmosphere and surface. Existence of the greenhouse effect as such is not disputed. The question is instead how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of particular greenhouse gases. The existence of the greenhouse effect as such is not disputed. Greenhouse gases create a natural greenhouse effect without which mean temperatures on Earth would be an estimated 30°C (54°F) lower so that Earth would be uninhabitable. Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 30°C (54°F), without which Earth would be uninhabitable, without which life as we know it would not exist, asserts the IPCC. Thus scientists do not "believe in" or "oppose" the greenhouse effect as such; rather, the debate concerns the net effect of the addition of greenhouse gases, while allowing for associated positive and negative feedback mechanisms, and increases in the severity of earthquakes. Rather, the debate centers on how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases in the atmosphere.

According to the National Center for Policy Analysis, water vapor comprises 95% of the greenhouse gases in the atmosphere, humans are responsible for just 3.4% of carbon dioxide emissions, and the human contribution to the greenhouse effect is 0.28% even though more than 400 species of life are known to thrive at the bottom of ocean at deep-sea hydrothermal vents; without the need for the greenhouse effect. On Earth, the major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%. Clouds also affect the radiation balance, but they are composed of liquid water or ice and so are considered separately from water vapor and other gases. Global changes in atmospheric composition occur from anthropogenic [human-caused] emissions of greenhouse gases, such as carbon dioxide that results from the burning of fossil fuels and methane and nitrous oxide from multiple human activities.

Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. Approximately 20% of methane gas emissions (which exert a stronger warming influence than carbon dioxide emissions) are due to direct release via livestock flatulence and eructation. A recent UN report indicates livestock generate more greenhouse gases on a global scale than vehicles. A senior UN official and co-author of the report Henning Steinfeld said "Livestock are one of the most significant contributors to today's most serious environmental problems". President of the National Academy of Sciences Ralph Cicerone (an atmospheric scientist), has indicated the contribution of methane by livestock flatulence and eructation to global warming is a "serious topic" Cicerone states "Methane is the second-most-important greenhouse gas in the atmosphere now. The population of beef cattle and dairy cattle has grown so much that methane from cows now is big. This is not a trivial issue". Approximately 5% of the methane is released via the flatus, whereas the other 95% is released via eructation. Vaccines are under development to reduce the amount introduced through eructation. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago. "About three-quarters of the anthropogenic man-made emissions of CO2 to the atmosphere during the past 20 years are due to fossil fuel burning. Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. The rest of the anthropogenic emissions are predominantly due to land-use change, especially deforestation". However, the human contribution to the greenhouse effect is minimal. CO2, a "trace gas", makes up only 38 out of every 100,000 molecules of air; and five year's worth of human greenhouse gas emissions adds just 1 molecule of CO2 to every 100,000 molecules of air.

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume. Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used. Research by NASA climate scientist James Hansen indicates the 0.75° rise in average global temperatures over the last 100 years has been driven mainly by greenhouse gases other than carbon dioxide. The Swedish utility Vattenfall did a study of full life cycle emissions of Nuclear, Hydro, Coal, Gas, Solar Cell, Peat and Wind which the utility uses to produce electricity. The net result of the study was that nuclear power produced 3.3 grams of carbon dioxide per KW-Hr of produced power. This compares to 400 for natural gas and 700 for coal (according to this study). The study also concluded that nuclear power produced the smallest amount of CO2 of any of their electricity sources.

The December 2007 global warming conference in Bali contributed to global warming in the following ways:
 * A November 25, 2007 article in Times Online reported that it was estimated that that year's conference would release the equivalent of 100,000 tons of carbon dioxide.
 * A December 18, 2007 article in the Sydney Morning Herald revealed new information that brought this total even higher. According to the article, a special custom air conditioning system was installed specifically for the conference. The air conditioning system used hydrochlorofluorocarbons, an outdated refrigerant gas that is especially bad for the problem of global warming. According to the article, the air conditioning used during the conference released the equivalent of 48,000 tons of carbon dioxide. The article stated, "... the refrigerant is a potent greenhouse gas, with each kilogram at least as damaging as 1.7 tonnes of carbon dioxide. Investigators at the Balinese resort complex at Nusa Dua counted 700 cylinders of the gas, each of them weighing 13.5 kilograms, and the system was visibly leaking".

The elements of Global Warming herein identified are symptoms of a deeper cause. In March 2006, Popular Science reported that the earth received from meteorites alone over two hundred tons of mass debris daily; this does not include other forms of mass increase. Knowing this and applying the mass accumulation into E=mc2, we know that an acceleration is taking place. Unless some phenomenon intervenes, then logically and very much realistically, earth can accelerate itself into energy. On this path to energy, primarily due to acceleration, heat will manifest. This is the real cause of global warming. As doctors in the medical field mask symptoms because they fail to discover cause, so scientists today are dealing with the mere symptoms of global warming, rather than the real issue, which is earthly mass increase that causes acceleration, which acceleration is attended by heat necessary for transitional phases from mass to energy.

Positive (reinforce) feedback effects such as the expected release of CH4 from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes) may lead to significant additional sources of greenhouse gas emissions not included in climate models cited by the IPCC.



They are called greenhouse gases because they trap radiant energy from the sun that would otherwise be re-radiated back into space, by analogy with the glass panes in a greenhouse. The analogy, however, is a false one: the effects are different: see greenhouse effect. (The fact that a natural greenhouse effect occurs is well-known and is not at issue in the debate over global warming. Without it, temperatures would drop by approximately 30°C, the oceans would freeze and life as we know it would be impossible). What climatologists are concerned about, rather, is that increased levels of greenhouse gases in the atmosphere might cause more heat to be trapped. Scientists and industrialists alike are not opposed to the natural greenhouse effect that tempers the earth's climate, as it is responsible for maintaining the current habitable temperature: without it, temperatures would be approximately 30°C lower. However, there is a concern by climatologists that increasingly higher levels of greenhouse gases, produced by human activity in the atmosphere might cause excessive temperature rises.

Concentrations of greenhouse gases in the atmosphere are currently at approximately 25% above pre-industrial values. The atmospheric concentration of carbon dioxide (CO2) and methane (CH4) has increased by 31% and 149% respectively above pre-industrial levels since 1750. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data exists, from ice cores. From less direct geological evidence it is believed that values this high were last attained 40 million years ago. "The longest continuous instrumental measurement of CO2 mixing ratios began in 1958 at Mauna Loa. Increases in CO2 measured since 1958 at Mauna Loa show a monotonically increasing atmospheric concentration of CO2. In fact, it is clear that the increase is faster than linear. On March 21, 2004, it was reported that the concentration in ppm reached 376ppm in 2003. Since then, the annually averaged value has increased monotonically from 315 ppmv (see the Keeling Curve). The concentration reached 376 ppmv in 2003. South Pole records show similar growth. The monthly measurements display small seasonal oscillations due to the change of seasons. Methane is produced biologically and released from gas pipelines. Some biological sources are "natural" such as termites and others are attributable to human activity such as agriculture, e.g., rice paddies. Recent evidence suggests that forests may also be a source. Recent evidence indicates that methane concentrations have begun to stabilize, perhaps due to reductions in leakage from fuel transmission and storage facilities. Note that this is a contribution to the natural greenhouse effect, and not to the anthropogenic greenhouse effect. Other greenhouse gases, such as methane, are produced naturally. Scientists have recently shown evidence that trees, while helping to remove carbon dioxide from the atmosphere, actually release methane, which is a more potent greenhouse gas than carbon dioxide. Also, trees have a low albedo, meaning their dark leaves absorb sunlight, which is released as heat. At some latitudes, this results in a net warming affect, rather than the cooling affect previously predicted. Another important greenhouse gas, methane, is produced biologically. Also, at higher latitudes afforestation may increase the albedo (due largely to the effects of winter snow); at these latitudes, this results in a net warming affect.



Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage, though the actual trajectory will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels and land-use change. The IPCC Special report on emissions scenarios gives a wide range of future carbon dioxide scenarios, ranging from 541 to 970 parts per million by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar, or Methane clathrates sands are extensively used. Carbon sink ecosystems (forests and oceans) are being degraded by pollutants. Degradation of major carbon sinks results in higher atmospheric carbon dioxide levels.

Sources of greenhouse gas emissions
Globally, the majority of anthropogenic greenhouse gas emissions arise from fuel combustion. The remainder is accounted for largely by "fugitive fuel" (consumed in the production and transport of fuel), emissions from industrial processes (excluding fuel combustion), and agriculture: these contributed 5.8%, 5.2%, and 3.3% respectively in 1990. Current figures are broadly comparable. Around 17% of emissions are accounted for by the combustion of fuel for the generation of electricity. A small percentage of emissions come from natural and anthropogenic biological sources, with approximately 6.3% derived from agriculturally produced methane and nitrous oxide.

Climate sensitivity is a measure of the equilibrium response to increased GHGs and other anthropogenic and natural climate forcings. It is found by observational and model studies. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere, which, according to the 2001 IPCC report, is estimated to be between 1.5 and 4.5°C (2.7; 8.1°F) (with a statistical likelihood of 66-90%). This should not be confused with the expected temperature change by a given date, which also includes a dependence on the future GHG emissions and a delayed response due to thermal lag, principally from the oceans. Models referenced by the Intergovernmental Panel on Climate Change (IPCC), using a range of SRES scenarios, project that global temperatures will increase between 1.4 and 5.8°C (2.5 to 10.5°F) between 1990 and 2100. Positive feedback (reinforcing) effects, such as the expected release of possibly as much as 70,000 million tonnes of methane from permafrost peat bogs in Siberia, which have started melting due to the rising temperatures, may lead to significant additional sources of greenhouse gas emissions. Note that anthropogenic emissions of other pollutants - notably sulphate aerosol - can exert a cooling effect; this can account for the plateau/cooling seen in the temperature record in the middle of the 20th century, though this may also be due to intervening natural cycles not included in IPCC's climate models.

Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs, and nitrous oxide. In 2011, the concentrations of CO2 and methane had increased by about 40% and 150% respectively since pre-industrial times, with CO2 readings taken at the world's primary benchmark site in Mauna Loa surpassing 400 ppm in 2013 for the first time. These levels are much higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores. Less direct geological evidence indicates that CO2 values haven't been this high for millions of years.

Global anthropogenic greenhouse gas emissions in 2010 were 49 billion tonnes of carbon dioxide-equivalents per year (using the most recent global warming potentials over 100 years from the AR5 report). Of these emissions, 65% was carbon dioxide from fossil fuel burning and industry, 11% was carbon dioxide from land use change, which is primarily due to deforestation, 16% was methane, 6.2% was nitrous oxide, and 2.0% were fluorinated gases. Using life-cycle assessment to estimate emissions relating to final consumption, the dominant sources of 2010 emissions were: food (26–30% of emissions); washing, heating and lighting (26%); personal transport and freight (20%); and building construction (15%).

The magnitude of solar effects
Modelling studies reported in the IPCC Third Assessment Report (TAR) found that volcanic and solar forcings may account for half of the temperature variations prior to 1950, but the net effect of such natural forcings has been roughly neutral since then. The IPCC Fourth Assessment Report (AR4) gives a best estimate for radiative forcing from changes in solar activity of +0.12 watts per square metre. This is less than half of the estimate given in the TAR. For comparison the combined effects of all human activity are estimated to be an order of magnitude greater at +1.6 watts per square meter.

In a review of existing literature, Foukal et al. (2006) determined both that the variations in solar output were too small to have contributed appreciably to global warming since the mid-1970s and that there was no evidence of a net increase in brightness during this period. Some scientists assert that a warming of the stratosphere, which has not been observed, would be expected if there were a significant increase in solar activity.

Other alternative theories
There have been attempts to measure the degree of scientific consensus that "most of the warming observed over the last 50 years is attributable to human activities". The extent of the scientific consensus on global warming; that "most of the observed warming over the last 50 years is likely to have been attributable to human activities". In the journal Science in December 2004, a study by UCSD Professor of History Dr. Naomi Oreskes considered the abstracts 928 referred scientific articles in the ISI citation database identified with the keywords "global climate change" and published from 1993–2003. Dr. Oreskes concluded: Of these 928 articles 75% were either explicitly or implicitly accepting of the consensus view while 25% covered methods or paleoclimate and did not take any stance on recent climate change. The study did not report how many of the 928 abstracts explicitly accepted the hypothesis of human-induced warming, but none of the 928 articles surveyed accepted any other hypothesis. In 2007, the Intergovernmental Panel on Climate Change concluded that human actions are "very likely" the cause of global warming, meaning a 90% or greater probability. In spite of this evident consensus, various other hypotheses have been proposed, including, but not limited to. Some of these hypotheses (listed here without comment on their validity or lack thereof) include:
 * 1) The current increase in temperature is predicted by the Milankovitch cycles theory, in which gradual changes in the Earth's orbit around the Sun and changes in the Earth's axial tilt affect the amount of solar energy reaching the Earth.


 * 1) The warming is within the range of natural variation and needs no particular explanation
 * 1) The warming is a consequence of coming out of a prior cool period - the Little ice age - and needs no other explanation.


 * 1) The warming is primarily a result of variances in solar irradiance, possibly via modulation of cloud cover. It is similar in concept to the operating princples of the Wilson cloud chamber, however acting on a global scale, where earth's atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of Cloud condensation nuclei. Henrik Svensmark says that fewer cosmic rays entering the atmosphere are causing a natural period of low cloud cover.


 * 1) The warming trend itself has not been clearly established, and therefore does not need any explanation. This does not seem to be correct; the urban heat island (often cited as a problem with the surface record) appears to be a small effect.

Some skeptics would claim that the warming trend itself is not valid, and therefore does not need any explanation.
 * 1) The observed warming actually reflects the Urban Heat Island, as most readings are done in heavily populated areas which are expanding with growing population.

At present, none of these has more than a small number of supporters within the climate science community as an explanation for recent warming. Indeed, as reported in the journal Science, researcher Naomi Oreskes took a sampling of the abstracts of 928 articles on climate change, selected objectively using the key phrase "global climate change", from the published peer-reviewed scientific literature. Oreskes concluded that of the 75% of abstracts that in any way considered the question of attribution, all of them (100%) either explicitly or implicitly supported the consensus view that a significant fraction of recent climate change is due to human activities. However, the strong scientific support for man-made global warming implies that such alternative opinions are not widely held. In the journal Science, an essay by N. Oreskes considered the abstracts of all 928 scientific articles in the ISI citation database identified with the keyword "global climate change". Dr. Oreskes concluded that none of these abstracts attempt to refute the position that man-made emissions of greenhouse gases are a substantial contributor to recent warming. There are several "fingerprints" as called by Ben Santer, that show through models that global warming is human induced, such as higher altitudes getting warmer faster than lower altitudes, land warming faster than the ocean, which refute the claim that warming is the result of solar irradiance.

Climate variability
The Earth's climate changes in response to external forcing, including greenhouse gases, variations in its orbit around the Sun (orbital forcing), changes in solar luminosity, and volcanic eruptions, all examples of the earth's own variation in temperatures, for which the UNFCCC uses the term climate variability.

The solar variation theory


Variations in solar output have likely been the cause of past climate changes, and to contribute to global warming during the industrial era. The most accepted influence of solar variation on the climate is through direct radiative forcing. Solar forcing estimated by global circulation models is too small to explain significant temperature change. Although solar forcing is generally thought to be too small to account for a significant part of global warming in recent decades, a few studies disagree, such as recent phenomenological analyses that indicate the contribution of solar forcing may be underestimated, and offer a mechanism of action for this larger than anticipated solar cycle climate forcing. However, a phenomenological analysis indicated that the contribution of solar forcing may be underestimated.

Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere. Observations show that temperatures in the stratosphere have been steady or cooling since 1979, when satellite measurements became available. Radiosonde (weather balloon) data from the pre-satellite era show cooling since 1958, though there is greater uncertainty in the early radiosonde record.

The direct radiative effects of solar luminosity variations on global climate are complex and difficult to model accurately. Yet there is no doubt that the total solar irradiance of the Earth defines the climate regime. A related hypothesis is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate. Other research has found no relation between warming in recent decades and cosmic rays. A recent study, proposed by Henrik Svensmark concluded that the influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.

Studies in 2011 have indicated that solar activity may be slowing, and that the next solar cycle could be delayed. To what extent is not yet clear; Solar Cycle 25 is due to start in 2020, but may be delayed to 2022 or even longer. It is even possible that the Sun could be heading towards another Maunder Minimum. While there is not yet a definitive link between solar sunspot activity and global temperatures, the scientists conducting the solar activity study believe that global greenhouse gas emissions would prevent any possible cold snap. In line with other details mentioned above, director of NASA's Goddard Institute for Space Studies James Hansen says that the sun is not nearly the biggest factor in global warming. Discussing the fact that low amounts of solar activity between 2005 and 2010 had hardly any effect on global warming, Hansen says it is more evidence that greenhouse gases are the largest culprit; that is, he supports the theory advanced by "nearly all climate scientists" including the IPCC. Yet recent analyses indicate solar forcing may have been a prominent, even dominant forcing during the industrial era and that the modeling of solar forcing in GCM's underestimates the solar contribution to the present-day climate change. Claims of major solar and cosmic ray effects on climate change within the last century have a long history, and an associated refutational literature. The reader is cautioned to take care in evaluating the resulting complex literature. The overwhelming majority of those who study climate have concluded that, in the words of the IPCC AR4, "most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations". Various hypotheses have been proposed to explain the apparent solar correlation with temperatures that some assert appear to be stronger than can be explained by direct irradiation and the first order positive feedbacks to increases in solar activity. While the consensus is that solar forcing of climate change has not had a significant role in recent warming, the topic continues to be an active field of research. Hypotheses have been proposed that variations in solar output, possibly amplified by cloud feedbacks, have been suggested as a possible cause of recent warming. A difference between this mechanism and greenhouse warming is that an increase in solar activity should warm the stratosphere while greenhouse warming should cool the stratosphere. Cooling in the lower stratosphere has been observed since at least 1960, which would not be expected if solar activity were the main contributor to recent warming.

NASA says sunspot activity is now at a 100 year low. "This is the quietest Sun we’ve seen in almost a century", according to NASA solar scientist David Hathaway. According to NASA research, there is a cause-and-effect relationship between sunspot activity and measured changes in global temperatures on Earth. This lack of sun activity has resulted in no net change in temperature for 12 years, according to former Virginia state climatologist Patrick Michaels. Scientists at the Center for Sun-Climate Research at the Danish Space Institute are testing a theory that may explain the connection between a lack of solar activity and cooling: the Svensmark Cosmic Ray Theory. Temperatures cooling by a few degrees would be far more catastrophic to world food supplies and human health than moderate warming. A related hypothesis is that cosmic rays may affect climate by influencing the generation of cloud condensation nuclei.

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature warming over the period 1900–2000, and about 25–35% between 1980 and 2000. A paper by Peter Stott and other researchers suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated. Stott and coauthors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated. Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.

One predicted effect of an increase in solar activity would be a warming of most of the stratosphere, whereas greenhouse gas theory predicts cooling there; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming. (Reduction of stratospheric ozone also has a cooling influence, but although substantial ozone depletion did not occur until the late 1970s.) Natural phenomena such as solar variation combined with changes in volcanic activity have probably had an enhanced warming effect from pre-industrial times to 1950, but a small cooling effect since 1950, but a cooling effect since. In 2006, a team of scientists and researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles lead to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.

A 2007 paper by Lockwood and Fröhlich found that in the last 20 years no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays coming to Earth and cloudiness and temperature. Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich. Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner. This theory is enforced as Venus, Mars, Jupiter, Saturn, and Pluto are also experiencing increasing temperatures. The debate is complicated by the lack of reliable measures of solar output, even over the 30 years of satellite record; further back requires proxies such as sunspot count of cosmogenic isotopes, which are believed to (partly) correlate to solar output. In general, the IPCC describes the level of scientific understanding of the contribution of variations in solar irradiance to historical climate changes as "low". A difference between this mechanism and greenhouse warming is that an increase in solar activity should produce a warming of the stratosphere while greenhouse warming should produce a cooling of the stratosphere. Stratospheric warming has not been observed.



The present level of solar activity during the last 60 to 70 years has been exceptionally high, the highest in more than 8,000 years. Various hypotheses and theories have been proposed to link terrestrial temperature variations to variations in solar output. However, most records say that there has been no increase over the last 30 years. Since 1750, solar variation is estimated to be less than one-tenth of the forcing from greenhouse gases.

Estimates of recent solar forcing vary. Modeling studies reported in the IPCC Third Assessment Report (TAR) found that volcanic and solar forcings may account for half of the temperature variations prior to 1950, but the net effect of such natural forcings has been roughly neutral and cooling since then. In a review of existing literature, Foukal et al. (2006) determined both that the variations in solar output were too small to have contributed appreciably to global warming since the mid-1970s and that there was no evidence of a net increase in brightness during this period. The IPCC Fourth Assessment Report (AR4) gives a best estimate for radiative forcing from changes in solar activity of +0.12 watts per square metre. This is less than half of the estimate given in the TAR. For comparison, the combined effects of all human activity are estimated to be an order of magnitude greater at +1.6 watts per square meter. Additionally, in 2005, researchers at Duke University have found that 10-30% of the global surface temperature warming over the last two decades may be due to increased solar output. Solar variation has probably had a relatively small effect on recent global warming, compared with anthropogenic effects. However, some research has suggested that the Sun's contribution may have been underestimated. Some researchers (e.g. Stott et al. 2003) believe that the effect of solar forcing is being underestimated and propose that solar forcing accounts for 16% or 36% of of the recent warming due to the greenhouse effect. Others (e.g. Marsh and Svensmark 2000) have proposed that feedback from clouds or other processes enhance the direct effect of solar variation, which if true would also suggest that the effect of solar variability was being underestimated.

In the April 7, 2007 issue of Science News [and elsewhere] it was reported that "There is global warming on Mars too". From the late 1970’s to the present [the same period of time in which global warming has be detected on the earth] the "average air temperature of the planet surface [Mars] has increased by 0.65°C higher" and that Mars’ southern polar ice cap has been shrinking. The April 5, 2007 issue of Nature reported that "the planet’s [Mars] southern ice cap, air temperatures might have increased by as much as 4°C".

It appears likely that solar variations are too small to directly explain a significant fraction of the observed warming. Various researchers, notably Nigel Marsh and Henrik Svensmark, have proposed that feedback from clouds or other processes enhance the direct effect of solar variation. A warming of the stratosphere, which has not been observed, would be expected if there were a significant increase in solar activity.

In the thousand years prior to the last few decades, changes in reconstructed global temperature and solar activity as reconstructed by proxies such as sunspot number or related measures show a coarse fit. This has led many to suspect variation in solar inputs may be a primary source of global warming. Despite these long term trends, the level of scientific understanding of the variance in direct solar irradience is low. Although the majority of scientists believe that recent direct variations in solar output appear too small to have substantially affected the climate; nonetheless some researchers have proposed that feedbacks from clouds or other processes enhance the effect.

The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century. The solar variations theory also has counter evidence in the form of measured global dimming which shows that the sunlight at the earths surface actually reduced during the 20th century. To reconcile theories of an increase in solar radiation with the measurements would require other changes either in the spectrum of the sun (which has not been observed) or in the absorption profile of the atmosphere, which would probably imply some kind of climate change. Sami Solanki, the director of the Max Planck Institute for Solar System Research in Gottingen, Germany said:
 * The sun has been at its strongest over the past 60 years and may now be affecting global temperatures.... He continued: the brighter sun and higher levels of so-called "greenhouse gases" both contributed to the change in the Earth's temperature, but it was impossible to say which had the greater impact.

Willie Soon and Sallie Baliunas of the Harvard Observatory correlated historical sunspot counts with temperature proxies. They report that when there are fewer sunspots, the earth cooled (see Maunder Minumum, Little Ice Age); and that when there are more sunspots the earth warmed (see Medieval Warm Period, though since sunspot numbers were only counted from 1700 the link to the MWP warmth is at best speculative). However, if 1700 had been chosen as the comparison date instead of 1750, the increase in solar forcing would have been twice as high and in general the level of scientific understanding of the variance in direct solar irradiance is very low. Although the majority of scientists believe that direct variations in solar output appear too small to have substantially affected the climate, some researchers have proposed that feedbacks from clouds or other processes enhance the effect. Proxy studies indicate that the level of solar activity during the last 60-70 years has probably been the highest in more than 8,000 years. Solanki (2004) estimates that there is only an 8% probability that this current period of high activity can last another 50 years. Solanki concluded based on their analysis that (1) there is a 92% probability that solar activity will decrease over the next 50 years, and (2) that the impact of such a decline will probably be small compared to the impact of concurrent increases in greenhouse gas. In addition, researchers at Duke University (2005) have found that 10-30% of the warming over the last two decades may be due to increased solar output. In a review of existing literature, Foukal et al. (2006) determined both that the variations in solar output were too small to have contributed appreciably to global warming since the mid-1970s and that there was no evidence of a net increase in brightness during this period. In the IPCC Third Assessment Report TAR, it was reported that volcanic and solar forcings might account for half of the temperature variations prior to 1950, but that the net effect of such natural forcings was roughly neutral since then. In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be 8 times larger than the change in forcing due to increasing solar activity over the same period. Since the TAR various studies have suggested that irradiance changes over pre-industrial are less by a factor of 3-4 than in the reconstructions used in the TAR. Work published since the IPCC TAR, has revised the assessment of solar contribution to the post 1950 warming. One paper states that "The best estimate of the warming from solar forcing is estimated to be 16% or 36% of greenhouse warming depending on the solar reconstruction". However, this attributed solar warming is not distributed evenly, being more important towards 1950 and less important today. Since 1980 one estimate of the solar contribution is 10% to 30% however a competing reconstruction of variation in solar output measured by satellite shows a negligible trend since 1980 (see Solar variation). Scafetta and West derive climate sensitivities to solar variation that are 1.5 to 3 times greater than models predict.

A number of studies have suggested that additional solar variation feedbacks may exist which have not been incorporated in the present models or that the relative importance of solar variation may be underestimated. Such claims are dispute, but form an active area of current research. The outcome of this debate may play a key role in determining how much climate change is attributed to human vs. natural factors.

The theories have usually been one of three types:


 * Solar irradiance changes directly affecting the climate. This is generally considered unlikely, as the variations seem to be small.


 * Variations in the ultraviolet component having an effect. The UV component varies by more than the total.


 * Effects mediated by changes in cosmic rays (which are affected by the solar wind, which is affected by the solar output) such as changes in cloud cover.

The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century. Although often correlations are presented and the mechanism is speculative. Many of these effects have fared badly over time, and in a paper "Solar activity and terrestrial climate: an analysis of some purported correlations". Peter Laut demonstrates problems with some of the most popular, notably those by Svensmark and by Lassen (below).

In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and temperature changes throughout the northern hemisphere. Initially, they used sunspot and temperature measurements from 1861 to 1989, but later found that climate records dating back four centuries supported their findings. This relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period. Sallie Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has been among the supporters of the theory that changes in the sun "can account for major climate changes on Earth for the past 300 years, including part of the recent surge of global warming".

On May 6, 2000, however, New Scientist magazine reported that Lassen and astrophysicist Peter Thejll had updated Lassen's 1991 research and found that while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4°C since 1980. "The curves diverge after 1980", Thejll said, "and it's a startlingly large deviation. Something else is acting on the climate. ... It has the fingerprints of the greenhouse effect".

Later that same year, Peter Stott and other researchers at the Hadley Centre in the United Kingdom published a paper in which they reported on the most comprehensive model simulations to date of the climate of the 20th century. Their study looked at both natural forcing agents (solar variations and volcanic emissions) as well as anthropogenic forcing (greenhouse gases and sulphate aerosols). Like Lassen and Thejll, they found that the natural factors accounted for gradual warming to about 1960 followed by a return to late 19th-century temperatures, consistent with the gradual change in solar forcing throughout the 20th century and volcanic activity during the past few decades. These factors alone, however, could not account for the warming in recent decades. Similarly, anthropogenic forcing alone was insufficient to explain the 1910-1945 warming, but was necessary to simulate the warming since 1976. They found that "solar effects may have contributed significantly to the warming in the first half of the century although this result is dependent on the reconstruction of total solar irradiance that is used. In the latter half of the century, we find that anthropogenic increases in greenhouses gases are largely responsible for the observed warming, balanced by some cooling due to anthropogenic sulphate aerosols, with no evidence for significant solar effects". Stott's team found that combining all of these factors enabled them to closely simulate global temperature changes throughout the 20th century. They predicted that continued greenhouse gas emissions would cause additional future temperature increases "at a rate similar to that observed in recent decades". It should be noted that their solar forcing included "spectrally-resolved changes in solar irradiance" and not the indirect effects mediated through cosmic rays for which there is still no accepted mechanism - these ideas are still being fleshed out. In addition the study notes "uncertainties in historical forcing", i.e., past natural forcing may still be having a delayed warming effect, most likely due to the oceans. A graphical representation of the relationship between natural and anthropogenic factors contributing to climate change appears in "Climate Change 2001: The Scientific Basis," a report by the Intergovernmental Panel on Climate Change (IPCC).

Too Many People
The "Too many people" theory of global warming states that an increase in technologically advanced world populations will increase global temperatures through heat radiation by not only humans but also by technology. Humans are warm-blooded; therefor every person on earth must maintain a body temperature above that of the environment. Humans have also been increasing livestock to feed the increasing world population. Humans and living organisms propagated by mankind must create and radiate massive amounts of body heat. World populations of large mammals (humans included), has rises exponentially over the past hundred years. Not only is the population of large bodied mammals increasing but also in energy usage. It is the usage of this energy not just the creation of energy that radiates heat into our atmosphere. All humans on earth travel to some degree. They either drive a car that radiates heat from the engine and emissions, go by foot where they radiate heat directly through increased energy consumption, or by public transit that also radiates heat through engines and electrical motors. In wintertime much of the world population turns on heaters that either uses electrical resistance or the burning of a fuel to generate heat. Electrical resistance heaters, light bulbs, computers, and anything with an electrical circuit also radiate heat. The electricity to power these appliances is generated in power plants that generate massive amounts of heat or is created by alternative energies that also create massive amounts of heat. Solar panels and wind turbines still create and radiate heat.

Forcing and Feedback Mechanisms/Climate change feedback


The forces that drive climate change are said to be operating in a system called forcing. None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur. The effects of forcing agents on the climate are complicated by various feedback processes. A positive feedback is a process that amplifies some change. Thus, when a warming trend results in effects that induce further warming, its process is known as a positive feedback; when the effects induce cooling, the process is referred to as a negative feedback. The primary positive feedback involves water vapor. The primary negative feedback in global warming is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature. Moreover, an indirect consequence of global warming is the stimulation of increased biological photosynthesis; due both to one of the causes of global warming (atmospheric carbon dioxide), and also the consequences of global warming whereby increased temperatures and water vapour, on average, increases the speed at which the carbon cycle occurs. This provides a powerful negative feedback which stabilizes the climate system over time. One of the most pronounced feedback effects relates to the evaporation of water. CO2 injected into the atmosphere causes a warming of the atmosphere and the earth's surface. The warming causes more water to be evaporated into the atmosphere. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming by these gases will cause more water to be evaporated into the atmosphere. This feedback effect is reversed only as the CO2 is slowly removed from the atmosphere. This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

There is also a counteracting negative feedback from evaporative cooling that becomes stronger with surface temperature. At 300K over tropical oceans this is roughly 7.2W m-2 K-1. The dependence of evaporative cooling on surface temperature is strongly stabilizing, and this estimate would indicate that it is large enough in magnitude to overcome the destabilizing effect of surface longwave cooling. Feedback effects due to clouds are an area of ongoing research and debate.




 * Water vapor feedback : If the atmosphere is warmed the saturation vapour pressure increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor (a positive feedback), to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached at a slight increase in humidity and with a much larger greenhouse effect than that due to CO2 alone. (Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.)


 * Cloud feedback : Warming is expected to change the distribution and type of clouds. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect. Seen from above, the same clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details were poorly observed before the advent of satellite data and are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models (about 125 to 500 km for models used in the IPCC Fourth Assessment Report). Increased global water vapor concentration may or may not cause an increase in global average cloud cover. The net effect of clouds thus has not been well modeled, however, cloud feedback is second only to water vapor feedback and is positive in all the models that contributed to the IPCC Fourth Assessment Report.


 * Stefan-Bolzmann feedback feedback : The most important negative feedback regulating global temperature is the dependence of the outgoing longwave radiation on temperature. In simple terms: if the planet gets warmer, it radiates more heat back out to space. This limits any increase in temperature caused by forcing.


 * Lapse rate : A subtler feedback process relates to changes in the lapse rate as the atmosphere warms. The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation emitted from the upper atmosphere is less than that emitted from the lower atmosphere. Most of the radiation emitted from the upper atmosphere escapes to space, while most of the radiation emitted from the lower atmosphere is re-absorbed by the surface or the atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height: if the rate of temperature decrease is greater the greenhouse effect will be stronger, and if the rate of temperature decrease is smaller then the greenhouse effect will be weaker. Both theory and climate models indicate that warming will reduce the decrease of temperature with height, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.


 * Ice-albedo feedback : Another important feedback process is ice-albedo feedback. The increased CO2 in the atmosphere warms the Earth's surface and leads to melting of ice near the poles. When global temperatures increase, ice near the poles melt at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. Rapid Arctic shrinkage is already occurring, with 2007 being the lowest ever recorded area and 2008 being possibly the lowest ever recorded volume. It has been suggested in unpublished research that the Arctic ocean may be ice free as early as 2013. As well as ice-albedo feedback, Arctic shrinkage may also induce other positive feedback mechanisms.


 * Arctic methane release : Warming is also the triggering variable for Arctic methane release from sources such as permafrost and clathrates, making both of these potential feedback effects. Lawrence et al (2008) suggests that a rapid melting of the sea ice may up a feedback loop that rapidly melts arctic permafrosts. Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming. Possible positive feedback due to CH4 release from melting seabed ices is a further mechanism to be considered. Similarly, a massive release of CH4 from methane clathrates in the ocean could cause rapid warming, according to the clathrate gun hypothesis. Methane discharge from permafrost is presently under intensive study. Warmer deep ocean temperatures, likewise, could release the greenhouse gas methane from the 'frozen' state of the vast deep ocean deposits of methane clathrate/methane hydrate, according to the Clathrate Gun Hypothesis. Estimates of the size of the total carbon reservoir in Arctic permafrost and clathrates vary widely. It is suggested that is at least 900 gigatonnes of carbon in permafrost worldwide. Further, there are believed to be around and another 400 gigatonnes of carbon in methane clathrates in permafrost regions alone. However, Buffett and Archer predict a much higher release of between 2,000 and 4,000 gigatonnes as a result of expected human-induced warming, as they include some deep-ocean clathrate stores in the expected release. Should the more conservative estimate be correct or at least too low, then 1300 gigatonnes of carbon may potentially be released as methane from permafrost and clathrates as a result of human activity. As methane is a potent greenhouse gas, this is equivalent to a release of carbon dioxide very roughly 50 times the total emitted by humans since the Industrial Revolution. Unless the methane is by some means destroyed, a release on this scale will create catastrophic climate change and is likely to lead to a complete collapse of human civilisation. This scenario may be referred to in mass media as an example of runaway climate change. A mechanism which produces a response which varies by the existing state is called a feedback mechanism. Feedback mechanisms involved with global warming can be either positive feedback, where the feedback causes an unstable increase in temperature, or negative feedback, where the feedback causes a stabilizing effect on temperature. Methane discharge from permafrost is presently under intensive study.


 * Reduced absorption of CO2 by the oceans : The ocean's ecosystems' ability to sequester carbon is expected to decline as it warms. This is because the resulting low nutrient levels of the mesopelagic zone (about 200 to 1000 m depth) limits the growth of diatoms in favour of smaller phytoplankton that are poorer biological pumps of carbon. There is also a further possible positive-feedback effect from oceans, known as the anti-CLAW hypothesis.


 * Gas release : Release of gases of biological origin may be affected by global warming, but research into such effects is at an early stage. Some of these gases, such as Nitrous oxide released from peat, directly affect climate. Others, such as Dimethyl sulfide released from oceans, have indirect effects.


 * Desertification : Relatively small climate changes could result in abrupt changes to vegetative cover. In 2006, Woods Hole Research Center, reporting on the second consecutive year of drought in the Amazon basin and an experiment that had been running since 2002, said that the Amazon forest in its present form could survive only three successive years of drought before potentially turning into desert. Scientists at the Brazilian National Institute of Amazonian Research argued that this drought response were pushing the rainforest towards a "tipping point". It concluded that the forest is on the brink of being turned into savanna or desert, with catastrophic consequences for CO2 in the atmosphere. According to the World Wide Fund for Nature, the combination of climate change and deforestation increases the drying effect of dead trees that fuels forest fires.

Recent research carried out in 2008 in the Siberian Arctic has shown millions of tons of methane being released, apparently through perforations in the seabed permafrost, with concentrations in some regions reaching up to 100 times normal. The excess methane has been detected in localized hotspots in the outfall of the Lena River and the border between the Laptev Sea and the East Siberian Sea. Some melting may be the result of geological heating, but more thawing is believed to be due to the greatly increased volumes of meltwater being discharged from the Siberian rivers flowing north. Shakhova et al (2008) estimate that not less than 1,400 Gt of Carbon is presently locked up as methane and methane hydrates under the Arctic submarine permafrost, and 5-10% of that area is subject to puncturing by open taliks. They conclude that "release of up to 50 Gt of predicted amount of hydrate storage [is] highly possible for abrupt release at any time". That would increase the methane content of the planet's atmosphere by a factor of twelve, equivalent in greenhouse effect to a doubling in the current level of CO2.

Positive feedback mechanisms, due to their dramatic potential, have been popularized in mainstream media such as Al Gore's book Earth in the Balance or the 2004 movie The Day After Tomorrow. Theories predicting positive feedback propose that the Earth is in an unstable equilibrium, and that slight increases in temperature will result in responses which trigger even larger increases in temperature. One example of this, as used by all climate models, is that increases in temperature caused by carbon dioxide will cause significant increases in water vapour, which will cause even stronger increases in temperature. Such a feedback mechanism is essential to many climate model predictions because water vapor alone is between three to twenty times more significant as a greenhouse gas than carbon dioxide.

Negative feedback mechanisms typically fall into two categories. The first category involves "carbon dioxide sinks", which are natural repositories which absorb carbon dioxide from the atmosphere, such as the ground, ocean, or biomass. This class of feedback mechanisms simply predicts that only some of the human emitted carbon dioxide will reside in the atmosphere, but makes no claims about the variation of temperature with relation to atmospheric carbon dioxide levels. The second category of negative feedback mechanisms involves the relationship between human-controlled greenhouse gases and temperature. One such theory is the Infrared Iris model, which proposes that the water vapor levels actually act as a negative feedback mechanism powerful enough to over-time stabilize increases in temperature due to carbon dioxide. There are also some models which predict neutral feedback, and propose that the contributions of water vapor to the greenhouse effect are nearly neutral, leaving only the contributions from carbon dioxide. Positive and negative feedbacks are not imposed as assumptions in the models, but are instead emergent properties that result from the interactions of basic dynamical and thermodynamic processes.

More studies on the extent of warming caused by changes in the brightness of the sun have been conducted. Scientists from the United States, Germany, and Switzerland under Heliophysics, Inc. and the National Science Foundation calculated that there has been no net increase of brightness over the last thousand years. All increases in brightness were the result of solar cycles, and the small increase in brightness over the last 30 years is .07 percent. Scientists say that this small increase is far too minute to contribute to global warming. Lockwood and Frolich find that "that the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified". In 2006, a team of scientists from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles lead to a small increase of 0.07% in brightness over the last 30 years. This effect is far too minute to contribute significantly to global warming.



The response of the climate system to an initial forcing is increased by positive feedbacks and reduced by negative feedbacks. The main negative feedback to global temperature change is radiative cooling to space as infrared radiation, which increases strongly with increasing temperature. The main positive feedbacks are the water vapour feedback, the ice–albedo feedback, and probably the net effect of clouds. Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.

As air gets warmer, it can hold more moisture. After an initial warming due to emissions of greenhouse gases, the atmosphere will hold more water. As water is a potent greenhouse gas, this further heats the climate: the water vapour feedback. The reduction of snow cover and sea ice in the Arctic reduces the albedo (reflectivity) of the Earth's surface. More of the sun's energy is now absorbed in these regions, contributing to Arctic amplification, which has caused Arctic temperatures to increase at almost twice the rate of the rest of the world. Arctic amplification also causes methane to be released as permafrost melts, which is expected to surpass land use changes as the second strongest anthropogenic source of greenhouse gases by the end of the century.

Cloud cover may change in the future. To date, cloud changes have had a cooling effect, with NASA estimating that aerosols produced by the burning of hydrocarbons have limited warming by half from 1850 to 2010. An analysis of satellite data between 1983 and 2009 reveals that cloud tops are reaching higher into the atmosphere and that cloudy storm tracks are shifting toward Earth's poles, suggesting clouds will be a positive feedback in the future.

Carbon dioxide stimulates plant growth so the carbon cycle has been a negative feedback so far: roughly half of each year's CO2 emissions have been absorbed by plants on land and in oceans, with an estimated 30% increase in plant growth from 2000 to 2017. The limits and reversal point for this feedback are an area of uncertainty. As more CO2 and heat are absorbed by the ocean it is acidifying and ocean circulation can change, changing the rate at which the ocean can absorb atmospheric carbon. On land, greater plant growth will be constrained by nitrogen levels and can be reversed by plant heat stress, desertification, and the release of carbon from soil as the ground warms.

A 2018 study tried to identify such a planetary threshold for self-reinforcing feedbacks and found that even a 2 C-change increase in temperature over pre-industrial levels, may be enough to trigger such a hothouse Earth scenario.

A climate change feedback is a phenomena that increases or decreases the rate of change, and the best way to understand this phenomenon is through some very plausible examples:
 * Higher global temperatures mean dryer forests in some areas, which means more forest fires, which means more CO2 and ash in the air, which raises global temperature, which means more forest fires, which means …


 * Higher global temperatures mean melting ice, which exposes darker areas (dirt, rock, water) that reflect less solar energy than ice, which means higher global temperatures, which means more melting ice, which means …

Both models and observations indicate that a strong positive feedback is produced by the tendency of warming to increase the amount of atmospheric water vapor, a significant greenhouse gas. The main negative feedback is radiative cooling, which increases as the fourth power of temperature; the amount of heat radiated from the Earth into space increases with the temperature of Earth's surface and atmosphere. Different models treat positive and negative feedbacks in different ways, and not all feedbacks are sufficiently constrained to be incorporated into the models. Imperfect understanding of feedbacks is a major cause of uncertainty and concern about global warming. A wide range of potential feedback processes exist, such as Arctic methane release and ice-albedo feedback. Consequentially, potential tipping points may exist, which may have the potential to cause abrupt climate change. For example, the "emission scenarios" used by IPCC in its 2007 report primarily examined greenhouse gas emissions from human sources. In 2011, a joint study by NSIDC-(US) and NOAA-(US) calculated the additional greenhouse gas emissions that would emanate from melted and decomposing permafrost, even if policymakers attempt to reduce human emissions from the currently-unfolding A1FI scenario to the A1B scenario. The team found that even at the much lower level of human emissions, permafrost thawing and decomposition would still result in 190 ± 64 Gt C of permafrost carbon being added to the atmosphere on top of the human sources. Importantly, the team made three extremely conservative assumptions: (1) that policymakers will embrace the A1B scenario instead of the currently-unfolding A1FI scenario, (2) that all of the carbon would be released as carbon dioxide instead of methane, which is more likely and over a 20 year lifetime has 72x the greenhouse warming power of CO2, and (3) their model did not project additional temperature rise caused by the release of these additional gases. These very conservative permafrost carbon dioxide emissions are equivalent to about 1/2 of all carbon released from fossil fuel burning since the dawn of the Industrial Age, and is enough to raise atmospheric concentrations by an additional 87 ± 29 ppm, beyond human emissions. Once initiated, permafrost carbon forcing (PCF) is irreversible, is strong compared to other global sources and sinks of atmospheric CO2, and due to thermal inertia will continue for many years even if atmospheric warming stops. A great deal of this permafrost carbon is actually being released as highly flammable methane instead of carbon dioxide. Other research published in 2011 found that increased emissions of methane, which over 20 years has 72x the greenhouse warming power of CO2, could instigate significant feedbacks that amplify the warming attributable to the methane alone. The researchers found that a 2.5-fold increase in methane emissions would cause indirect effects that increase the warming 250% above that of the methane alone. For a 5.2-fold increase, the indirect effects would be 400% of the warming from the methane alone.
 * Higher global temperatures mean tundra permafrost melts, releasing CO2 and methane from rotted organic material, which means higher global temperature, which means more permafrost melting, which means.

Historical temperature record
Climate scientists generally agree that Earth has undergone several cycles of global warming and global cooling in the last 20 thousand years, with the average air temparature fluctuating within a range of about 3 Celsius degrees, or 5 Fahrenheit degrees, over this time period. During the Medieval Warm Period, for example, Vikings colonized Greenland, but the colonies were abandoned during the Little Ice Age that followed. Some scientists, retired scientists and GW "skeptics" argue that 34 years of upper-atmosphere records have already disproven the GW theory. A report by the National Research Council states,: "Data collected by satellites and balloon-borne instruments since 1979 indicate little if any warming of the low- to mid- troposphere - the atmospheric layer extending up to about 5 miles from the Earth's surface. Climate models generally predict that temperatures should increase in the upper air as well as at the surface if increased concentrations of greenhouse gases are causing the warming." Over the past 20,000 years the dominant temperature signal has been the end of the last ice age, approximately 12,000 years ago. Since then the temperature has been quite stable, though with various fluctuations, e.g. Medieval Warm Period or Little Ice Age.

Land use change
Changing the type of vegetation in a region impacts the local temperature by changing how much sunlight gets reflected back into space and how much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, and causes it to reflect more sunlight: an increase in albedo. Humans change the land surface mainly to create more agricultural land. Since the pre-industrial era, albedo increased due to land use change, which has a cooling effect on the planet. Other processes linked to land use change however have had the opposite effect, so that the net effect remains unclear.

Aerosols particulates, and soots
Solid and liquid particles known as aerosols – from volcanoes, plankton and human-made pollutants – reflect incoming sunlight, cooling the climate. From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed, a phenomenon popularly known as global dimming, typically attributed to aerosols from biofuel and fossil fuel burning. Aerosol removal by precipitation gives tropospheric aerosols an atmospheric lifetime of only about a week, while stratospheric aerosols can remain in the atmosphere for a few years. Global aerosols have been declining since 1990, removing some of the masking of global warming that aerosols had been providing.

The albedo of the Earth is a measure of how much solar energy is reflected and how much is absorbed. Various human activities have changed the Earth's albedo significantly in certain regions. Albedo effects include the creation of black carbon, which may not have been properly considered in some climate models. Additionally, aerosol pollution causes cooling. Efforts to reduce this pollution may have unintended effects on global warming: "Strong aerosol cooling in the past and present would then imply that future global warming may proceed at or even above the upper extreme of the range projected by the Intergovernmental Panel on Climate Change".

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially counteracted global warming during the period 1960-1990. Human-caused aerosols likely precipitated this effect. The main cause of this dimming is particulates produced by volcanoes and human made pollutants, which exerts a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO2 greenhouse gases such as methane. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the warming effect of increasing greenhouse gases. Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century, though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have, for the short term, largely offset one another, so that net warming in recent decades has been driven mainly by non-CO2 greenhouse gases such as methane. Radiative forcing due to aerosols is temporally limited due to wet deposition which causes aerosols to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in aerosol concentrations will only delay climate changes due to carbon dioxide.

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets, a phenomenon known as the Twomey effect. This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect. Indirect effects of aerosols are the largest uncertainty in radiative forcing. Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Aerosols, particularly indirect effects, represent the largest uncertainty in radiative forcing.

Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. Regionally but not globally, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds. Atmospheric soot always contributes additional warming to the climate system. When deposited, especially on glaciers, or on ice in arctic regions, the lower surface albedo can also directly heat the surface. The influences of aerosols, including black carbon, will be most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases will be dominant in the extratropics and southern hemisphere.

While aerosols typically limit global warming by reflecting sunlight, if black carbon in soot falls on snow or ice, it can also increase global warming. Not only does it increase the absorption of sunlight, it also increases melting and sea level rise. Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 degrees Celsius by 2050. When soot is suspended in the atmosphere, it directly absorbs solar radiation, heating the atmosphere and cooling the surface. In areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds. The influences of atmospheric particles, including black carbon, are most pronounced in the tropics and northern mid-latitudes, with the effects of greenhouse gases dominant in the other parts of the world.

Incoming sunlight
As the Sun is Earth's primary energy source, changes in incoming sunlight directly affect the climate system. Solar irradiance has been measured directly by satellites since 1978, but indirect measurements are available beginning in the early 1600s. There has been no upward trend in the amount of the Sun's energy reaching the Earth, so it cannot be responsible for the current warming. Physical climate models are also unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity.

Another line of evidence for the warming not being due to the Sun is the temperature changes at different levels in the Earth's atmosphere. According to basic physical principles, the greenhouse effect produces warming of the lower atmosphere (the troposphere), but cooling of the upper atmosphere (the stratosphere). If solar variations were responsible for the observed warming, warming of both the troposphere and the stratosphere would be expected, but that has not been the case.

While variations in solar activity have not produced recent global warming, variations in solar output over geologic time (millions to billions of years ago) are believed to have caused major changes in the earth's climate. The 11-year solar cycle of sunspot activity also introduces climate changes that have a small cyclical effect on annual global temperatures. The tilt of the Earth's axis and the shape of its orbit around the Sun vary slowly over tens of thousands of years in a phenomenon known as the Milankovitch cycles. This changes climate by changing the seasonal and latitudinal distribution of incoming solar energy at the Earth's surface, resulting in periodic glacial and interglacial periods over the last few million years. During the last few thousand years, this phenomenon contributed to a slow cooling trend at high latitudes of the Northern Hemisphere during summer, a trend that was reversed by greenhouse-gas-induced warming during the 20th century.

Climate models




Scientists have studied this issue with computer models of the climate (see below). These models are based on physical principles including fluid dynamics, thermodynamics, and radiative transfer. Although they attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. All modern climate models are in fact combinations of models for different parts of the Earth. These include an atmospheric model for air movement, temperature, clouds, and other atmospheric properties; an ocean model that predicts temperature, salt content, and circulation of ocean waters; models for ice cover on land and sea; and a model of heat and moisture transfer from soil and vegetation to the atmosphere. Some models also include treatments of chemical and biological processes. Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models. The majority of climate models can only simulate the observed warming since the 1970s by increasing greenhouse gases; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes in the models. These models are accepted by the scientific community as being valid only after it has been shown that they do a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Niño. Before a climate model is accepted by the scientific community, it has to be validated against observed climate variations. As of 2006, models with sufficiently high-resolution are able to successfully simulate summer/winter differences, the North Atlantic Oscillation, and El Niño. Once models which fail these tests are rejected, the remaining models all predict that the net effect of adding greenhouse gases will be a warmer climate in the future. However, even when the same assumptions of fossil fuel consumption and CO2 emission are used, the amount of predicted warming varies by model, and there still remains a considerable range of climate sensitivity predicted by the models which survive these tests, however, which probably reflects the way different models depict clouds differently. Part of the technical summary of the IPCC TAR includes a recognition of the need to quantify this uncertainty: "In climate research and modeling, we should recognize that we are dealing with a coupled non-linear system, and therefore that the prediction of a specific future climate is not possible. Rather the focus must be on the probability distribution of the system's possible future states by the generation of ensembles of model solutions". An example of a study which aims to do this is the project; their methodology is to investigate the range of climate sensitivities predicted for the 21st century by those models which have first been shown to give a reasonable simulation of late 20th century climate change.

As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4°C to 5.8°C (2.5°F; 10.4°F) between 1990 and 2100. They have also been used to help determine the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors. In addition to having their own characteristic climate sensitivity, models have also been used to derive independent assessments of climate sensitivity. Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1°C to 6.4°C (2.0°F to 11.5°F) by the end of the 21st century, relative to 1980–1999. Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

As of 2012, current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions. Not all effects of global warming are accurately predicted by the climate models used by the IPCC. Observed Arctic shrinkage has been faster than that predicted. Precipitation increased proportional to atmospheric humidity, and hence significantly faster than current global climate models predict.

Global climate model projections of future climate are forced by imposed greenhouse gas emission scenarios, most often from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback. The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.

A recent study by David Douglass, John Christy, Benjamin Pearson and Fred Singer comparing the composite output of 22 leading global climate models with actual climate data finds that the models do not accurately predict observed changes to the temperature profile in the tropical troposphere. The authors note that their conclusions contrast strongly with those of recent publications based on essentially the same data.

The most recent climate models produce a good simulation of the global temperature change over the last century. Climate simulations do not unambiguously show that warming occurring from 1910 to 1945 can be explained by internal and natural forcing (variation in solar radiation) only, but warming occurring from 1976 to 2000 needs anthropogenic greenhouse gases emissions to be explained. The majority of the scientific community is now convinced that a significant proportion of observed global warming is caused by anthropogenic emissions of greenhouse gases. Adding simulation of the ability of the environment to sink carbon dioxide suggested that rising fossil fuel emissions would decrease absorption from the atmosphere, amplifying climate warming beyond previous predictions, although "Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses". Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release the potent greenhouse gas, methane, that is trapped in large quantities in permafrost and ice clathrates ''To do: give details of "warming" from 1976 to 2000. Is this from land-based weather stations only? Were the data corrected for the urban heat island effect? If so, explain how. Also, compare land-based reading to satellite readings since 1979 (which show a 0.1 degree drop)''. These conclusions depend on the accuracy of the models used and on the correct estimation of the external factors, such as direct and indirect solar forcing. According to the IPCC, the majority of scientists agrees that important climate features are incorrectly accounted by the climate models, but these scientists don't think that better models would change the conclusion.

Critics point out that there are flaws in the models and external factors not taken into consideration that could change the conclusion above. The critics say that the climate simulations are unable to model the cooling effects of the particles, fitting the water vapor feedback and handling clouds. Critics also point out that the Sun may have a share of responsibility for the observed global warming greater than now thought by the majority of the scientific community. Some unidentified critics say that the climate simulations are unable to fit the water vapor feedback, and handle clouds. Some indirect solar effects may be very important and are not accounted by the models. Or then again, they might not be important at all. On the other hand, some skeptics arbitrarily assume all model flaws bias results in a positive manner, however most skeptics reason, based on larger solar coupling than can be accounted for by direct insolation, that models that don't take this into account, yet still tune their parameters to account for recent climate warming, will attribute too much of that warming to greenhouse gases, and will necessarily be biased to predict greater warming from future greenhouse gas increases. Unspecified bias can occur in either direction and the models may equally well be underestimating climate sensitivity. So, the share of global warming caused by anthropogenic greenhouse gases may be lower than thought. The models are used to estimate the relative importance of the various factors mentioned above. Most models show that warming occurring from 1976 to 2000 needs anthropogenic greenhouse gas emissions to be explained. The IPCC (see below), a United Nations science and public policy organization, published a report saying that scientists believe that anthropogenic greenhouse gases "play an important role in global warming".

Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds. The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small-scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considers climate models "to be suitable tools to provide useful projections of future climates".

In December 2005, Bellouin et al suggested in Nature that the reflectivity effect of airborne pollutants was about double that previously expected, and that therefore some global warming was being masked. If supported by further studies, this would imply that existing models underpredict future global warming. These results imply that as air quality improves additional global warming may be unmasked.

The IPCC says ''Complex physically-based climate models are required to provide detailed estimates of feedbacks and of regional features. Such models cannot yet simulate all aspects of climate (e.g., they still cannot account fully for the observed trend in the surface-troposphere temperature difference since 1979) and there are particular uncertainties associated with clouds and their interaction with radiation and aerosols. Nevertheless, confidence in the ability of these models to provide useful projections of future climate has improved due to their demonstrated performance on a range of space and time-scales''. A climate model is a representation of the physical, chemical and biological processes that affect the climate system. Computer models are run on supercomputers to reproduce and predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere. There are more than two dozen scientific institutions that develop climate models. Model forecasts vary due to different greenhouse gas inputs and different assumptions about the impact of different feedbacks on climate sensitivity.

A subset of climate models add societal factors to a simple physical climate model. These models simulate how population, economic growth and energy use affect – and interact with – the physical climate. With this information, scientists can produce scenarios of how greenhouse gas emissions may vary in the future. Scientists can then run these scenarios through physical climate models to generate climate change projection.

Climate models include different external forcings for their models. For different greenhouse gas inputs four RCPs (Representative Concentration Pathways) are used: "a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0) and one scenario with very high GHG emissions (RCP8.5). Models also include changes in the Earth's orbit, historical changes in the sun's activity and volcanic forcing. RCPs only look at concentrations of greenhouse gases, factoring out uncertainty as to whether the carbon cycle will continue to remove about half of the carbon dioxide from the atmosphere each year.

A model for the phenomenon that yields intuitive results, and using only basic thermodynamics is to at first place the Earth and Sun in a state of thermal equilibrium. For initial understanding, no land masses are included in the model. The Earth can emit or absorb enough heat that it is warm at the equator and cool near the poles, which have ice caps. The total amount of ice remains constant, initially. The power of this model arises when we add a warming phenomenon that then sets the model out of equilibrium; burning wood, ethanol, fossil fuels†, radioactivity, more sunlight etc. The first effect is not a dramatic increase in the Earth's temperature (counter-intuitively), but a gradual reduction in the amount of ice near the poles. The glacial run-off, or heat absorbed by the ice melting (specific heat of ice) tends to keep the temperature of the Earth constant. Both of these phenomena have been observed: The non-drastic temperature change, contributing to controversy, is referenced throughout this article. While satellite models and geological surveys have demonstrated reduction in polar ice. The model may be improved by the addition of land masses and geographic features. For example, the nearness of the glaciers in the Pacific Northwest caused a dramatic change in its climate during the 1990's; unusually cold winters and snow. The continued retreat of the glaciers in recent years has caused a return to its former climate, as glacier water now warms before it reaches the Gulf of Alaska and the Pacific Coast. Another example is provided by the expansion of deserts-directly related to more water being driven from those regions by the increased heat and approach to a new equilibrium. Note that the model predicts non-dramatic temperature change due to Arctic Ice melting, when this ice is gone, new dynamics must replace it. Though simple, this model has the advantage of accurately demonstrating and predicting effects of the Global Warming phenomenon. The burning of fossil fuels is the release of yesterday’s sunshine, effectively adding more sunlight or heat to the Earth.

A climate model is a computerized representation of the five components of the climate system: Atmosphere, hydrosphere, cryosphere, land surface, and biosphere. Such models are based on physical principles including fluid dynamics, thermodynamics and radiative transfer. There can be components which represent air movement, temperature, clouds, and other atmospheric properties; ocean temperature, salt content, and circulation; ice cover on land and sea; the transfer of heat and moisture from soil and vegetation to the atmosphere; chemical and biological processes; and others.

Although they attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. Results from models can also vary due to different greenhouse gas inputs and the model's. For example, the uncertainty in IPCC's 2007 projections is caused by (1) the use of multiple models with differing sensitivity to greenhouse gas concentrations, (2) the use of differing estimates of humanities' future greenhouse gas emissions, (3) any additional emissions from climate feedbacks that were not included in the models IPCC used to prepare its report, i.e., greenhouse gas releases from permafrost.

The models do not assume the climate will warm due to increasing levels of greenhouse gases. Instead the models predict how greenhouse gases will interact with radiative transfer and other physical processes. One of the mathematical results of these complex equations is a prediction whether warming or cooling will occur. Recent research has called special attention to the need to refine models with respect to the effect of clouds and the carbon cycle, but a new technique relying on satellite-based measurements of atmospheric humidity may fill this need. Initial results suggest that the climate is highly sensitive and that the models used by the IPCC which show the greater warming are most likely to be correct. Global climate model projections of future climate most often have used estimates of greenhouse gas emissions from the IPCC Special Report on Emissions Scenarios (SRES).

Models are also used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1970 is dominated by man-made greenhouse gas emissions. The physical realism of models is tested by examining their ability to simulate contemporary or past climates.{vn|date=November 2011}} Past models have underestimated the rate of Arctic shrinkage and underestimated the rate of precipitation increase. Sea level rise since 1990 was underestimated in older models, but now agrees well with observations. The 2017 United States-published National Climate Assessment notes that "climate models may still be underestimating or missing relevant feedback processes". Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate. While a 2007 study by David Douglass and colleagues found that the models did not accurately predict observed changes in the tropical troposphere, a 2008 paper published by a 17-member team led by Ben Santer noted errors and incorrect assumptions in the Douglass study, and found instead that the models and observations were not statistically different. Not all effects of global warming are accurately predicted by the climate models used by the IPCC. For example, observed Arctic shrinkage has been faster than that predicted. Precipitation increased proportional to atmospheric humidity, and hence significantly faster than current global climate models predict. Efforts are underway to reanalyze historical weather data and provide estimates of global tropospheric variability back to 1871, to allow climate scientists to evaluate past climate variations relative to recent IPCC climate model simulations.

Revegetation of desert lands, and solar panels in desert cities
Another proposal is to replant native vegetation, like the local trees and perennial grasses in hot desert lands of the world, to insulate the barren soils from absorbing the heat from the sun, to limit the amount of heat reflecting back into the atmosphere, and to utilize the native perennial grasses to start the process of carbon sequestration with their roots. This replanting would serve three purposes-- to decrease global warming, to start some carbon sequestration, and to increase the annual precipitation to these lands. Additionally, all towns and cities in desert areas, like Los Angeles, California, or Phoenix Arizona, or Riyadh in the Kingdom of Saudi Arabia, every buildings that can support solar panels, could install them to provide two functions--to produce carbon-free electricity and to shade the roof, to significantly cut the use of electricity to cool the buildings during the summer.

Potential Effects/Attributed and expected environmental effects/Natural systems
"Detection" is the process of demonstrating that climate has changed in some defined statistical sense, without providing a reason for that change. Detection does not imply attribution of the detected change to a particular cause. "Attribution" of causes of climate change is the process of establishing the most likely causes for the detected change with some defined level of confidence. Global warming has been detected in natural, ecological or social systems as a change having statistical significance. Attribution of these changes e.g., to natural or human activities, is the next step following detection. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on temperature changes. Rising sea levels and observed decreases in snow and ice extent are consistent with warming. The Earth is at what would have been the mid warming level between two ice ages. During an ice age sea levels drop 120 m and ice sheets 1,000 m thick cover northern and southern continents. Prior to a few million years ago, the Earth did not oscillate between ice ages, and was much warmer than today during the age of dinosaurs, 100 million years ago. During the last inter ice age period, 125,000 years ago, sea levels reached a level about 7 m higher than those of today. Natural mechanisms caused the Earth to oscillate between ice ages and warming periods, first over a 41,000 year period, and more recently over a 100,000 year period. Positive feedback mechanisms both move the Earth into and out of ice ages. The end point of each is (normally) set by natural limits. Most of the increase in global average temperature since the mid-20th century is, with high probability, atttributable to human-induced changes in greenhouse gas concentrations. Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades. Over the course of the 21st century, increases in emissions at or above their current rate as of 2012 would very likely induce changes in the climate system larger than those observed in the 20th century.

In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21st century (the year 2090-2099, relative to 1980-1999) range from 0.18 to 0.59 m. These estimates, however, were not given a likelihood due to a lack of scientific understanding, nor was an upper bound given for sea level rise. On the timescale of centuries to millennia, the melting of ice sheets could result in even higher sea level rise. Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, could contribute 4–6 metres (13 to 20ft) or more to sea level rise.

Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean. Snow cover area and sea ice extent are expected to decrease, with the Arctic expected to be largely ice-free in September by the 2037. The frequency of hot extremes, heat waves, and heavy precipitation will very likely increase. It is calculated that, with high statistical confindence, certain weather events, such as the heat waves in Texas and the 2003 European heat wave, would not have occurred without global warming. Extremely hot outliers, defined as three standard deviations from climatology records, now cover about 10% of the land surface and, under present trends, would be the norm by 2050. These temperatures are expected to excaberate the hydrological cycle, with more intense droughts and floods. Though it is difficult to connect specific weather events to global warming, such changes may increase the frequency and intensity of extreme weather events such as floods, droughts, heat waves, hurricanes. A few individual scientists (including several eminent scientific leaders in their fields) disagree with some of these conclusions as well. This is expected to result in other climate changes including rises in glacial retreat, Arctic shrinkage, including long-term shrinkage of Greenland ice sheet and worldwide sea level as well as changes in the amount and pattern of precipitation, likely leading to an expanse of tropical areas and increased pace of desertification resulting in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events, although prediction of such effects is highly uncertain. Other consequences include higher or lower of agricultural yields, addition of new trade routes, trade routes, cause glacier retreat, reduced summer streamflows, or even cause biological mass species extinctions and increases in the range of disease vectors, and reduced mortality rates in the United States. Among the positive effects of global warming include the potential for increased crop yields in certain areas, as well as improved quality of life for tribal communities in colder regions. Warming and related changes will vary from region to region around the globe, with projections being more robust in some areas than others If global mean temperature increases to 4 C-change above preindustrial levels, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits of adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved. Policy responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future geoengineering. Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate objective is to prevent dangerous anthropogenic (i.e., human-induced) climate change.

Many public policy organizations and government officials are concerned that the current warming has the potential for harm to the environment in various ways. As well as the intrinsic value attached by them to the preservation of the environment, there are wide range of possible consequences for humans. These include rising sea levels, declining output of global agriculture, increased extreme weather, and the spread of disease. In some cases, the effects may already be being experienced, although it is impossible to attribute specific incidents of natural phenomena to long-term global warming. Since the mid-1970s, the total power of hurricanes has increased markedly because they have been more intense and have lasted longer; in addition, there has been a high correlation of hurricane power with tropical sea-surface temperature. In particular strong evidence, the relationship between global warming and hurricanes is still being debated. Three new papers correlating climate change with increased hurricane intensity seem to be making the case that the two phenomena are linked; a draft WMO statement acknowledges the different viewpoints on this issue.

Some effects on both the natural environment and human life are already being attributed at least in part to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of hurricanes and extreme weather events, are being attributed at least in part to global warming. While changes are expected for overall patterns, intensity, and frequencies, it is difficult or impossible to attribute specific events (such as Hurricane Katrina) to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, adverse health effects from warmer temperatures, and the spread of disease.

Increasing extreme weather catastrophes are primarily due to an increase in population, and are partly due to increasing severe weather. The World Meteorological Organization said that scientific assessments indicate as global temperatures continue to warm, the number and intensity of extreme events might increase. Hoyos et al. (2006), find that the increasing number of category 4 and 5 hurricanes is directly linked to increasing temperatures. Kerry Emmanuel in Nature writes that hurricane power dissipation is highly correlated with temperature, reflecting global warming. Thomas Knutson and Robert E. Tuleya of NOAA stated in 2004 that warming induced by greenhouse gas may lead to increasing occurrence of highly destructive category 5 storms.

Increasing deaths, displacements, and economic losses caused by extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas. Social and economic effects of global warming may be exacerbated by growing population densities in affected areas. Temperate regions are projected to experience some benefits, such as fewer cold-related deaths, but many more deaths from heat exposure. A summary of probable effects and recent understanding can be found in the report of the IPCC Working Group II; the newer AR4 summary reports, "There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones". Vecchi and Soden, however, find that hurricane frequency and intensity in Atlantic and East Pacific are largely due to natural cycles. The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature (see Atlantic Multidecadal Oscillation), but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones. Climate change may turn native plants into "plant refugees" in the next century as they seek more suitable habitats.

Mark Lynas's book Six Degrees: Our Future on a Hotter Planet explains how the release of methane hydrate and the release of methane from melting permafrost could unleash a major extinction event. Carbon cycle feedbacks, the demise of coral, the destruction of the Amazon rainforest, and extreme desertification are also described, with five or six degrees of warming potentially leading to the complete inhabitability of the tropics and subtropics, as well as extreme water and food shortages, possibly leading to mass migration of billions of people.

A December 2, 2007 article from the British publication The Times states, "Average annual deaths from weather-related events in the period 1990-2006 – considered by scientists to be when global warming has been most intense – were down by 87% on the 1900-89 average. The mortality rate from catastrophes, measured in deaths per million people, dropped by 93%. The number of deaths had fallen sharply because of better warning systems, improved flood defences and other measures. Poor countries remained most vulnerable".

Some additional anticipated effects include sea level rise of up to 14 metres, repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity, in some locations, and frequency of hurricanes and extreme weather events, lowering of ocean pH, the spread of diseases such as malaria and dengue fever, and mass extinction events. One study predicts 18 to 35 percent of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections. Mechanistic studies have documented extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change. However, Parmesan states, "Few mechanic studies have been conducted at a scale that encompasses an entire species" and McLaughlin et al. agree "few mechanistic studies have linked extinctions to recent climate change" and some researchers believe the projected rates of extinction are overestimated or uncertain. A 2008 scientific paper showed that the increases in economic damage from hurricanes is the result of increases in population, development, and wealth, and not the result of increases in hurricane frequency or intensity. MIT professor of meteorology Dr. Kerry Emanuel, publishing in the March 2008 issue of the Bulletin of the American Meteorological Society, states that new climate modeling data indicates "global warming should reduce the global frequency of hurricanes". The new work suggests that, even in a dramatically warming world, hurricane frequency and intensity may not substantially rise during the next two centuries.

Mechanistic studies have documented extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change. Parmesan states, "Few studies have been conducted at a scale that encompasses an entire species" and McLaughlin et al. agreed "few mechanistic studies have linked extinctions to recent climate change". Two UK scientists supporting the mainstream scientific opinion on global warming criticize what they call the "catastrophism and the 'Hollywoodisation'" of some of the expected effects. They argue the sensationalized claims cannot be justified by the science.

The extent and likelihood of these consequences is a matter of considerable controversy, with environmentalist groups typically emphasizing the possible dangers and groups close to industry questioning the climate models and consequences of global warming; and funding scientists to do so. A summary of possible effects and our current understanding, can be found in the report of the IPCC Working Group II;the newer AR4 summary reports, "There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones". Two British scientists supporting the mainstream scientific opinion on global warming criticize what they call the "catastrophism and the 'Hollywoodisation'" of some of the expected effects. They argue that sensationalized claims cannot be justified by science. Some scientists have concluded that global warming is already causing death and disease across the world through flooding, environmental destruction, heatwaves and other extreme weather events, according to a Lancet study. Much remains to be learned, however.

Due to potential effects on human health and economy due to the impact on the environment, global warming is the cause of great concern. Some important environmental changes have been observed and linked to global warming.

The examples of secondary evidence cited above (lessened snow cover, rising sea levels, weather changes) are examples of consequences of global warming - and funding scientists to do so that may influence not only human activities, but also the ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions while others may spread. Few of the terrestrial ecoregions on Earth could expect to be unaffected. The predicted effects of global warming are many and various, both for the environment and for human life. The primary effect (indeed, the primary symptom) of global warming is increasing carbon dioxide and increasing global average temperature. From this flow a variety of secondary effects, including sea level rise, impacts on agriculture, reductions in the ozone layer (see below), increased extreme weather, and the spread of disease. In some cases, the effects may already be being experienced, although it is generally difficult to attribute specific natural phenomena to long-term global warming.

Physical environmental
Recent studies indicate that globally, natural ecosystems are already responding to climate change. For some species, these responses appear to part of coping strategies, for others adverse effects including localized population extinctions have been observed. The environmental effects of global warming are broad and far-reaching. The predicted effects of global warming are many and various, both for the environment and for human life. The primary effect of global warming is increasing carbon dioxide and increasing global average surface temperature. From this flow a variety of secondary effects, including sea level rise, impacts on agriculture, reductions in the ozone layer (see below), increased intensity and frequency of extreme weather events, and the spread of disease. In some cases, the effects may already be being experienced, although it is generally impossible to attribute specific natural phenomena to long-term global warming. In particular the relationship between global warming and hurricanes is still being debated. Three new papers correlating climate change with increased hurricane intensity seem to be making the case that the two phenomena are linked. They include the following diverse effects:

Arctic sea ice decline, sea level rise, retreat of glaciers: global warming has led to decades of shrinking and thinning of the Arctic sea ice, making it vulnerable to atmospheric anomalies. Projections of declines in Arctic sea ice vary. Recent projections suggest that Arctic summers could be ice-free (defined as an ice extent of less than 1 million square km) as early as 2025–2030. Over the 21st century, the IPCC projects (for a high emissions scenario) that global mean sea level could rise by 52–98 cm. The rate of ice loss from glaciers and ice sheets in the Antarctic is a key area of uncertainty since Antarctica contains 90% of potential sea level rise. Polar amplification and increased ocean warmth are undermining and threatening to unplug Antarctic glacier outlets, potentially resulting in more rapid sea level rise.

Ecological systems/Food security
In terrestrial ecosystems, the earlier timing of spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs. It is expected that most ecosystems will be affected by higher atmospheric CO2 levels, combined with higher global temperatures., Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems. Dissolved CO2 increases ocean acidity. This process is known as ocean acidification and has been called the "equally evil twin" of global climate change. Increased ocean acidity decreases the amount of carbonate ions, which organisms at the base of the marine food chain, such as foraminifera, use to make structures they need to survive. The current rate of ocean acidification is many times faster than at least the past 300 million years, which included four mass extinctions that involved rising ocean acidity, such as the Permian mass extinction, which killed 95% of marine species. By the end of the century, acidity changes since the industrial revolution would match the Palaeocene-Eocene Thermal Maximum, which occurred over 5000 years and killed 35–50% of benthic foraminifera. As has been seen in the previous brief interglacials, the period is ending with a brief very warm period wherein peat bogs form at high latitudes to lock in carbon for an extended ice age. It appears that human activities have merely delayed and not stopped this process. Under present trends, by 2030, maize production in Southern Africa could decrease by up to 30% while rice, millet and maize in South Asia could decrease by up to 10%. By 2080, yields in developing countries could decrease by 10% to 25% on average while India could see a drop of 30% to 40%. By 2100, while the population of three billion is expected to double, rice and maize yields in the tropics are expected to decrease by 20–40% because of higher temperatures without accounting for the decrease in yields as a result of soil moisture and water supplies stressed by rising temperatures. Future warming of around $3 °C$ (by 2100, relative to 1990–2000) could result in increased crop yields in mid- and high-latitude areas, but in low-latitude areas, yields could decline, increasing the risk of malnutrition. A similar regional pattern of net benefits and costs could occur for economic (market-sector) effects. Warming above $3 °C$ could result in crop yields falling in temperate regions, leading to a reduction in global food production.

Species migration
In 2010, a gray whale was found in the Mediterranean Sea, even though the species had not been seen in the North Atlantic Ocean since the 18th century. The whale is thought to have migrated from the Pacific Ocean via the Arctic. Climate Change & European Marine Ecosystem Research (CLAMER) has also reported that the Neodenticula seminae alga has been found in the North Atlantic, where it had gone extinct nearly 800,000 years ago. The alga has drifted from the Pacific Ocean through the Arctic, following the reduction in polar ice. In the Siberian sub-arctic, species migration is contributing to another warming albedo-feedback, as needle-shedding larch trees are being replaced with dark-foliage evergreen conifers which can absorb some of the solar radiation that previously reflected off the snowpack beneath the forest canopy.

Expected Social system effects
Vulnerability of human societies to climate change mainly lies in the effects of extreme weather events rather than gradual climate change. Impacts of climate change so far include adverse effects on small islands, adverse effects on indigenous populations in high-latitude areas, and small but discernable effects on human health. Over the 21st century, climate change is likely to adversely affect hundreds of millions of people through increased coastal flooding, reductions in water supplies, increased malnutrition and increased health impacts. However there are also many benefits from a warmer planet. Previously pristine arctic areas will now be available for resource extraction and transportation.

Future warming of around 3°C (by 2100, relative to 1990-2000) could result in increased crop yields in mid- and high-latitude areas, but in low-latitude areas, yields could decline, increasing the risk of malnutrition. A similar regional pattern of net benefits and costs could occur for economic (market-sector) effects. Warming above 3 °C could result in crop yields falling in temperate regions, leading to a reduction in global food production. Most economic studies suggest losses of world gross domestic product (GDP) for this magnitude of warming.

Some areas of the world would start to surpass the wet-bulb temperature limit of human survivability with global warming of about 6.7°C (12°F) while a warming of 11.7°C (21°F) would put half of the world's population in an uninhabitable environment. In practice the survivable limit of global warming in these areas is probably lower and in practice some areas may experience lethal wet bulb temperatures even earlier, because this study conservatively projected the survival limit for persons who are out of the sun, in gale-force winds, doused with water, wearing no clothing, and not working. There is some evidence of regional climate change affecting systems related to human activities, including agricultural and forestry management activities at higher latitudes in the Northern Hemisphere. Future climate change is expected to particularly affect some sectors and systems related to human activities. These include:


 * Water resources in some dry regions at mid-latitudes, the dry tropics, and areas that depend on snow and ice melt

It is expected that some regions will be particularly affected by climate change, including the Arctic, Africa, small islands, and Asian and African megadeltas. Some people, such as the poor, young children, and the elderly, are particularly at risk, even in high-income areas.
 * Agriculture in low latitudes
 * Low-lying coastal systems
 * Human health in populations with limited capacity to adapt to climate change

Reuters have reported that the US military is spending millions of dollars a year on nuclear submarine patrols and torpedo tests in the Arctic. This is with a view to global warming leading to Arctic ice disappearing during the summers from the mid-2030s onwards, which in turn will mean that they expect vast new oil and gas reserves to become accessible and commercial shipping to make increased use of shorter passages via the Bering Strait. They report that the US is "jockeying for position" with Russia, China, and other countries to benefit from such new business opportunities in the area. The arctic is expected to be largely ice-free during summer months by the 2030's, allowing increased commercial shipping thorough the northwest passage.

Sea level rise
Another cause of great concern is sea level rise. Sea levels appear to be rising 1 to 2 mm/y this century, although satellite data show a rate of 3 mm/y since 1992. The sea levels are rising 0.01 to 0.02 centimeters (around an inch) per decade and some small countries in the Pacific Ocean are expressing concerns that if this rise in sea level doesn't stop, they soon will be entirely under water. Some Pacific Ocean island nations, such as Tuvalu, are working out the details of their expected eventual evacuation. For historical reasons to do with trade, many of the world's largest and most prosperous cities are on the coast, and the cost of building better coastal defenses is likely to be considerable. Some countries will be more affected than others - low-lying countries such as Bangladesh and the Netherlands will be worst hit by any sea level rise, in terms of floods or the cost of preventing them. There is significant variance of measured sea level rise depending on which land mass the rise is measured from.

Global warming causes the sea level to rise mainly because sea water expands, but some scientists are concerned that in the future, the polar ice caps and glaciers may melt. As a consequence, the sea level could rise several meters. At the moment, scientists are not expecting any major ice melting in the next 100 years. Additionally, this introduction of freshwater into the ocean can have an effect on the thermohaline conveyor, an oceanic circulatory system which plays a large part in regulating Earth's climate. Some researchers have found a negative correlation between sea level rise and average global temperature; water evaporates more quickly than it expands. The detailed causes of this change remain an active field of research, but the scientific consensus identifies greenhouses gases as the primary cause of the recent warming. This conclusion can be controversial, especially outside the scientific community.

More extreme weather
As the climate gets hotter, evaporation will increase. This will cause heavier rainfall and more erosion. Many people think that it could result in more extreme weather as global warming progresses. The IPCC TAR says: "...global average water vapour concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected". ''To do: describe the opposite theory that rising atmosphere temperatures cause sea level to drop due to increased evaporation. Also, describe other theories of sea-level rise''.

Stop of drift/Decline of agriculture
For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of CO2 in photosynthesis. This may still be true in some regions (such as Siberia), but recent evidence is that global yields will be negatively affected. "Rising atmospheric temperatures, longer droughts and side-effects of both, such as higher levels of ground-level ozone gas, are likely to bring about a substantial reduction in crop yields in the coming decades, large-scale experiments have shown".

Clathrate Gun Hypothesis
Methane clathrate, also known as methane hydrate, was once believed to only exist in space, as extremes of cold are required for its formation. Around 6.4 trillion tonnes (6.4 teratonnes/Tt) of methane. The Clathrate gun hypothesis states that warmer deep ocean temperatures can release the methane (CH4) from the deep ocean deposits of methane clathrate. It is theorized that this was responsible for two extinction events in earth's history; the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. A concentration of 5-15% methane in the atmosphere, which would occur at and around the area of a CH4 release, is explosive; the products of such an explosion would be 2 parts H2O to one of CO2. CH4 dissipates faster than carbon dioxide (C02), but even after 20 years has a 62 times greater Global warming potential index. Methane escaping from the sea floor to the atmosphere has been a popular suspect for causing rapid climate changes during and at the end of the last ice age. The most recent research, however, seems to disprove the idea.

Methane (CH4) is a much stronger greenhouse gas than carbon dioxide. It is usually released from swamps or through biomass burning. But it is also trapped in huge amounts in some ocean-floor sediments, where it lies buried in an ice-like structure known as 'methane clathrate'. These clathrates are stable only within a certain range of temperatures and pressures; when brought to the surface, they melt rapidly and release inflammable gas to the air. The fear is that if global sea temperatures were to rise enough, or rise too quickly, then this could destabilise the methane clathrate. If it were destabilised, it could cause a catastrophic release of trillions of tons of methane very quickly into the atmosphere which would cause the weather system to go out of control. The oceans would appear to boil as the gas was released and the resulting effects on global weather could be devastating. The clathrate gun hypothesis has been controversial from the onset. All Quaternary warming episodes seem to have been accompanied by increased abundance of atmospheric methane. But many climate scientists think this is an effect, rather than the trigger, of warming climates. In most cases, there is evidence that the methane values only began to rise several decades after the temperature started to climb.

None of this means that marine methane hydrates don't occasionally erupt, however. Hinrichs has used fossil remnants of bacteria that flourish only under high methane concentrations to show that large quantities of the gas must have been released in the Santa Barbara Basin off California during an event some 44,000 years ago. According to Hinrichs, this gas did not necessarily escape to the atmosphere, but it did come from underwater ice. Researchers are now exploring the isotopic values of gas bubbles trapped in ice cores going back some 900,000 years, to find out where methane came from in the past. How the world's methane hydrates will respond to future global warming and other disturbances is uncertain. Seafloor reservoirs currently contain twice as much methane as all known conventional fossil-fuel reserves.

Effects on ecosystems
Both primary and secondary effects of global warming; such as higher temperatures, lessened snow cover, rising sea levels, weather changes; provides examples of consequences of global warming that may influence not only human activities but also the ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may flourish. Similarly, changes in timing of life patterns, such as annual migration dates, may alter regional predator-prey balance. The effect of advanced spring arrival dates in Scandanavia of birds that overwinter in sub-Saharan Africa has been ascribed to evolutionary adaptation of the species to climactic warming. Few of the terrestrial ecoregions on Earth could expect to be unaffected. Many of the species at risk are arctic fauna such as polar bears, emperor penguins, many salt wetland flora and fauna species, and any species that inhabit the low land areas near the sea (goodbye New Orleans!). Species that rely on cold weather conditions such as gyrfalcons, and snowy owls that prey on lemmings that use the cold cold winter to their advantage will be hit hard. Ocean pH is lowering as a result of increased carbon dioxide levels. Lowering of ocean pH, along with changing water temperature and ocean depth will have a direct impact on coral reefs.

Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release significant amounts of the potent greenhouse gas methane that is trapped in permafrost and ice clathrate compounds.

There are also ecological effects of melting polar ice: for example, polar bears use sea ice to reach their prey, and they must swim to another ice floe when one breaks up. Ice is now becoming further separated, and dead polar bears have been found in the water, believed to have drowned. More recently, some scientists have suggested that the observed cannibalistic behavior in polar bears may be the result of food shortages brought on by global warming.

Impact on Glaciers
Global warming has led to negative glacier mass balance, causing glacier retreat around the world. Oerlemans (2005) showed a net decline in 142 of the 144 mountain glaciers with records from 1900 to 1980. Since 1980 global glacier retreat has increased significantly. Similarly, Dyurgerov and Meier (2005) averaged glacier data across large scale regions (e.g. Europe) and found that every region had a net decline from 1960 to 2002, though a few local regions (e.g. Scandinavia) have shown increases. Some glaciers that are in disequilibrium with present climate have already disappeared and increasing temperatures are expected to cause continued retreat in the majority of alpine glaciers around the world. Upwards of 90% of glaciers reported to the World Glacier Monitoring Service have retreated since 1995.



Of particular concern is the potential for failure of the Hindu Kush and Himalayan glacial melts. The melt of these glaciers is a large and reliable source of water for China, India, and much of Asia, and these waters form a principal dry-season water source. Increased melting would cause greater flow for several decades, after which "some areas of the most populated region on Earth are likely to 'run out of water'".

Miniature Rock Glaciers
Rock glaciers - a cache of ice under a pile of boulders - are among other water signs such as drying meadows and warning lakes that scientists are studying in the Sierra's in the western United States. Connie Millar searches for the rock glaciers in the Yosemite area of the Sierra crest. She hypothesizes that rock glaciers will be predictors of how ecosystems change with rising temperatures. Millar is leading an effort (the Consortium for Integrated Climate Research in Western Mountains) to co-ordinate the work of many scientists to see how the pieces of the Global Warming puzzle may fit.

Destabilisation of ocean currents/Sea level rise and environmental refugees


There is also some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift. The chances of this occurring are unclear.

The major global system of ocean currents is known as the thermohaline circulation or, more colloquially, the ocean conveyor belt. Warm water from the equator is transported via the gulf stream into arctic regions. During the transport, its temperature drops and its salinity increases, due to evaporation. The salinity further increases when ocean ice is formed. The remaining cold, high salinity, and hence very dense water sinks, and, following the ocean floor, flows back south. Increasing temperatures will decrease the formation of pack ice, and may furthermore cause frozen fresh water to melt and thus reduce the salinity of the arctic ocean. This effect could weaken or even temporarily stop the thermohaline circulation and hence the gulf stream that provides much of northwestern Europe with a relatively mild climate. It would also cause a greater temperature imbalance between the polar and equatorial regions, and might thus lead to stronger storms to restore equilibrum.

Rising global temperatures will melt glaciers and expand the water of the seas through the mechanism of thermal expansion, leading to sea level rise. Even a relatively small rise in sea level would make some densely settled coastal plains uninhabitable and create a significant refugee problem. If the sea level were to rise in excess of 4 metres (13ft) almost every coastal city in the world would be severely affected, with the potential for major impacts on world-wide trade and economy. Presently, the IPCC predicts sea level rise is most probable to be just short of half a metre, and at least between 9 and 88 cm (less than 1 meter (3ft) through 2100, but they also warn that global warming during that time may lead to irreversible changes in the Earth's glacial system and ultimately melt enough ice to raise sea level many meters over the next millenia. It is estimated that around 200 million people could be affected by sea level rise, especially in Vietnam, Bangladesh, China, India, Thailand, Philippines, Indonesia, Nigeria, and Egypt. An example of the ambiguous nature of environmental refugees is the emigration from the island nation of Tuvalu, which has an average elevation of approximately one meter above sea level. Tuvalu already has an ad hoc agreement with New Zealand to allow phased relocation and many residents have been leaving the islands. However, it is far from clear that rising sea levels from global warming are a substantial factor - best estimates are that sea level has been rising there at approximately 1-2 millimeters per year (~1/16 in/yr), but that shorter timescale factors - ENSO, or tides - have far larger temporary effects. While there are other social and economic factors that drive emigration from Tuvalu, concern about rising sea levels is also a factor. They are concerned about the possibility of an eventual evacuation, as flood defense may become economically inviable for them. Tuvalu already has an ad hoc agreement with New Zealand to allow phased relocation.

The creation of biomass by plants is influenced by the availability of water, nutrients, and carbon dioxide. Part of this biomass is used (directly or indirectly) as the energy source for nearly all other life forms, including feed-stock for domestic animals, and fruits and grains for human consumption. It also includes timber for construction purposes.

A rise in atmospheric carbon dioxide can increase the efficiency of the metabolism of most plants, potentially allowing them to create more biomass. While it's thought that an increase in carbon dioxide levels should speed up plant growth, which would slow down the effects of global warming, a new study has found the opposite to be true. Scientists at Stanford have found that "elevated atmospheric carbon dioxide actually reduces plant growth when combined with other likely consequences of climate change -- namely, higher temperatures, increased precipitation or increased nitrogen deposits in the soil". A rising temperature can also increase the growing season in colder regions. It is sometimes argued that these effects can create a greener, richer planet, with more available biomass. However, there are many other factors involved, and it is currently unclear if plants benefiting from global warming is a realistic scenario. Plant growth can be limited by a number of factors, including soil fertility, water, temperature, and carbon dioxide concentration. Ocean plants (phytoplankton) are actually harmed by global warming, presumably with negative impact on ocean ecosystems.

Spread of disease
Global warming will probably extend the favourable zones for vectors conveying infectious disease, such as Dengue fever, Malaria, West Nile virus, St. Louis encephalitis, or Yellow fever. A warmer environment boosts the reproduction rate of mosquitos and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse. An example of this may be the recent extension to the north Mediterranean region of bluetongue disease in domesticated ruminants associated with mite bites. Another is the increase of hantavirus infection, Crimean-Congo hemorrhagic fever, tularemia, and rabies in wide areas of Russia during 2004–2005 associated with a population explosion of rodents and their predators. Some of this, however is blamed on breakdowns in governmental vaccination and rodent control programs. However, it has been pointed out that despite the disappearance of infectious disease most temperate regions, the indigenous mosquitoes that transmitted it were never eliminated and remain common in some areas. Thus, although temperature is important in the transmission dynamics of malaria, many other variable factors are of equal or greater importance.

Financial effects
In an October, 2006, report entitled the Stern Review by the former Chief Economist and Senior Vice-President of the World Bank, Nicholas Stern, he states that climate change could affect growth which could be cut by one-fifth unless drastic action is taken. Stern has warned that one percent of global GDP is required to be invested in order to mitigate the effects of climate change, and that failure to do so could risk a recession worth up to twenty percent of global GDP. Stern’s report suggests that climate change threatens to be the greatest and widest-ranging market failure ever seen. The report has had significant political effects: Australia reported two days after the report was released that they would allott AU$60 million to projects to help cut greenhouse gas emissions. The Stern Review has been criticized by economists, saying that Stern used an incorrect discount rate in his calculations, and that stopping or significantly slowing climate change will require deep emission cuts everywhere.

According to a 2005 report from the Association of British Insurers, limiting carbon emissions could avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s. A June 2004 report by the Association of British Insurers declared "Climate change is not a remote issue for future generations to deal with. It is, in various forms, here already, impacting on insurers' businesses now". It noted that weather risks for households and property were already increasing by 2–4 % per year due to changing weather, and that claims for storm and flood damages in the UK had doubled to over £6 billion over the period 1998-2003, compared to the previous five years. The results are rising insurance premiums, and the risk that in some areas flood insurance will become unaffordable for some. In the U.S., according to Choi and Fisher (2003) each 1% increase in annual precipitation could enlarge catastrophe loss by as much as 2.8%. Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warned in a 2002 study that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost US$150 billion each year in the next decade. These costs would, through increased costs related to insurance and disaster relief, burden customers, taxpayers, and industry alike. Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warn in a joint study that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost US$150 billion each year in the next decade. These costs would, through increased costs related to insurance and disaster relief, burden customers, taxpayers, and industry alike.

According to the Association of British Insurers, limiting carbon emissions could avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s. According to Choi and Fisher (2003) each 1% increase in annual precipitation could enlarge catastrophe loss by as much as 2.8%. The United Nation’s Environmental Program recently announced that severe weather around the world this year has made 2005 the most costly year on record, although there is no way to prove that [a given hurricane] either was, or was not, affected by global warming, global warming is thought to increase the probability of hurricanes emerging. Preliminary estimates presented by the German insurance foundation Munich Re put the economic losses at more than US$200 billion, with insured losses running at more than US$70 billion.

Nicholas Stern in the Stern Review has warned that one percent of global GDP is required to be invested in order to mitigate the effects of climate change, and that failure to do so could risk a recession worth up to twenty percent of global GD]. Stern’s report suggests that climate change threatens to be the greatest and widest-ranging market failure ever seen. The report has had significant political effects: Australia reported two days after the report was released that they would allott AU$60 million to projects to help cut greenhouse gas emissions. Tony Blair said the Stern Review showed that scientific evidence of global warming was "overwhelming" and its consequences "disastrous".

Destabilisation of local climates
Global warming might also have other, less obvious effects. The North Atlantic drift, for instance, is affected by salinity changes. It seems that it is diminishing as the climate grows warmer, and there has been speculation that areas like Scandinavia and Britain that are warmed by the drift might face a colder climate in spite of the general global warming. Some even fear that global warming may be able to trigger the type of abrupt massive temperature shifts which bracketed the Younger Dryas period. (See the discussion of chaos theory for related ideas). However, in coupled AOGCMs the warming effects outweigh the cooling, even locally: the IPCC TAR notes that even in models where the THC weakens, there is still a warming over Europe. A study (published in Science) of changes to Siberia's permafrost suggests that it is gradually disappearing in the southern regions, leading to the loss of nearly 11% of Siberia's nearly 11,000 lakes since 1971.

Reducing ozone layer
One of the lesser-known effects of global warming is a reduction in the thickness of the ozone layer, which protects life on Earth from harmful radiation. Global warming appears to be partially countering the improvements in the ozone layer caused by the reduction in the use of ozone-destroying chemicals achieved through the Montreal Protocol. "What appears to have caused the further loss of ozone is the increasing number of stratospheric clouds in the winter, 15 miles above the earth. These clouds, in the middle of the ozone layer, provide a platform which makes it easier for rapid chemical reactions which destroy ozone to take place". A reduced ozone layer has negative impacts on human health (notably skin cancer and eye problems such as cataracts) and on ecosystems. However, the net effect of the thinning of the ozone layer on human health may be positive. Research by Dr. Edward Giovannucci, a Harvard University professor of medicine and nutrition who gave a keynote lecture at a recent American Association for Cancer Research, suggests that vitamin D might help prevent 30 deaths for each one caused by skin cancer. Vitamin D is nicknamed the "sunshine vitamin" because the skin makes it from ultraviolet rays. The ecological impact may have further knock-on effects, as it reduces photosynthesis in plants (with potential impacts on agriculture) and damages the DNA of plankton, which play a significant role in the world's carbon cycle. On the technical details of the two-way interaction between the ozone layer and global warming, see the relation of global warming and ozone depletion, below.

Possible positive effects
However, global warming can also have positive effects, since higher temperatures and higher CO2 concentrations improve the ecosystems' productivity. Plants form the basis of the biosphere. Using photosynthesis, they utilize the sun's energy to convert water, nutrients, and CO2 into usable biomass. Plant growth can be limited by a number of factors, including soil fertility, water, temperature, and CO2 concentration. Lack of carbon dioxide can induce photorespiration, which can destroy existing sugars. Thus, an increase in temperature and atmospheric CO2 can stimulate plant growth in places where these are the limiting factors. Similarly, from the human economic viewpoint, an increase in total biomass but a decrease in crop harvests would be a net disadvantage. In addition, IPCC models predict that higher CO2 concentrations would only spur growth of flora up to a point, however, because in many regions the limiting factors are water or nutrients, not temperature or CO2; after that, though greenhouse effects and warming would continue there would be no compensatory increase in growth. Despite the limiting factor of water, an increase in CO2 concentration has the direct effect of increasing the transpiration efficiency of most plants so that they actually produce more net biomass per unit of water used by the plant. Satellite data shows that the productivity of the Northern Hemisphere has increased from 1982 to 1991. However, more recent studies found that from 1991 to 2002, wide-spread droughts had actually caused a decrease in summer photosynthesis in the mid and high latitudes of the northern hemisphere. Moreover, on the other hand, just the fact that the total amount of biomass produced increases is not necessarily all good, since biodiversity can still decrease even though a small number of species are flourishing.

A possible counter-argument to this is the claim that suppression of plant growth is caused by a shortage of atmospheric carbon dioxide, which is rare in comparison to oxygen (21%). This carbon dioxide starvation becomes apparent in photorespiration, where there is so little carbon dioxide, that oxygen can enter a plant's chloroplasts and takes the place where carbon dioxide normally would be in the Calvin Cycle. This causes the sugars being made to be destroyed, badly suppressing growth.

Opening up of the Northwest Passage in summer
Melting Arctic ice may open the Northwest Passage in summer in approximately ten years, which would cut 5,000 nautical miles (9,300 km) from shipping routes between Europe and Asia. This would be of particular relevance for supertankers which are too big to fit through the Panama Canal and currently have to go around the tip of Africa. According to the Canadian Ice Service, the amount of ice in Canada's eastern Arctic Archipelago decreased by 15 percent between 1969 and 2004. A similar opening is possible in the Arctic north of Siberia, allowing much faster East Asian to Europe transport.

Adverse effects of the melting of ice include a potential increase in the rate of global warming, since ice reflects 90% of solar heat, while open water absorbs 90%. Negative impacts of the melting of ice include a potential increase in the rate of global warming, as that ice reflects more sunlight than the open water which is replacing it. There are also ecological effects of melting polar ice: for example, polar bears use sea ice to reach their prey, and swim to another ice floe when one breaks up. Ice is now becoming further separated, and dead polar bears are being found in the water, believed to have drowned. More recently, observed cannibalistic behavior in Polar Bears has been suggested by some scientists to be the result of food shortages brought on by global warming. Extreme weather, extreme events, tropical cyclones: Globally, many regions have probably already seen increases in warm spells and heat waves and it's virtually certain these changes continue over the 21st century. Data analysis of extreme events from 1960 until 2010 suggests that droughts and heat waves appear simultaneously with increased frequency. Extremely wet or dry events within the monsoon period have increased since 1980. Studies have also linked the rapidly warming Arctic to extreme weather in mid-latitudes as the jet stream becomes more erratic. Maximum rainfall and wind speed from hurricanes and typhoons are likely increasing.

Changes in ocean properties: increases in atmospheric CO2 concentrations have led to an increase in dissolved CO2 and as a consequence ocean acidity. Furthermore, oxygen levels decrease because oxygen is less soluble in warmer water, an effect known as ocean deoxygenation.

Long-term effects of global warming: On the timescale of centuries to millennia, the magnitude of global warming will be determined primarily by anthropogenic CO2 emissions. This is due to carbon dioxide's very long lifetime in the atmosphere. Long-term effects also include a response from the Earth's crust, due to ice melting and deglaciation, in a process called post-glacial rebound, when land masses are no longer depressed by the weight of ice. This could lead to landslides and increased seismic and volcanic activities. Tsunamis could be generated by submarine landslides caused by warmer ocean water thawing ocean-floor permafrost or releasing gas hydrates.

Abrupt climate change, tipping points in the climate system: Climate change could result in global, large-scale changes. Some large-scale changes could occur abruptly, i.e. over a short time period, and might also be irreversible. Examples of abrupt climate change are the rapid release of methane and carbon dioxide from permafrost, which would lead to amplified global warming. Another example is the possibility for the Atlantic Meridional Overturning Circulation to slow or to shut down (see also shutdown of thermohaline circulation). This could trigger cooling in the North Atlantic, Europe, and North America.

Further global warming (positive feedback)
Some effects of global warming themselves contribute directly to further global warming, in a vicious circle, the nature of which may be difficult to predict in advance.


 * The melting of permafrost and ice caps appears to be causing the release of large amounts of additional carbon dioxide or methane from decaying vegetation trapped beneath. In addition, methane clathrate deposits on the ocean floor might release more methane (the clathrate gun hypothesis).


 * There have been predictions, and some evidence, that global warming might cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO2 levels.


 * Melting could also lead to increased heat absorption because ice reflects more solar radiation (i.e., it has higher albedo) than land or water. Because sea ice and seasonal snow cover are more reflective than the underlying sea, any meltback may lead to further warming.


 * Warmer temperatures in the oceans reduce the productivity (growth) of ocean phytoplankton (algae). This is expected to reduce the amount of carbon dioxide taken up by photosynthesis in the ocean, which would again increase the effects of anthropogenic CO2 releases on the overall amount of CO2 in the atmosphere, and hence increase the greenhouse effect. This is a concern because ocean photosynthesis is as large a part of the planet's overall carbon balance as land photosynthesis.

Global warming controversy
These competing views typically weigh the benefits of limiting emissions of greenhouse gases against the costs. However, Neil Paul Cummins claims that the appropriate response to global warming is not determined by such a cost-benefit analysis. Rather, the appropriate policy response to global warming is geoengineering because this is the only response capable of maintaining the temperature of the atmosphere given the human and the non-human forces which are already set to lead to significant future warming. Furthermore, Cummins claims that such an outcome is necessary for the future survival of life on Earth, so geoengineering is in the interests of life on Earth. Geoengineering should therefore be seen as a positive event for the planet, an outcome to be celebrated. In general, it seems likely that climate change will impose greater damages and risks in poorer regions. The global warming controversy refers to a variety of disputes, significantly more pronounced in the popular media than in the scientific literature, regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature, especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, whether humankind has contributed significantly to it, and whether the increase is wholly or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, and what the consequences of global warming will be. Exact measured percentages of warming, with margins of error, from each possible cause, is not precisely known. For example, the exact measured percentage of warming from natural solar events is not known. Predictions are also not exact, similar to how the weather is not exactly predicted for the future year. In the scientific literature, there is a strong consensus that global surface temperatures have increased in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases. No scientific body of national or international standing disagrees with this view, though a few organisations hold non-committal positions. From 1990–1997 in the United States, conservative think tanks mobilized to undermine the legitimacy of global warming as a social problem. They challenged the scientific evidence; argued that global warming will have benefits; and asserted that proposed solutions would do more harm than good, although most scientists have challenged these views as being based on little to no evidence, and some scientists state that these conservatives have a religious or corporate agenda. In his 2012 speech to the Republican National Convention accepting their nomination as a candidate for President, Republican Mitt Romney used a rhetorical device which, according to Forbes, he "milked for a few long seconds": "President Obama promised to slow the rise of the oceans and to heal the planet...."

Within scientific organizations, many prominent scientists, including Nobel Prize winners, have registered objections to the oft repeated view that global warming theories are "incontrovertible". A petition to that effect was presented to the American Physical Society by some of its members, and Freeman Dyson notes the irony that some physicists ready to entertain the view that basic physical constants may change over time simultaneously reject questioning global warming orthodoxy. The hypothesis that man’s activities are the most important cause of global warming remains controversial. In the past the Earth has been both much warmer and much colder than now, and for most of it was warmer, sufficiently so that there were no global ice caps. During many periods when the Earth was as warm as now, atmospheric carbon dioxide was as high. Many scientific organizations take no positive on global warming. Others do, generally in favor of the man caused hypothesis. However, many scientists object to scientific organizations issuing statements advocating the view that man makes the primary contribution to global warming. Some, including Nobel Prize winners, have resigned from the American Physical Society in protest. Ordinarily science progresses by gradual accumulation of evidence rather than by scientific groups advocating a position. Some see such positions as political or quasi-religious, rather than scientific. The scientific findings surrounding global warming have led to the involvement of politicians, energy industry funding of critics and skeptical politicians, participation by environmental groups, and active discussions by pundit and media personalities over the cause, effects, and mitigations. Governments have taken stances, often widely divergent at national and local levels. U.S. officials, such as Philip Cooney, have repeatedly edited scientific reports from U.S. government scientists, many of whom, such as Thomas Knutson, have been ordered to refrain from discussing climate change and related topics. Fossil fuel companies such as ExxonMobil have spent large sums of money for public relations to downplay its importance, according to the Union of Concerned Scientists, while environmental groups have launched far-reaching campaigns attempting to emphasize its dangers. The global warming controversy is an ongoing dispute about what effect humans have on the global climate and what policies should be followed to mitigate any current detrimental effects, and prevent future detrimental effects. Although not fully settled, the current consensus from the official scientific communities on climate change is that recent warming is largely human-caused. According to the Third Assessment Report of the IPCC, most of the observed warming over the last 50 years is 66% to 90% certain to have been due to radiative forcing from increases in greenhouse gas concentrations. Undersecretary of State Tim Wirth said that "the science is settled," yet there is not a broad scientific consensus that human activities are causing global warming. Benjamin Santer, lead author of chapter 8 of the [IPCC] report says, ". . . few scientists would say the attribution issue was a done deal." Dr. Bert Bolin, Chairman of the IPCC, says that the science is not settled. When told that Undersecretary of State Tim Wirth had said the science was settled, Dr. Bolin replied: "I've spoken to [Tim Wirth], I know he doesn't mean it".

However, there is growing evidence that the consensus among scientists is beginning to be questioned. A 2006 report by the Laviosier Group openly questioned that human activity has contributed to global warming. The report cites a survey of 1117 peer-reviewed papers on a scientific database between 1993 and 2003, which mentioned the phrase "global climate warming". Of these, "only 13 (1 per cent) explicitly endorsed the consensus view". However, 34 of them "rejected or questioned the view that human activities are the main driving force of [global warming]". The rest were inconclusive or did not explicitly endorse or question the "consensus view".

The relation between global warming and ozone depletion

 * The same carbon dioxide radiative forcing that produces near-surface global warming is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion]] and the frequency of ozone holes. See ozone depletion for more.


 * Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposed effects: reduced ozone allows more solar radiation to penetrate, thus warming the troposphere. But a colder stratosphere emits less long-wave radiation, tending to cool the troposphere. Overall, the cooling dominates: the IPCC concludes that observed stratospheric O3 losses over the past two decades have caused a negative forcing of the surface-troposphere system of about 8211; 0.15 ± 0.10 watts per square meter (W/m2).


 * One of the strongest predictions of the global warming theory is that the stratosphere should cool. However, although this cooling has been observed, it is difficult to use it for attribution (for example, one of the difficulties to this conclusion include the fact that warming induced by increased solar radiation would not have this upper cooling effect), however, because similar cooling is caused by ozone depletion. Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of greenhouse gases and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the NOAA Geophysical Fluid Dynamics Laboratory show that above 20km (12.4 miles), the greenhouse gases dominate the cooling.


 * Ozone depleting chemicals are also greenhouse gases, representing 0.34 ± 0.03 W/m2, or about 14% of the total radiative forcing from well-mixed greenhouse gases.


 * Decreased ozone leads to an increase in ultraviolet levels. Ultraviolet (UV) radiation may be responsible for the death of ocean algae, which operate as a carbon dioxide sink in the ocean. Increased UV, therefore, may lead to a decrease in carbon dioxide uptake, thereby raising global carbon dioxide levels.

Global Warming Scam
Founder of the Weather Channel, John Coleman, has said that Global Warming is the "Greatest Scam in History". He claims that reports of global warming have been highly exaggerated by politicians such as Al Gore. Reports say that most scientists do not believe that we as a single race on Earth could effect an entire global climate. Earth is getting hotter because the Sun is burning more brightly than it has been at any time during the past millennium, according to new research. German and Swiss scientists suggest that that increasing radiation from the sun has caused the recent climate change, not the increased CO2 emissions.

Dangerous global warming
Although global warming has been seen as potentially dangerous for some time, the first international attempt to define what constitutes a 'dangerous' level occurred at the Avoiding Dangerous Climate Change scientific conference in February 2005. This took place in Exeter, United Kingdom under the UK presidency of the G8. At the conference it was said that increasing damage was forecast if the globe warms to about 1 to 3°Celsius (1.8 to 5.4°Fahrenheit) above pre-industrial levels. It was concluded that the stabilisation of greenhouse gasses at the equivalent of 450 ppmv CO2 would provide a 50% likelihood of limiting global warming to the average figure of 2°C (3.6°F). Stabilisation below 400 ppm would give a relatively high certainty of not exceeding 2°C, while stabilisation at 550 ppm would mean it was likely that 2°C would be exceeded. It was stated that unless 'urgent and strenuous mitigation actions' were taken in the next 20 years, it was almost certain that by 2050 global temperatures will have risen to between 0.5 and 2°C (0.9 and 3.6°F) above current levels. With carbon dioxide levels currently around 381 ppm and rising by 2ppm per year, without such action greenhouse gasses are likely to reach to reach 400ppm by 2016, 450ppm by 2041, and 550ppm by around 2091.

Possible compounding effects
Another concern is the possibility of a positive feedback loop: i.e., that global warming might cause further global warming in a vicious circle, the nature of which may be difficult to predict in advance. For example, the melting of ice caps may lead to the release of large amounts of additional carbon dioxide or methane from decaying vegetation trapped beneath; it could also lead to increased heat absorption because ice reflects more solar heat (has higher albedo) than land or water.

Ocean acidification
The increase in the percentage of carbon dioxide in the atmosphere causes an increased amount of carbon dioxide to dissolve into the ocean. This ameliorates the greenhouse effect. Unfortunately, dissolving carbon dioxide in water creates carbonic acid and other acids, so this increases the acidity of the ocean. This causes damage to ocean ecosystems; it has already been linked to coral bleaching. It should be noted that this is not caused by global warming; it is simply caused by carbon dioxide emissions. Increased atmospheric carbon dioxide increases the amount of CO2 dissolved in the oceans. This ameliorates the greenhouse effect by removing the greenhouse gas from the atmosphere. Unfortunately, carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid. The net effect, even accounting for warming of the oceans, is an observed and accelerating ocean acidification. Since biosystems are adapted to a narrow range of pH this is a very serious concern directly driven by increased atmospheric CO2 and not global warming. Ocean acidification would not be created by any other postulated climate change mechanisms. In fact, the result of these other forcings would be to decrease the acidity of the oceans as the sea temperature warmed. This is exactly contrary to what has been observed. The chemistry of the process was first described by Roger Revelle and Hans Suess.

The relationship between global warming and global dimming
Scientists have stated with 66-90% confidence that the effects of volcanic and human-caused aerosols have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for this effect.


 * For comparison of the relative significance of these factors:


 * The best estimate for the magnitude of radiative forcing from the long-lived greenhouse gases CO2, CH4, and N2O alone is +2.3 watts/m^2.


 * Radiative forcing from the halocarbon class of long-lived greenhouse gases is about +0.34 watts/m^2.


 * The cooling effects of aerosols are estimated to be:


 * Direct cooling effects of -0.5 watts/m^2


 * Cloud albedo cooling effects of -0.7 watts/m^2


 * Total warming effects from post-industrial human activity including the above and other cooling and warming factors are estimated at +1.6 watts/m^2.

Some scientists now consider that the effects of the recently recognized phenomena of global dimming (the reduction in sunlight reaching the surface of the planet, possibly due to aerosols) may have masked some of the effect of global warming. If this is so, the indirect aerosol effect is stronger than previously believed, which would imply that the climate sensitivity to CO2 is also stronger, although possibly outweighed by a non-linear coupling to the aerosols. Concerns about the effect of aerosol on the global climate were first researched as part of concerns over global cooling in the 1970s. Understanding of global dimming is at an early stage, and the implications are not clear. Initial work to incorporate the effects of global dimming suggests that world temperatures may rise by 2°C by 2030, and as much as 10°C by 2100; this is a doubling of the speculated figure of a 5 degree rise in global temperature over the coming century, if one assumes that global dimming is only temporary and any coupling to greenhouse warming is less than linear.

A further speculation is that an increase in global dimming would prevent such a rise in temperature that might trigger a rapid and irreversible release of the huge deposits of methane hydrates currently locked beneath the ocean floor, releasing methane - a similar theory is one of those that has been proposed to explain the Permian-Triassic extinction event approximately 252 million years ago.

Global warming as a possible Ice Age trigger
Perhaps more dangerous than the direct effects of global warming is its possible effect in triggering an ice age as theorized by some scientists. There is some speculation that global warming could trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. The chances of this occurring are unclear, but probably not large, based in climate model results.

Normal circulation (current)/Previous events
Under the currently well known global circulation of ocean water heat is distributed from the tropics to more extreme latitudes by the circulation of global currents, with warm water near the surface and cold water at deeper levels. The best known segment of this circulation is the Gulf Stream. Warm water from the Carribean is transported to the North Atlantic, where its effect in warming the atmosphere keeps northern Europe habitable. The evaporation of ocean water in the North Atlantic increases the salinity (relative saltiness) of the water as well as cooling it, both actions increasing the density of water at the surface. The formation of sea ice further increases the salinity. This dense water then sinks and the now circulation stream continues in a southerly direction toward Africa. During the previous glacial, a series of Dansgaard-Oerschger events - rapid climate fluctuations - may be attributed to freshwater forcing at high latitude interrupting the THC.

Interupted circulation scenario (hypothetical)
The melting of archaic ice in northern regions is currently releasing a large amount of fresh water into the northern oceans. Modeling and computer simulations are currently being used to investigate a disturbing theory: that this fresh water, by reducing the density of the ocean surface water at the North Atlantic sink may interrupt the global ocean circulation, or modify it in such a way that the northern European regions would no longer be warmed by the Gulf Stream. This could clearly be much more devastating for certain regions than would be the supposed benefits of warming at high latitudes. As historically (as revealed by ice core examination) warm periods are relatively short (about 10,000 years) and ice ages are quite long (more than 100,000 years), and the earth has been in an extended warm period for about 10,000 years, the global circulation system may be close to changing and a switch could be brought on early by the effects described. Global warming could lead to an increase in freshwater in the northern oceans, by melting in Greenland and by increasing precipitation. It is by no means clear that sufficient freshwater could be provided to interrupt the THC - climate models indicate not, but research continues.

Ice free Arctic Ocean scenerio (hypothetical)
Some theories posit that the Arctic Ocean is ice free during an ice age. This would have the effect of releasing a large amount of moisture into the atmosphere over the ocean, which could in turn greatly increase the amount of precipitation in the form of snow where permanent ice on land may form. The formation of extensive ice fields requires only that snow that falls in the winter is not completely melted in the summer. The high reflectivity of such snow results in further localized cooling. Over a period of many years such snow accumulation results in the formation if glacial ice in large icefields, which in response to gravity can form moving glaciers.

Jurassic Global Warming/Pre-human global warming
The IPCC says ''Complex physically-based climate models are required to provide detailed estimates of feedbacks and of regional features. Such models cannot yet simulate all aspects of climate (e.g., they still cannot account fully for the observed trend in the surface-troposphere temperature difference since 1979) and there are particular uncertainties associated with clouds and their interaction with radiation and aerosols. Nevertheless, confidence in the ability of these models to provide useful projections of future climate has improved due to their demonstrated performance on a range of space and time-scales''. The Earth has experienced natural global warming and cooling many times in the past, and these processes can offer useful insights into present processes. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles with interglacial warming periods much hotter than current temperatures. The chart also shows the time of the last glacial maximum about 20,000 years ago. It is though by some geologists that a rapid buildup of greenhouse gases caused the Earth to experience global warming in the early Jurassic period, with average temperatures rising by 5° degrees C Celsius (9° Fahrenheit). Research by the Open University published in Geology indicates that this caused the weathering of rocks to be speeded up by 400%, a process that took around 150,000 years to return carbon dioxide levels to normal. Rock weathering locks away Carbon in calcite (CaCO3) and dolomite, which are minerals with various degrees of carbon oxides.



Sudden release of methane clathrate from clathrate compounds (the clathrate gun hypothesis) (a greenhouse gas) from its ice complex (clathrate) has been hypothesized as both a cause for and an effect of other past global warming, because methane is a greenhouse gas. Two events possibly linked in this way are the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last glacial period is thought to not be due to clathrate release. Instead natural variations in the Earth's orbit are believed to have triggered the retreat of ice sheets by changing the amount of solar radiation received at high latitude and led to deglaciation.

The greenhouse effect has also been invoked to explain how the Earth made it out of the Snowball Earth period. During this period all silicate rocks were covered by ice, thereby preventing them to combine with atmospheric carbon dioxide. The atmospheric carbon dioxide level gradually increased until it reached a level that could have been as much as about 350 times the current levels. At this point temperatures were raised hot enough to melt the ice, even though the reflective ice surfaces had been reflecting most sunlight back into space. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere. Thick layers of abiotic carbonate sediment which can be found on top the glacial rocks from this period are believed to be formed by this rapid carbon dioxide removal process.

Snowball Earth
The greenhouse effect is also invoked to explain how the Earth made it out of the proposed Snowball Earth period 600 million years ago. During this period all silicate rocks would have been covered by ice, thereby preventing them from combining with atmospheric carbon dioxide. As a result, the atmospheric carbon dioxide level would have gradually increased until it reached a level that could have been as much as 350 times the current level. At this point temperatures would have increased enough to melt the ice, even though the reflective ice surfaces would have been reflecting most sunlight back into space. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere. The thick layers of abiotic carbonate sediment that have been found on top of the glacial rocks from this period support this theory.

Pre-industrial global warming?
Using paleoclimate data for the last 500 million years concluded that long-term temperature variations are only weakly coupled to CO2 variations. Most paleoclimatologists believe this is because other factors, such as continental drift and mountain building have larger effects in determining very long term climate. However, Shaviv and Veizer extended this by arguing that the biggest long-term influence on temperature is actually the solar system's motion around the galaxy, and the ways in which this influences the atmosphere by altering the flux of cosmic rays received by the Earth. Afterwards, they argued that over geologic time a change in CO2 concentrations comparable to doubling preindustrial levels, only results in about 0.75 degrees C (1.3; degrees F) warming rather than the usual 1.5-4.5 degrees C (2.7; 8.1; degrees F) reported by climate models. In turn, Veizer's recent work has been discussed and criticised. They acknowledge (Shaviv and Veizer 2004), however, that this conclusion may only be valid on multi-million year time scales when glacial and geological feedback have had a chance to establish themselves. Rahmstorf et al. 2004 argue that S+V have highly and arbitrarily tuned their data, and that their conclusions are unreliable. Leading palaeoclimatologist William Ruddiman has argued that human influence on the global climate actually began around 8000 years ago with the development of agriculture. In his opinion, this prevented CO2 (and later methane) levels falling as rapidly as they would have done otherwise. Ruddiman argues that without this effect, the Earth would be entering, or already have entered, a new ice age. However other work in this area argues that the present interglacial is most analogous to the interglacial 400,000 years ago that lasted approximately 28,000 years, in which case there is no need to invoke the spread of agriculture for having delayed the next ice age. He contends that forest clearing explains the rise in carbon dioxide levels in the current interglacial that started 8,000 years ago, contrasting with the decline in carbon dioxide levels seen in the previous three interglacials and 5,000 years ago with the start of Asian rice irrigation. He further contends that the spread of rice irrigation explains the breakdown in the last 5,000 years of the correlation between the Northern Hemisphere solar radiation and global methane levels, which has been maintained over at least the last 11 22,000-year cycles. Ruddiman argues that without these effects, the Earth would be nearly 2°C cooler and "well on the way" to a new ice age. Ruddimann's viewpoint is a minority one, however; and his interpretation of the historical record, with respect to the methane data, has been disputed.

Recent findings, developments
Note: In addition to being a scientific concept, global warming is also a societal issue. This section will be for documenting various recent findings and statements by various groups. If you prefer to focus on the scientific aspects, then that is fine. However, please do not veto others' ideas about might be valid here. Thanks for your help.

International organizations
In November 2006, the World Meteorological Organization released a Statement on Tropical Cyclones and Climate Change which provides "an updated assessment of the current state of knowledge of the impact of anthropogenically induced climate change on tropical cyclones".

In February 2007, the U.N. Intergovernmental Panel on Climate Change (IPCC) released a summary report for policymakers] stating that it is "very likely" (>90% assessed likelihood) that most of the observed increase in globally averaged temperature since the mid-20th century was caused by human activity. In 2006-7 the Earth experienced its warmest December-February since records began 128 years ago, a US agency, the NOAA, said in a report. The IPCC's position on hurricanes was a marked departure from a November 2006 statement by the World Meteorological Organization, which helped found the IPCC. The meteorological organization, after contentious debate, said it could not link past stronger storms to global warming. The debate about whether stronger hurricanes were linked to global warming was one division in the scientific community, which was otherwise largely united in agreeing that mankind is behind recent global warming

Joint declaration on global warming
In 2005, the national science academies of the G8 nations (including the U.S. National Academy of Sciences) - and Brazil, China and India, three of the largest emitters of greenhouse gases in the developing world, signed a statement on the global response to climate change. The statement stresses that the scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action.

Business opportunities and challenges
Perhaps the leading industry to accept global warming is the insurance industry. For example, a June 2004 report by the Association of British Insurers declared "Climate change is not a remote issue for future generations to deal with. It is, in various forms, here already, impacting on insurers' businesses now". It noted that weather risks for households and property were already increasing by 2-4 % per year due to changing weather, and that claims for storm and flood damages in the UK had doubled to over £6 billion over the period 1998 - 2003, compared to the previous five years. In the United States insurance losses have also greatly increased but those increases are attributed to increased population and property values in vulnerable coastal areas.

However, the threat posed by global warming also provides business opportunities. For example, a creation of a market for "trading" carbon dioxide emissions within the Kyoto Protocol, it is possible that London financial markets would be the centre for this potentially highly lucrative business, and the New York and Chicago stock markets would like a share (which is unlikely as long as the US rejects Kyoto). One of the biggest opponents of action on global warming has been the energy industry, and particularly the oil industry, such as ExxonMobil, which regularly publishes papers mitigating the threat of global warming. In 2005, it opposed a shareholders' resolution to explain the science behind its denial of global warming. Many studies link population growth with emissions and the effect of climate change. However, there are many other companies who do accept the possibilities of global warming, such as when Chairman of BP, John Browne, declared a need for action in 2002. Lord Oxburgh, non-executive chairman of Shell, said in a speech at the 2005 Hay-on-Wye Festival: "We have 45 years, and if we start now, not in 10 or 15 years' time, we have a chance of hitting those targets. But we've got to start now. We have no time to lose".

On 9 May, 2005 Jeff Immelt, the chief executive of General Electric (GE), announced plans to reduce GE's own emissions tied to global warming by one percent by 2012. GE said that given its projected growth, those emissions would have risen by 40 percent without such action.

In June 2005, the chief executive of BT became the first head of a British company to admit that climate change was already taking place, and affecting its business, and announced plans to source much of its substantial energy use from renewable sources. He noted that, "Since the beginning of the year, the media has been showing us images of Greenland glaciers crashing into the sea, Mount Kilimanjaro devoid of its ice cap and Scotland reeling from floods and gales. All down to natural weather cycles? I think not".

BT is also one of 23 multinational corporationss in the G8 Climate Change Roundtable, a business group formed at the January 2005 World Economic Forum. The group also includes Ford, Toyota, British Airways, and BP. On 9 June, 2005, the Group published a statement stating that there was a need to act on climate change and stressing the importance of market-based solutions. It called on governments to establish "clear, transparent, and consistent price signals" through "creation of a long-term policy framework" that would include all major producers of greenhouse gases. On 21 June, 2005, a group of leading airlines, airports, and aerospace manufacturers pledged to work together to reduce the negative environmental impact of the aviation industry, including limiting the impact of air travel on climate change by improving fuel efficiency and reducing carbon dioxide emissions of new aircraft by fifty percent per seat kilometre by 2020 from 2000 levels. The group aims to develop a common reporting system for carbon dioxide emissions per aircraft by the end of 2005, and pressed for the early inclusion of aviation in the European Union's carbon emission trading scheme.

Biosphere
Ecosystem changes: In terrestrial ecosystems, the earlier timing of spring events, as well as poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. It is expected that most ecosystems will be affected by higher atmospheric CO2 levels, combined with higher global temperatures. Expansion of deserts in the subtropics is probably linked to global warming. Ocean acidification threatens damage to coral reefs, fisheries, protected species, and other natural resources of value to society. Without substantial actions to reduce the rate of global warming, land-based ecosystems are at risk of major ecological shifts, transforming composition and structure.

Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems. Rising temperatures have been found to push bees to their physiological limits, and could cause the extinction of bee populations. Continued ocean uptake of CO2 may affect the brains and central nervous system of certain fish species, and that this impacts their ability to hear, smell, and evade predators.

Impacts on humans
The effects of climate change on human systems, mostly due to warming or shifts in precipitation patterns, or both, have been detected worldwide. The future social impacts of climate change will be uneven across the world. All regions are at risk of experiencing negative impacts, with low-latitude, less developed areas facing the greatest risk. Global warming has likely already increased global economic inequality, and is projected to do so in the future. Regional impacts of climate change are now observable on all continents and across ocean regions. The Arctic, Africa, small islands and Asian megadeltas are regions that are likely to be especially affected by future climate change. Many risks increase with higher magnitudes of global warming.

Food and water
Crop production will probably be negatively affected in low latitude countries, while effects at northern latitudes may be positive or negative. Global warming of around 4 °C relative to late 20th century levels could pose a large risk to global and regional food security. The impact of climate change on crop productivity for the four major crops was negative for wheat and maize, and neutral for soy and rice, in the years 1960–2013. Climate variability and change is projected to severely compromise agricultural production, including access to food, across Africa. By 2050, between 350 million and 600 million people are projected to experience increased water stress due to climate change in Africa. Water availability will also become more limited in regions dependent on glacier water, regions with reductions in rainfall and small islands.

Health and security
A number of groups have begun calling attention to the security implications of global warming, with particular attention being paid to this issue in 2007. On April 15, 2007, the Military Advisory Board, a panel of retired U.S. generals and admirals released a report entitled "National Security and the Threat of Climate Change". The report predicts that global warming will have significant security implications, in particular serving as a "threat multiplier" in already volatile regions. Just two days later, on April 17, the U.N. Security Council held a debate on the security implications of climate change, during which Britain's Foreign Secretary Margaret Beckett argued that "An unstable climate will exacerbate some of the core drivers of conflict, such as migratory pressures and competition for resources". And several weeks earlier, U.S. Senators Chuck Hagel (R-NB) and Richard Durbin (D-IL) introduced a bill in the U.S. Congress that would require federal intelligence agencies to collaborate on a National Intelligence Estimate to evaluate the security challenges presented by climate change. In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change. These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Generally impacts on public health will be more negative than positive. Impacts include the direct effects of extreme weather, leading to injury and loss of life; and indirect effects, such as undernutrition brought on by crop failures. There has been a shift from cold- to heat-related mortality in some regions as a result of warming. Temperature rise has been connected to increased numbers of suicides. Climate change has been linked to an increase in violent conflict by amplifying poverty and economic shocks, which are well-documented drivers of these conflicts. Links have been made between a wide range of violent behaviour including fist fights, violent crimes, civil unrest, or wars.

Livelihoods, industry, and infrastructure
In small islands and mega deltas, inundation as a result of sea level rise is expected to threaten vital infrastructure and human settlements. This could lead to issues of homelessness in countries with low-lying areas such as Bangladesh, as well as statelessness for populations in island nations, such as the Maldives and Tuvalu. Climate change can be an important driver of migration, both within and between countries.

Africa is one of the most vulnerable continents to climate variability and change because of multiple existing stresses and low adaptive capacity. Existing stresses include poverty, political conflicts, and ecosystem degradation.

Responses to global warming
Reducing the amount of future climate change is called mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere. Many countries, both developing and developed, are aiming to use cleaner, less polluting, technologies. Use of these technologies aids mitigation and could result in substantial reductions in CO2 emissions. Policies include targets for emissions reductions, increased use of renewable energy, and increased energy efficiency. Studies indicate substantial potential for future reductions in emissions. To limit warming to the lower range in the overall IPCC's "Summary Report for Policymakers" means adopting policies that will limit emissions to one of the significantly different scenarios described in the full report. This will become more and more difficult, since each year of high emissions will require even more drastic measures in later years to stabilize at a desired atmospheric concentration of greenhouse gases, and energy-related carbon-dioxide (CO2) emissions in 2010 were the highest in history, breaking the prior record set in 2008.

Since even in the most optimistic scenario, fossil fuels are going to be used for years to come, mitigation may also involve carbon capture and storage, a process that traps CO2 produced by factories and gas or coal power stations and then stores it, usually underground.

Kyoto Protocol
Emissions reduction cannot immediately stop or reverse global warming, as carbon dioxide has an atmospheric lifetime of decades to centuries. If emissions stopped immediately, surface global warming would worsen due to a reduction in reflective aerosols such as sulfate aerosols in the atmosphere, which primarily result from burning of various fuels. Low level (tropospheric) aerosols have a lifetime of several days. Currently, the cooling effect of aerosols is the same order of magnitude as greenhouse warming. Aerosols will continue to cool the planet as long as the pollution sources remain, but decreases in this atmospheric pollution would result in additional warming. This additional warming effect will commence quickly, due to the short lifetime of the aerosols. Modeling studies suggest that, regardless of whether aerosols remain or not, temperatures will continue to rise at least until 2100, due to the heat capacity of the oceans and the lifespan of greenhouse gases in the atmosphere. If global warming causes runaway climate change due to positive feedback effects, then emission reduction will no longer be able to prevent global warming from worsening, and global temperatures may continue to increase for thousands of years as methane from clathrates enters the atmosphere. A similar effect is believed to have occurred in the Paleocene-Eocene Thermal Maximum.

Models suggest that mitigation can quickly begin to slow global warming, but that temperatures will appreciably decrease only after several centuries. The world's primary international agreement on combating climate change is the Kyoto Protocol. The Kyoto Protocol is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC). The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions. Countries that ratify this protocol commit to reduce their emissions of carbon dioxide and five other greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases. Developing countries are exempt from meeting emission standards in Kyoto. This includes China and India, the second and third largest emitters of CO2, behind the United States. The International Energy Agency predicts China exceeded total U.S. emissions in 2007 or 2008.

Only the United States, (historically the world's largest greenhouse gas emitter; Australia; and Kazakhstan have refused to ratify the treaty. Citing "serious harm" to its economy, as well as the exemption of developing nations from 80 percent of the world, including major population centers, the treaty, the Bush administration contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns; in a ratification vote, the Protocol was defeated by a vote of 95-0 in the Senate. Bush has promoted improved energy technology as a means to combat climate change. In the absence of clear concerted action by the U.S. Federal government, various state, city, local, and regional governments have begun their own initiatives to indicate support and compliance with the Kyoto Protocol, on a local basis. For example, the Regional Greenhouse Gas Initiative (RGGI) is a state-level emissions capping and trading program involving eight Northeastern states, which was founded on December 20, 2005. The U.S. Climate Change Science Program is a joint program of over twenty U.S. federal agencies, all working together to investigate climate change; in June 2008, it stated in a report that weather would become more extreme, due to climate change. China (which is expected to soon overtake the US in greenhouse gas emissions) and India, two other large emitters, have ratified the treaty but, as developing countries, are exempt from its provisions. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming. This treaty expired in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.

Because the burning of coal to produce electricity is a primary cause of global warming, countries are trying to find clean alternatives to coal. According to the BBC in 2004, France shut down its last coal mine because it now gets almost all of its electricity from nuclear power. According to a 2007 story broadcast on 60 Minutes, nuclear power gives France the cleanest air of any industrialized country, and the cheapest electricity in all of Europe. According to that same 60 Minutes story, several other countries in western Europe use wind power, but when the wind isn't blowing sufficiently they import their electricity from France. According to a Frontline story on PBS, the people of France are well educated in science and understand the environmental benefits of nuclear power. The earth has enough uranium to provide all of our energy needs until the sun blows up in 5 billion years. Electric cars such as the Tesla Roadster can obtain their electricity from nuclear power. The IPCC's Working Group III is responsible for crafting reports on mitigation of global warming and the costs and benefits of different approaches. The 2007 IPCC Fourth Assessment Report concludes that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6 percent increase and three percent decrease in global gross domestic product.

Mitigation of global warming is accomplished through reductions in the rate of anthropogenic greenhouse gas release. Models suggest that mitigation can quickly begin to slow global warming, but that temperatures will appreciably decrease only after several centuries. The world's primary body for crafting a response to global warming is the Intergovernmental Panel on Climate Change (IPCC), a UN-sponsored activity organization which holds periodic meetings between national delegations on the problems of global warming, and issues working papers and assessments on the current status of the science of climate change, impacts, and mitigation. It convenes four different working groups examining various specific issues. For example, in May 2007, the IPCC held conferences in Bonn, Germany, and in Bangkok, Thailand. This organization was awarded the Nobel Peace Prize in 2007, along with former U.S. Vice-President Al Gore. Some further negotiations will be then necessary to achieve the drafting of an effective text for the next United Nations Conference of the Parties in Mexico next winter.

Developed nations range from the middle to highest emissions on both unadjusted emissions lists and per capita emissions lists. Developing nations, however, have very good ratings when emissions are divided by population. China and India, the world's two most populous countries, when listed by greenhouse gas emissions per capita (including land use change), have rankings of 121st largest per capita emitter at 3.9 Tonnes of CO2e and 162nd largest per capita emitter at 1.8 Tonnes of CO2e respectively, compared with for example the USA at position of the 14th largest per capita CO2e emitter at 22.9 Tonnes of CO2e. Similar statistics are found for pure carbon emissions, not considering equivalents.

Advocates for developing nations argue against developing and developed nations following the same timetable for emission reduction, when the developing nations are beginning industrial growth the developed nations began decades ago, are using technologies with lower emissions than the developed nations started with, and have made decades' less impact on total emissions: "'Looking at the industrialization process, to the extent that fossil fuel use is a necessary ingredient of economic development, as acknowledged by the UNFCCC, the emergence of the global climate change issue at this time effectively determines the distinction between the developed, Annex I nations and the developing, non-Annex I nations. For Annex I nations, that energy exploitation has been incorporated into their economies and is part of their baseline for considering any controls on greenhouse gases. For developing, non-Annex I nations, however, economic development will require expanded energy use, of which fossil fuels can be the least costly. Thus imposing limits on fossil energy use at this time could result in developing countries being relegated to a lower standard of living than those nations that developed earlier'. - The Encyclopedia of Earth"

Climategate
Scientists at the University of East Anglia have admitted throwing away much of the raw temperature data on which their predictions of global warming are based. It means that other academics are not able to check basic calculations said to show a long-term rise in temperature over the past 150 years. The UEA's Climatic Research Unit CRU was forced to reveal the loss following requests for the data under Freedom of Information legislation. The data were gathered from weather stations around the world and then adjusted to take account of variables in the way they were collected. The revised figures were kept, but the originals — stored on paper and magnetic tape — were dumped to save space when the CRU moved to a new building. The admission follows the leaking of a thousand private emails sent and received by Professor Phil Jones, the CRU's director. In them he discusses thwarting climate sceptics seeking access to such data. In a statement on its website, the CRU said: "We do not hold the original raw data but only the value-added (quality controlled and homogenized) data". The CRU is the world's leading center for reconstructing past climate and temperatures. Climate change sceptics have long been keen to examine exactly how its data were compiled. That is now impossible.

Rain
A very strong effect is that with evaporation increasing by 6.5% for each degree Celcius, global precipitation is expected to increase

Drivers of greenhouse gas emissions
Over the last three decades of the twentieth century, gross domestic product per capita, and population growth were the main drivers of increases in greenhouse gas emissions. CO2 emissions are continuing to rise due to the burning of fossil fuels and land-use change. Emissions can be attributed to different regions. Attribution of emissions due to land-use change are subject to considerable uncertainty.

Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological, technological, and natural developments. In some scenarios emissions continue to rise over the century, while others have reduced emissions. Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century. Emission scenarios, combined with modelling of the carbon cycle, have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future. Depending both on the Shared Socioeconomic Pathway (SSP) the world takes and the mitigation scenario, model suggest that by the year 2100, the atmospheric concentration of CO2 could range between 380 and 1400 ppm.

Reducing greenhouse gases
Near- and long-term trends in the global energy system are inconsistent with limiting global warming at below 1.5 or 2 °C, relative to pre-industrial levels. Current pledges made as part of the Paris Agreement would lead to about 3.0 °C of warming at the end of the 21st century, relative to pre-industrial levels. In limiting warming at below 2 °C, more stringent emission reductions in the near-term would allow for less rapid reductions after 2030, and be cheaper overall. Many integrated models are unable to meet the 2°C target if pessimistic assumptions are made about the availability of mitigation technologies.

Co-benefits of climate change mitigation may help society and individuals more quickly. For example, cycling reduces greenhouse gas emissions while reducing the effects of a sedentary lifestyle at the same time. The development and scaling-up of clean technology, such as cement that produces less CO2 is critical to achieve sufficient emission reductions for the Paris agreement goals.

It has been suggested that the most effective and comprehensive policy to reduce carbon emissions is a carbon tax or the closely related emissions trading. There are diverse opinions on how people could mitigate their carbon footprint. Some disagree with encouraging people to stop having children, saying that children "embody a profound hope for the future", and that more emphasis should be placed on overconsumption, lifestyle choices of the world's wealthy, fossil fuel companies and government inaction. Still others, such as Mayer Hillman, contend that both individual action and political action by national governments will not be enough, and only a global transition to zero GHG emissions throughout the entire economy and a reduction in human population growth will be sufficient to mitigate global warming.

Mitigating and adapting to the effects of global warming


Other policy responses include adaptation to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention. The ability to adapt (called "adaptive capacity") is closely linked to social and economic development. Even societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood. Reducing the amount of future climate change is called mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere. Many countries, both developing and developed, are aiming to use cleaner, less polluting, technologies. Use of these technologies aids mitigation and could result in substantial reductions in CO2 emissions. Policies include targets for emissions reductions, increased use of renewable energy, and increased energy efficiency. Studies indicate substantial potential for future reductions in emissions. To limit warming to the lower range in the overall IPCC's "Summary Report for Policymakers" means adopting policies that will limit emissions to one of the significantly different scenarios described in the full report.{{vn|date=November 2011} This will become more and more difficult, since each year of high emissions will require even more drastic measures in later years to stabilize at a desired atmospheric concentration of greenhouse gases, and energy-related carbon-dioxide (CO2) emissions in 2010 were the highest in history, breaking the prior record set in 2008.

Since even in the most optimistic scenario, fossil fuels are going to be used for years to come, mitigation may also involve carbon capture and storage, a process that traps CO2 produced by factories and gas or coal power stations and then stores it, usually underground. However, such technologies are still experimental and currently unproven, and in the end may prove overly complicated and expensive. Interim low-tech solutions to simply control temperatures until such time as we transition to non-CO2 producing energy alternatives already exist using relatively low-cost techniques. For example, it would be possible to cover vast areas of land (perhaps unused desert areas) with inexpensive reflective materials (such as the Mylar used in grocery store balloons) which could reject enough of the sun's energy to offset any undesired temperature gain (or, for that matter, using absorptive materials should we fear an incipient ice age). While not a long term solution by itself (CO2 emissions would still need to be reined in), the existence of available mitigating approaches demonstrate that popular "doomsday predictions" are not an immediate concern during the period of time it will take to develop and deploy better / cheaper solar and battery technologies to replace the burning of carbon emitting fuels. A wide variety of measures have been suggested for adaptation to global warming. These measures range from the trivial, such as the installation of air-conditioning equipment, to major infrastructure projects, such as abandoning settlements threatened by sea level rise. Measures including water conservation, water rationing, adaptive agricultural practices, including diversification, construction of flood defenses, changes to medical care, and interventions to protect threatened species have all been suggested. A wide-ranging study of the possible opportunities for adaptation of infrastructure has been published by the Institute of Mechanical Engineers, who state that some degree of climate change is inevitable. As Robert Kandel (member of the IPCC working group on science) says in his book Turning the tide on the Climate change, "to keep the world comfortable, we must reduce emissions. Rather than going back to nature, we must use ingenuity to live better with less". The broad consensus among climate scientists that global temperatures will increase has led nations, states, corporations and individuals to try to reduce its effects. Their actions and proposals fall into five categories


 * Impact of global climate changes on agriculture


 * Flood defence

There are five categories of actions that can be taken to mitigate global warming:


 * 1) Reduction of energy use (conservation)


 * 1) Shifting from carbon-based fossil fuels to alternative energy sources


 * 1) Carbon capture and storage


 * 1) Carbon sequestration


 * 1) Planetary engineering to cool the earth, including screening out sunlight or increasing albedo of the Earth.

Although the evidence of global warming is clear, many argue that predictions of short and long-term effects are exaggerated. There is significant controversy over whether global warming (even at its current rate) will significantly alter the lifestyle of the western world. The threat of possible global warming has led to attempts to "Mitigation of global warming" covers all actions aimed at reducing the extent negative effects or the likelihood of global warming. The Environmental Protection Agency (EPA) of the United States, determined that carbon dioxide, and five other greenhouse gases, "endanger public health and welfare" of the American people. These gases, they said, contribute to climate change, which is causing more heat waves, droughts and flooding, and is threatening food and water supplies. The possibility that global temperatures will continue to significantly increase has led others to propose means of mitigate global warming. The world's primary international agreement on combating climate change is the Kyoto Protocol. The consensus among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming. Some of the strategies that have been proposed for mitigation of global warming strategies include development of new technologies, wind power, nuclear power, renewable energy, biodiesel, electric cars (and hybrids automobiles), and fuel cells, Energy conservation, carbon taxes, enhancing natural carbon dioxide sinks; increased use of sulfate aerosols, which exhibit a cooling effect on the Earth, carbon offsets, population control, and carbon capture and storage; nanotechnology; and environmental vegetarianism schemes. Some environmentalist groups encourage individual action against global warming, often aimed at the consumer, and there has also been business action on climate change. Many environmental groups encourage individual action against global warming, often aimed at the consumer, but also by community and regional organizations, and others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions. These responses to global warming divide broadly between adapting to the effects of global warming and reducing or even reversing global warming itself. The latter is referred to as mitigation and includes both emission reduction and speculatively, geoengineering. Many environmental groups encourage individual action against global warming, as well as community and regional actions. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions. There has also been business action on climate change, including efforts at increased energy efficiency and (still limited) moves to alternative fuels. In January 2005, the European Union introduced its European Union Emission Trading Scheme, a greenhouse gas emissions trading scheme through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances. Australia announced its Carbon Pollution Reduction Scheme in 2008. The President elect of the United States Barack Obama has announced that he will introduce an economy wide cap and trade scheme. Some indigenous rights organizations, such as Survival International, Amazon Watch, and Cultural Survival, have raised concerns over the fact that not only climate change affects the tribal people most of all, as some measures to mitigate the problem are equally harmful for them. Survival international came to public with the report, The most inconvenient truth of all, which documents the impact of the biofuels industry, hydro-electric power, carbon-offsetting and forest conservation schemes on indigenous communities worldwide. The organization argues that some climate change mitigation measures have led to exploitation, violation and in some cases destruction of land recognized as belonging to indigenous communities. The International Forum of Indigenous Peoples on Climate Change has expressed similar concerns. Stephen Corry, director of Survival International, explains that "projects that victimise the people and harm the environment cannot be promoted or marketed as green projects".

Although the combination of scientific consensus and economic incentives (especially for Russia) is clear on the general conclusions - enough to persuade the governments of more than 150 countries, an amendment to the UNFCCC negotiated in 1997. The Protocol to ratify the Kyoto Protocol (notably excluding the US and Australia) - there are issues about just how much greenhouse gas emissions have warmed the planet. The uncertainties have in the past been exploited by politicians, corporations, and others with vested interests in opposing the activities needed to mitigate global warming; however, business opinion is increasingly changing to accept global warming as real and that action is needed. The scientific consensus is otherwise opposed by only a small minority of scientists. Uncertainties remain and have been emphasized by some politicians, and others questioning the costs needed to reduce future global warming; however, the business position on climate change is increasingly changing to accept global warming as both real and anthropogenic, and that action such as carbon emissions trading and carbon taxes is needed. The scientific consensus is questioned by a small minority of scientists. Some politicians (such as American president George W. Bush and Australian Prime Minister John Howard) and public intellectuals (such as Bjorn Lomborg and Ronald Bailey), have argued that the cost of mitigating global warming via the Kyoto protocol is too large to be justified. This view may be proving correct, as the signatories of the Kyoto protocol are currently struggling to meet their targets, including Europe and Japan. After only five years, Canada has given up entirely. Also, of the 163 countries that have signed and ratified Kyoto, only 31 are actually required to lower greenhouse emissions. It should also be noted that the world's two largest populations, India and China, are currently exempt from the requirements of the protocol. However, some segments of the business community have accepted both the reality of anthropogenic global warming and its attribution to anthropogenic causes, as well as the need for actions such as carbon emissions trading and carbon taxes.

Adaptation strategies accept some warming as a given foregone conclusion and focus on preventing or reducing undesirable consequences: for example defending against rising sea levels or ensuring food security.

Because human activity is strongly correlated with global warming, and is asserted to be the main cause, it is interesting to develop consideration about the possible (psycho) sociological reactions independently of the political arena with the topic of sociological considerations about greenhouse gases.



Mitigation of climate change is the reduction of greenhouse gas emissions, or the enhancement of the capacity of carbon sinks to absorb greenhouse gases from the atmosphere. There is a large potential for future reductions in emissions by a combination of activities, including energy conservation and increased energy efficiency; the use of low-carbon energy technologies, such as renewable energy, nuclear energy, and carbon capture and storage; decarbonizing buildings and transport; and enhancing carbon sinks through, for example, reforestation and preventing deforestation. A 2015 report by Citibank concluded that transitioning to a low carbon economy would yield positive return on investments.



Climate change adaptation is the process of adjusting to actual or expected climate and its effects. Humans can strive to moderate or avoid harm due to climate change and exploit opportunities. Examples of adaptation are improved coastline protection, better disaster management and the development of crops that are more resistant. The adaptation may be planned, either in reaction to or anticipation of global warming, or spontaneous, i.e., without government intervention.

The public section, private sector and communities are all gaining experience with adaptation and adaptation is becoming embedded within certain planning processes. While some adaptation responses call for trade-offs, others bring synergies and co-benefits. That is, the capacity and potential for humans to adapt, called adaptive capacity, is unevenly distributed across different regions and populations, and developing countries generally have less capacity to adapt. In June 2019, U.N. special rapporteur Philip Alston warned of a 'climate apartheid' situation developing, where global warming "could push more than 120 million more people into poverty by 2030 and will have the most severe impact in poor countries, regions, and the places poor people live and work". Yvo de Boer, Executive Secretary of the UN Framework Convention on Climate Change, warned scientists and government officials from some 130 countries that failure to act on climate change while there was time would be "criminally irresponsible". Other policy responses include adaptation to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention. The ability to adapt is closely linked to social and economic development. Even societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood.

Climate engineering/Geoengineering
Another policy response is engineering of the climate (geoengineering). This policy response is sometimes grouped together with mitigation. Geoengineering is largely unproven, and reliable cost estimates for it have not yet been published. Geoengineering encompasses a range of techniques to remove CO2 from the atmosphere or to block incoming sunlight. Little is known about the effectiveness, costs or potential side effects of geoengineering options. As most geoengineering techniques would affect the entire globe, the use of effective techniques, if they can be developed, would require global public acceptance and an adequate global legal and regulatory framework. Climate engineering (sometimes called geoengineering or climate intervention) is the deliberate modification of the climate. It has been investigated as a possible response to global warming, e.g. by NASA. Techniques under research fall generally into the categories solar radiation management and carbon dioxide removal, although various other schemes have been suggested. Research is at a generally early stage, with no large-scale schemes currently deployed. A study from 2014 investigated the most common climate engineering methods and concluded they are either ineffective or have potentially severe side effects and cannot be stopped without causing rapid climate change. Geoengineering would involve the deliberate modification of Earth's environment on a large scale "to suit human needs and promote habitability". It can be divided two major approaches. The first is remediation, in which greenhouse gases would be removed from the atmosphere, principally by carbon sequestration methods such as air capture. The second is solar radiation management, in which incoming solar radiation would be reduced, such as by the insertion of stratospheric sulfur aerosols or painting roofs of houses white. The slow pace of action to reduce greenhouse gas emissions have led some scientists, such as Ken Caldeira and the Nobel Prize winning Paul Crutzen to suggest geoengineering techniques, which can be employed to change the climate deliberately and thus control some of the effects of global warming. Whilst some cool roof and tree planting projects are underway, no planetary-scale geoengineering has yet been attempted. Fears of runaway climate change, which are partially based on observations of Arctic shrinkage, have been a major cause of recent interest in geoengineering. No geoengineering projects have been initiated or studied in great detail as yet, but various established experts and mainstream sources have recently suggested that beginning to study them at a higher level of urgency is warranted as a stopgap measure and for their different externalities and costs (even with the possibility of single-nation unilateralism in mind). No large-scale geoengineering projects have yet been undertaken.

Suggested techniques include:


 * Solar radiation management - changing the Earth's albedo to alter the amount of sunlight that's reflected back into space. Techniques such as space sunshade, creating stratospheric sulfur aerosols and cool roof technology all fall into this category.


 * Greenhouse gas remediation - techniques such as biomass energy with carbon capture and storage, using lasers to break up CFCs in the atmosphere and iron fertilisation of oceans to stimulate phytoplankton growth.


 * Arctic geoengineering - typically seeking to preserve sea ice or adjust thermohaline circulation by using methods such as diverting rivers to keep warm water away from sea ice, or tethering icebergs to prevent them drifting into warmer waters and melting.

UNFCCC
Mitigation of and adaptation to climate change are two complementary responses to global warming. Most countries are Parties to the United Nations Framework Convention on Climate Change (UNFCCC). The ultimate objective of the Convention is to prevent "dangerous" human interference of the climate system. As is stated in the Convention, this requires that GHGs are stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and economic development can proceed in a sustainable fashion.

The UNFCCC recognizes differences among countries in their responsibility to act on climate change. In the Kyoto Protocol to the UNFCCC, most developed countries (listed in Annex I of the treaty) took on legally binding commitments to reduce their emissions. Policy measures taken in response to these commitments have reduced emissions. For many developing (non-Annex I) countries, reducing poverty is their overriding aim.

The Framework Convention was agreed in 1992, but since then, global emissions have risen. During negotiations, the G77 (a lobbying group in the United Nations representing 133 developing nations): pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions. This was justified on the basis that: the developed world's emissions had contributed most to the stock of GHGs in the atmosphere; per-capita emissions (i.e., emissions per head of population) were still relatively low in developing countries; and the emissions of developing countries would grow to meet their development needs. This mandate was sustained in the Kyoto Protocol to the Framework Convention, which entered into legal effect in 2005.

In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to limit their emissions. These first-round commitments expire in 2012. US President George W. Bush rejected the treaty on the basis that "it exempts 80% of the world, including major population centers such as China and India, from compliance, and would cause serious harm to the US economy".

At the 15th UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC Parties produced the Copenhagen Accord. Parties agreeing with the Accord aim to limit the future increase in global mean temperature to below 2°C. The 16th Conference of the Parties (COP16) was held at Cancún in 2010. It produced an agreement, not a binding treaty, that the Parties should take urgent action to reduce greenhouse gas emissions to meet the 2 °C goal. It also recognized the need to consider strengthening the goal to a global average rise of 1.5 °C.

Issue debate and skepticism, social political processes, and laws/Political response/Economics and Social


Developed and developing countries have made different arguments over who should bear the burden of economic costs for cutting emissions. Developing countries often concentrate on per capita emissions, that is, the total emissions of a country divided by its population. Per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries. This is used to make the argument that the real problem of climate change is due to the profligate and unsustainable lifestyles of those living in rich countries.



On the other hand, commentators from developed countries point out that total carbon emissions, carrying capacity, efficient energy use and civil and political rights are very important issues. Land is not the same everywhere. Not only the quantity of fossil fuel use but also the quality of energy use is a key debate point. For example, efficient energy use supporting technological change might help reduce excess carbon dioxide in Earth's atmosphere. The use of fossil fuels for conspicuous consumption and excessive entertainment are issues that can conflict with civil and political rights. People{{who|date=March 2011} in developed countries argue that history has proven the difficulty of implementing fair rationing programs in different countries because there is no global system of checks and balances or civil liberties.

The Kyoto Protocol, which came into force in 2005, sets legally binding emission limitations for most developed countries. Developing countries are not subject to limitations. This exemption led the U.S. and Australia to decide not to ratify the treaty, although Australia did finally ratify the treaty in December 2007. Debate continued at the Copenhagen climate summit and the Cancún climate summit.

In a literature assessment, Smith and others concluded, with medium confidence, that:
 * climate change would increase income inequalities between and within countries


 * a small increase in global mean temperature (up to 2°C by 2100, measured against 1990 levels) would result in net negative market sector impacts in many developing countries and net positive market sector impacts in many developed countries


 * the aggregate market sector impact (i.e., total impacts across all regions) of a small increase in global mean temperature would amount to plus or minus a few percent of world GDP.

With high confidence, Smith and others concluded that a medium (2-3°C) to high (above 3°C) level of warming would result in more intense negative impacts, and that net positive impacts would begin to decline and eventually become negative. They found that most studies showed aggregate net damages at a global scale above a medium temperature increase, with further damages at higher temperatures. Depending on underlying assumptions, studies of the economic impacts of a doubling in atmospheric carbon dioxide (CO2) from pre-industrial levels conclude that this would have a slightly negative to moderately positive aggregate effect on the agricultural sector. This aggregate effect hides substantial regional differences, with benefits mostly predicted in the developed world and strongly negative impacts for populations poorly connected to regional and global trading systems. A number of other sectors will be affected by climate change, including the livestock, forestry, and fisheries industries. Other sectors sensitive to climate change include the energy, construction, insurance, tourism and recreation industries. The aggregate impact of climate change on most of these sectors is highly uncertain.

According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others. Stern in 2007 assessed climate change impacts using the basic economics of risk premiums. He found that unmitigated climate change could result in a reduction in welfare equivalent to a persistent average fall in global per-capita consumption of at least 5%. The study by Stern has received both criticism and support from other economists (see Stern Review). The IPCC in 2007 concluded that "Aggregate estimates of costs mask significant differences in impacts across sectors, regions and populations and very likely underestimate damage costs because they cannot include many non-quantifiable impacts". US intelligence believes fallout from global climate change over the next 20 years will boost global instability and may place new burdens on US military forces, according to a report delivered to Congress. Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe (the social cost of carbon). Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide). One measure of climate change impacts is the social cost of carbon (SCC). The SCC is a global aggregate estimate, and based on the marginal impact of emitting one more tonne of carbon (as carbon dioxide) at any point in time. The mean peer-reviewed SCC estimate is US$43 per tonne of carbon, with a standard deviation of US$83 per tonne. These values are from 2007 IPCC Assessment Report. SCC estimates are calculated using integrated assessment models. Not all climate change impacts are included in these models, and as a result, the true costs of climate change may larger or smaller than predicted. The IPCC concluded that climate change costs were probably underestimated by integrated assessment models. One problem with global aggregate estimates, like the SCC, is that they do not indicate of how the impacts of climate change will be distributed. According to the IPCC report, climate change impacts would likely be unevenly distributed between developing and developed countries. With modest warming (2 degrees Celsius global mean temperature increase above 1990 levels), many developing countries would be expected to suffer net negative market sector impacts, while net positive market sector impacts would be expected in many developed countries. Some population groups in developed countries are vulnerable to less than 2 degrees Celsius of warming. With 4 degrees Celsius of warming, net negative impacts would be expected in many developing and developed countries. One widely-publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst case scenario global consumption per head could fall 20%. The report's methodology, advocacy and conclusions has been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios, while others have supported the general attempt to quantify economic risk, even if not the specific numbers. Preliminary studies suggest that costs and benefits of mitigating global warming are broadly comparable in magnitude. According to Barker, the costs of mitigating climate change are 'insignificant' relative to the risks of unmitigated climate change.

According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others. Developing countries dependent upon agriculture will be particularly harmed by global warming. Recent climate research published in Nature Geoscience by National Oceanic and Atmospheric Administration meteorologist Tom Knutson indicates global warming is not responsible for the recent increase in Atlantic Hurricanes. Research published in the medical journal Lancet indicates that obese people are disproportionately responsible for greenhouse gas emissions, since they consume 18% more food energy (due to their greater body mass) and require increased quantities of fuel to transport themselves and the food they eat. The Global Humanitarian Forum projected in a 2009 study that in 2010 14,500 deaths due to droughts, temperature extremes, floods, storms, and wildfires will directly attributable to climate change, and 29,000 in 2050 if trends continue; 99% of deaths are within developing countries. Likewise in 2010, 87 million people will be serious affected, and 351 million in 2050 if current trends continue; those affected are disproportionately within developing countries susecptable to scocio-economic changes, changes in percipitation, and weather extremes—attributable to climate change. A July 2011 Rasmussen Reports poll found that 69% of adults in the USA believe it is at least somewhat likely that some scientists have falsified global warming research. In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.

In June 1976, in response to press reports predicting a Little Ice Age, the World Meteorological Organization issued a warning that a very significant warming of global climate was probable. Despite the overwhelming scientific consensus that global warming is occurring and that it is caused by humans, the lay population in America does not share such viewpoints. A variety of industries, especially the oil industry, have systematically funded think tanks and contrarian scientists to sow doubt about the scientific consensus and make the public think that the situation is not as severe as it is. Such attempts at denial have focused mainly on the United States and the fraction of the public which accepts the scientific consensus about global warming is much smaller in the United States than in other modern countries such as Japan. The global warming skepticism movement started by first claiming that the Earth was not warming. When that no longer worked, they claimed that the warming was not due to humans. Now, the claim is made that even if the warming is due to humans, the warming will be small and not possible to stop. Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed. Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been negligible compared to the developed world. There is some evidence of regional climate change affecting systems related to human activities, including agricultural and forestry management activities at higher latitudes in the Northern Hemisphere. Low-lying coastal systems are vulnerable to sea level rise and storm surge. Human health in populations with limited capacity to adapt to climate change. It is expected that some regions will be particularly affected by climate change, including the Arctic, Africa, small islands, and Asian and African megadeltas. Some people, such as the poor, young children, and the elderly, are particularly at risk, even in high-income areas. Some areas such as some Greenlanders welcome global warming because it provides increased opportunities for agriculture, commercial fishing, mining, and oil exploration due to the receding ice. In the Greater Mekong Subregion, which encompasses Myanmar, Yunnan in China, Lao PDR, Thailand, Vietnam and Cambodia, food production has risen steadily in the past two decades. At the same time, the population has also grown to 275 million and is anticipated to exceed 340 million by 2050. Climatic changes anticipated to affect agricultural production in this megadelta region include: increased temperature (potentially reducing crop yields); increased pests and diseases; increased water demand (scientists estimate demand for irrigation in semi-arid regions of Asia will increase by at least 10 per cent per 1°C temperature rise); and changes sea-level rise and saltwater intrusion. In some areas the effects on agriculture, industry and health could be mixed, or even beneficial in certain respects, but overall, it is expected that these benefits will be outweighed by negative effects.

On Tuesday, September 22, 2009, World leaders met to call for immediate and substantive steps to combat climate change, saying failure to act now would bring "irreversible catastrophe". Speaking to a special summit on climate change at the United Nations, United States President, Barack Obama pledged the full commitment of the United States to a global response after what he called years of responding too slowly to the magnitude of the challenge. "We seek sweeping but necessary change in the midst of a global recession, where every nation's most immediate priority is reviving their economy and putting their people back to work", Obama said. "And so all of us will face doubts and difficulties in our own capitals as we try to reach a lasting solution to the climate challenge. Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been negligible compared to the developed world, reports The New York Times. At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and have been used as part of its justification for continued non-ratification. In the Western world, the idea of human influence on climate and efforts to combat it has gained wider acceptance in Europe than in the United States. In 2010, some of the world's most eminent climate scientists said, "The bitter winter afflicting much of the Northern Hemisphere is only the start of a global trend towards cooler weather that is likely to last for 20 or 30 years". Prof Latif, who leads a research team at the renowned Leibniz Institute at Germany’s Kiel University, developed new methods for measuring ocean temperatures 3,000ft beneath the surface, where the cooling and warming cycles start. He and his colleagues predicted the new cooling trend in a paper published in 2008 and warned of it again at an IPCC conference in Geneva in September 2009.



This issue of climate change has sparked debate in the U.S. about weighing the benefits of limiting industrial emissions of greenhouse gases to reduce impacts to the climate, versus the effects on economic activity and also about the politic manipulation of scientific testimonies and reports. In the U.S., the political manipulation of scientific testimonies and reports has also become an issue. There has also been discussion in several countries about the cost of adopting alternate, cleaner energy sources in order to reduce emissions. Fossil fuel organizations and companies such as American Petroleum Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls, represented by individuals such as Philip Cooney, and some think tanks such as the Competitive Enterprise Institute and the Cato Institute have spent large sums of money for public relations to downplay the risks of climate change, while environmental groups and entertainers have launched campaigns emphasizing its impacts. One perspective is that use of the term "global warming" has posed some challenges to broader acceptance. The term "warm" or "warming" generally has positive associations. Moreover, the term gives an impression that how we will experience global climate change is a small, gradual increase in average temperatures while failing to convey the full extent of the negative consequences. In recent years, "global climate change" has become more widely used than "global warming". One blog promotes adoption of the term "global storming" to more accurately reflect how we are likely to experience global climate change -- more severe fluctuations in weather patterns including more severe storms and droughts.

Environmental groups and some public figures have launched campaigns emphasizing the risks. Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have recently scaled back such efforts or called for policies to reduce global warming.

Increased publicity of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the projected effects of global warming, while their emissions have been small compared to the developed world. The exemption of developing country exempt from Kyoto Protocol restrictions have been used as part of a rationale for non-ratification by the U.S. and criticism from Austria.

A third of the world population are unaware of the causes and issues of global warming, developing countries more unaware than developed. Of those who are aware, Latin America and develop countries in Asia lead in belief that the changes in global temperatures are a result of human activity while Eastern Europe, the Middle East, and Africa lead in the opposite. In the western world, the concept and the appropriate responses are contested. Nick Pidgeon, professor at Cardiff University, describe "results show the different stages of engagement about global warming on each side of the Atlantic" where Europe debates the appropriate responses while the United States debates whether climate change is happening.

In 2007-2008 the Gallup Polls surveyed 127 countries. Over a third of the world's population are unaware of global warming, developing countries more unaware than developed, Africa the lowest. However, awareness does not equate to belief that global warming is a result of human activities. Of those aware, Latin America leads in belief that temperature changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few countries from the Former Soviet Union lead in the opposite. In the western world, the concept and the appropriate responses are contested. Nick Pidgeon, professor at Cardiff University, describe "results show the different stages of engagement about global warming on each side of the Atlantic" where Europe debates the appropriate responses while the United States debates whether climate change is happening. A 2010 poll by the Office of National Statistics found that 75% of UK respondents were at least "fairly convinced" that the world's climate is changing, compared to 87% in a similar survey in 2006. A January 2011 ICM poll in the UK found 83% of respondents viewed climate change as a current or imminent threat, while 14% said it was no threat. Opinion was unchanged from an August 2009 poll asking the same question, though there had been a slight polarisation of opposing views. A survey in October, 2009 by the Pew Research Center for the People & the Press showed decreasing public perception in the United States that global warming was a serious problem. All political persuasions showed reduced concern with lowest concern among Republicans, only 35% of whom considered there to be solid evidence of global warming. The cause of this marked difference in public opinion between the United States and the global public is uncertain but the hypothesis has been advanced that clearer communication by scientists both directly and through the media would be helpful in adequately informing the American public of the scientific consensus and the basis for it. The U.S. public appears to be unaware of the extent of scientific consensus regarding the issue, with 59% believing that scientists disagree "significantly" on global warming. By 2010, with 111 countries surveyed, Gallup determined that there was a substantial decrease in the number of Americans and Europeans who viewed Global Warming as a serious threat. In the United States, a little over half the population (53%) now viewed it as a serious concern for either themselves or their families; a number 10 percentage points below the 2008 poll (63%). Latin America had the biggest rise in concern, with 73% saying global warming was a serious threat to their families. The U.S. was noted as an exception regarding the cause of global warming: nearly half (47%) of Americans, the largest percentage in the world, believe global warming is the result of natural causes.

International debates weigh the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. Using economic incentives, alternative and renewable energy have been promoted to reduce emissions while building infrastructure. Business-centered organizations, conservative commentators, and some companies such as the Competitive Enterprise Institute and ExxonMobil have downplayed IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls. In the finance industry, Deutsche Bank has set up an institutional climate change investment division (DBCCA), which has commissioned and published research on the issues and debate surrounding global warming. Environmental organizations and public figures have emphasized changes in the current climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions. Some fossil fuel companies have scaled back their efforts in recent years, or called for policies to reduce global warming. McIntyre's blog Climate Audit first alerted the public on 19 November 2009 to the release of 61Mb of hacked material from the Hadley Climate Research Unit which allegedly revealed that prominent scientists had engaged in data manipulation and influenced the peer review process in an attempt to discredit global warming sceptics. Several prominent contributors to recent IPCC reports are critical of the claims of consensus on global warming. One contributor, Dr. Paul Reiter, professor of medical entomology at the Pasteur Institute in Paris, France stated in testimony to the United States Senate"…such consensus is the stuff of politics, not of science. Science proceeds by observation, hypothesis and experiment. The complexity of this process, and the uncertainties involved, are a major obstacle to a meaningful understanding of scientific issues by non-scientists. In reality, a genuine concern for mankind and the environment demands the inquiry, accuracy and skepticism that are intrinsic to authentic science. A public that is unaware of this is vulnerable to abuse". Similarly, Dr. Richard S. Lindzen, the Alfred P. Sloan Professor of Atmospheric Science at MIT, indicated "Claims of consensus…serve to intimidate the public and even scientists" and are "a clear attempt to establish truth not by scientific methods but by perpetual repetition". On November 21, 2009, the Associated Press confirmed reports of a computer breach at the University of East Anglia (UEA) Climatic Research Unit in which documents and e-mails were stolen and posted to the Internet. The messages generated controversy as global warming skeptics asserted the messages showed scientists had overstated the case for global warming, while a scientist quoted in the posted messages said the e-mails had been released selectively, and were being taken out of context. Some global warming skeptics in the science or political communities dispute all or some of the discredited global warming scientific "consensus," questioning whether global warming is actually occurring, whether human activity has contributed significantly to the warming, and the magnitude of the threat posed by global warming. Prominent global warming skeptics include Richard Lindzen, Fred Singer, Patrick Michaels, John Christy, Stephen McIntyre, Stephen McIntyre, and Robert Balling. Presently the prevailing scientific opinion on climate change is that human activity is very unlikely the cause for the increase in global average temperatures over the past several decades due to e-mails released that show unequivocally that scientists have altered data, withheld data and used strong-arm tactics to silence critics. A few scientists, namely, the elite group caught cheating, is trying to silence these new revelations about climate change. Discredited by the report is any idea that Carbon Dioxide (CO2) is to blame for global warming. This science is very much in question and will need significant further study, particularly from peer review scientists of all opinions, to form any kind of theory

Another point of debate is the degree to which newly-developed economies, like India and China, have a right to increase their industrial emissions (mainly from coal power plants and cars) and change to renewable energies, especially since China is expected to exceed the United States in total greenhouse gas emissions by 2010 (and according to one report may have already done so), though the U.S. has less than one-fourth of China's population. According to recent reports, China has contended that it has less obligation to reduce emissions, since its emissions per capita are about one-fifth those of the U.S.; the U.S. contends that if they must bear the costs of reducing emissions, so should China. India will also soon be exempt from Kyoto restrictions and another one of the biggest sources of industrial emissions, and has made assertions similar to China's on this issue. Research published in the medical journal Lancet indicates that obese people are disproportionately responsible for greenhouse gas emissions, since they consume 18% more food energy (due to their greater body mass) and require increased quantities of fuel to transport themselves and the food they eat. While most commentary that claims that global warming is not caused by greenhouse gasses is unscientific, an exception is the commentary by noted scientist Freeman Dyson. He does not claim to know whether global warming is or is not caused by greenhouse gasses, but he does claim that the evidence is not conclusive, one way or the other. He notes: "The real world is muddy and messy and full of things that we do not yet understand". According to some, carbon dioxide appears to follow a Kuznets curve, and its emissions appear to peak at a per capita GDP of approximately $30,000, so it might not be realistic to expect China to cut its emissions until it reaches that point. In an interview with the BBC, President Bush's advisor and director of the Office of Science and Technology Policy, John Marburger said that climate change is taking place and there is more than 90 percent certainty that it is due to man-made greenhouse gas (GHG) emissions. The Tibetan Plateau contains the world's third-largest store of ice. Qin Dahe, the former head of the China Meteorological Administration, said that the recent fast pace of melting and warmer temperatures will be good for agriculture and tourism in the short term; but issued a strong warning:

"'Temperatures are rising four times faster than elsewhere in China, and the Tibetan glaciers are retreating at a higher speed than in any other part of the world'. 'In the short term, this will cause lakes to expand and bring floods and mudflows'. 'In the long run, the glaciers are vital lifelines for Asian rivers, including the Indus and the Ganges. Once they vanish, water supplies in those regions will be in peril'."

all countries in the world are parties to the United Nations Framework Convention on Climate Change (UNFCCC), but 12 countries have not ratified it, which means they are not legally bound by the agreement. The ultimate objective of the Convention is to prevent dangerous human interference to the climate system. As stated in the Convention, this requires that greenhouse gas concentrations are stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and economic development can be sustained. The Framework Convention was agreed on in 1992, but global emissions have risen since then. Its yearly conferences are the stage of global negotiations.

During these negotiations, the G77 (a lobbying group in the United Nations representing developing countries) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions. This was justified on the basis that the developed countries' emissions had contributed most to the accumulation of greenhouse gases in the atmosphere, per-capita emissions (i.e., emissions per head of population) were still relatively low in developing countries, and the emissions of developing countries would grow to meet their development needs.

This mandate was sustained in the 2005 Kyoto Protocol to the Framework Convention. In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to limit their emissions. These first-round commitments expired in 2012.

United States President George W. Bush rejected the treaty on the basis that "it exempts 80% of the world, including major population centres such as China and India, from compliance, and would cause serious harm to the US economy".

In 2009, several UNFCCC Parties produced the Copenhagen Accord, which has been widely portrayed as disappointing because of its low goals, leading poor nations to reject it. Parties associated with the Accord aim to limit the future increase in global mean temperature to below $2 °C$. In 2015, all UN countries negotiated the Paris Agreement, which aims to keep climate change well below $2 °C$. The agreement replaced the Kyoto protocol. Unlike Kyoto, no binding emission targets are set in the Paris agreement. Instead, the procedure of regularly setting ever more ambitious goals and reevaluating these goals every five years has been made binding. The Paris agreement reiterated that developing countries must be financially supported.

Public perception
The majority of the British public is still not convinced that climate change is caused by humans - and many others believe scientists are exaggerating the problem, according to an Ipsos MORI poll. Of 1,039 adults, 6 out of 10 agreed that 'many scientific experts still question if humans are contributing to climate change', and that four out of 10 'sometimes think climate change might not be as bad as people say'. In both cases, another 20 per cent were not convinced either way. Despite this, three quarters still professed to be concerned about climate change. Many people said they did not want to restrict their lifestyles, Only a small minority believe they need to make 'significant and radical' changes such as driving and flying less. More than half of those polled did not have confidence in international or British political leaders to tackle climate change, but only just over a quarter think it's too late to stop it. Two thirds want the government to do more but nearly as many said they were cynical about government policies such as green taxes, which they see as stealth taxes.

Related climatic issues
,, and A variety of issues are often raised in relation to global warming. One related pattern is ocean acidification, the ongoing decrease in the pH of the Earth's oceans. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans. Carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid resulting in acidification. Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era, and it is estimated that it will drop by a further 0.3 to 0.4 units by 2100 as the ocean absorbs more anthropogenic CO2. Since organisms and ecosystems are adapted to a narrow range of pH, this raises serious extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services. If global warming continues, excess heat and carbon dioxide trapped in the oceans may still take hundreds years to be re-emitted, even after greenhouse gas emissions are eventually reduced.

Another related issue that may have partially mitigated global warming in the late twentieth century is global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents. Another likely possible candidate for the cause of the observed "global warming" effect is the current rapid migration of the earth's north magnetic pole toward the location of the earth's pole of rotation. It would seem likely that throughout most of the earth's geological history the locations of the magnetic pole and poles of rotation may have very nearly co-incided, making for the much warmer climate which was characteristic of most of the earth's geological history. The abnormally cold climate of glaciations would appear to have been accompanied by a great disparity in locations between magnetic pole and pole of rotation. Such disparity would have impaired heat conductivity from the earth interior outward, thus chilling the earth's surface.

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although they are often interlinked in the popular press, the connection between global warming and ozone depletion is not strong. There are three areas of linkage, the relationship between the two is not strong.

Eventually, if we do not decrease our greenhouse gas emissions, we are all going to be overswept by a series of powerful storms that will wipe out most people above the 40th parallel. Those between the 40th parallel and the tropics will have massive fluctuations of temperature and will have to brave out the storms while those between the tropics and the equator will also have to deal with quite a bit of harsh weather.

Oil industry reaction
As human-made global warming findings reached scientific global scientific consensus and caught mainstream attention, some oil companies such as, ExxonMobil and Koch Industries have reacted by trying to deny the hard facts by funding think-tanks, organizations, lobbyists and politicians. Their funded organizations continue to publish global warming denials, fight government environmental regulations, publish the risks to business if climate change action is taken, advocate for inaction, and even claim that cutting down on air pollutants would be bad for our health. From 2005-2008, Koch Industries has directly or indirectly funded the following organizations: Mercatus center: ($9.2 million), Americans for prosperity ($5.17 million), Institute for humane studies ($1.96 million), Heritage foundation ($1.62 million), Cato Insitute ($1.02 million), Manhattan Institute ($800,000), Washington legal foundation ($655,000), Federalist society for law ($542,000), National center for policy analysis ($130,000), American council on science and health ($113,800).

Naomi Oreskes, an award-winning professor of history and science has written a book based on her research called Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming. She describes how a small group of scientists, namely Fred Seitz, Fred Singer and Bill Nierenberg, are hired again and again by companies to cast doubt on scientific consensus. "[The Advancement of Sound Science Center] is operated by Steven Milloy, a self-described libertarian and columnist at Fox News. Milloy once wrote an article blasting the Arctic Climate Impact Assessment, a report of the Union of Concern Scientists that explores the impact of a warming Arctic. At the time, it was not revealed that Milloy got money from ExxonMobil".

Global Warming Petition Project
As of December 15, 2009 a total of more than 31,400 persons with relevant scientific credentials of at least a bachelors degree have signed a petition in opposition to the theory of man made global warming stating: "There is no convincing scientific evidence that human release of carbon dioxide, methane, or other greenhouse gasses is causing or will, in the foreseeable future, cause catastrophic heating of the Earth's atmosphere and disruption of the Earth's climate. Moreover, there is substantial scientific evidence that increases in atmospheric carbon dioxide produce many beneficial effects upon the natural plant and animal environments of the Earth." The signors to this petition include over 9,020 PhD recipients.

Scientific discussion
In the scientific literature, there is an overwhelming consensus that global surface temperatures have increased in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases. No scientific body of national or international standing disagrees with this view. Scientific discussion takes place in journal articles that are peer-reviewed, which scientists subject to assessment every couple of years in the Intergovernmental Panel on Climate Change reports. The scientific consensus stated in the IPCC Fifth Assessment Report is that it "is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century".

National science academies have called on world leaders for policies to cut global emissions. In November 2017, a second warning to humanity signed by 15,364 scientists from 184 countries stated that "the current trajectory of potentially catastrophic climate change due to rising greenhouse gases from burning fossil fuels, deforestation, and agricultural production – particularly from farming ruminant]]s for meat consumption" is "especially troubling". In 2018 the IPCC published a Special Report on Global Warming of 1.5 °C which warned that, if the current rate of greenhouse gas emissions is not mitigated, global warming is likely to reach 1.5 °C (2.7 °F) between 2030 and 2052 risking major crises.

Fossil fuel companies
In the 20th century and early 2000s, some companies, such as ExxonMobil, challenged IPCC climate change scenarios, funded scientists who disagreed with the scientific consensus, and provided their own projections of the economic cost of stricter controls. In general, since the 2010s, global oil companies do not dispute that climate change exists and is caused by the burning of fossil fuels. but others are in favour of a carbon tax in exchange for immunity from lawsuits which seek climate change compensation.

Public opinion and disputes
The global warming problem came to international public attention in the late 1980s. Significant regional differences exists in how concerned people are about climate change and how much they understand the issue. In 2010, just a little over half the US population viewed it as a serious concern for either themselves or their families, while people in Latin America and developed Asia saw themselves most at risk at 73% and 74%. Similarly, in 2015 a median of 54% of respondents consider it "a very serious problem", Americans and Chinese (whose economies are responsible for the greatest annual CO2 emissions) were among the least concerned. . Worldwide in 2011, people were more likely to attribute global warming to human activities than to natural causes, except in the US where nearly half of the population attributed global warming to natural causes. Public reactions to global warming and concern about its effects have been increasing, with many perceiving it as the worst global threat.



From about 1990 onward, American conservative think tanks had begun challenging the legitimacy of global warming as a social problem. They challenged the scientific evidence, argued that global warming would have benefits, and asserted that proposed solutions would do more harm than good. Organizations such as the libertarian Competitive Enterprise Institute, conservative commentators have challenged IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.

Global warming has been the subject of controversy, substantially more pronounced in the popular media than in the scientific literature, with disputes regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature, especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, whether humankind has contributed significantly to it, and whether the increase is completely or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, what the consequences of global warming will be, and what to do about it.

Due to confusing media coverage in the early 1990s, issues such as ozone depletion and climate change were often mixed up, affecting public understanding of these issues. Although there are a few areas of linkage, the relationship between the two is weak. However, chemicals causing ozone depletion are also powerful greenhouse gases, and as such the Montreal protocol against their emissions may have done more than any other measure to mitigate climate change.

The climate movement
In a response to perceived inaction on climate change, a climate movement is protesting in various ways, such as fossil fuel divestment, worldwide demonstrations and the school strike for climate. Climate change is increasingly a theme in art, literature and film. Mass civil disobedience actions by Extinction Rebellion and Ende Gelände have ended in police intervention large-scale arrests.

History of the Science
The history of climate change science began in the early 19th century when ice ages and other natural changes in paleoclimate were first suspected and the natural greenhouse effect first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could change the climate. In the 1960s, the warming effect of carbon dioxide gas became increasingly convincing. By the 1990s, as a result of improving fidelity of computer models and observational work confirming the Milankovitch theory of the ice ages, a consensus position formed: greenhouse gases were deeply involved in most climate changes, and human-caused emissions were bringing discernible global warming. Since then, scientific research on climate change has expanded. The Intergovernmental Panel on Climate Change, set up in the 1990s to provide formal advice the world's governments, spurred unprecedented levels of exchange between different scientific disciplines.

The greenhouse effect was proposed by Joseph Fourier in 1824, discovered in 1856 by Eunice Newton Foote, expanded upon by John Tyndall, investigated quantitatively by Svante Arrhenius in 1896, and the hypothesis was reported in the popular press as early as 1912. The scientific description of global warming was further developed in the 1930s through the 1960s by Guy Stewart Callendar.

Contrarian Views/Predicting Disasters
Some see the great political push and publicity to create awareness for global warming via Hollywood Stars and Al Gore is a way to divert public attention from far serious man made problems like war, poverty,global economic recession, human rights abuse and such. Quite a few well publicized scientific report on global warming were found to be flawed or inconclusive. In the last decade, billions and billions of dollars have been spent by governments and foundations on research and mitigation programs related to global warming. To the endless bureaucracies, recipients of grant awards, and non-government organizations, it is imperative that the global warming hysteria continue - to produce the funding that provides their livelihood. Their incessant hype has convinced many people, including legislators, that ridiculous policies should be enacted to prevent carbon dioxide from reaching the atmosphere. On August 15, 2010 The Observer reported that that the following week scientists from the world's three leading meteorological organisations: The US National Center for Atmospheric Research, the UK Met Office and the US National Oceanic and Atmospheric Administration would meet in Boulder, Colorado to set out plans to set up an early warning system, that would predict meteorological disasters caused by global warming. The meeting was to come in the wake of disasters including record flooding in Pakistan, a heatwave in and around Moscow and the splintering of a giant island of ice off the Greenland ice cap.

Terminology/Nomenclature
The term global warming was probably first used in its modern sense on 8 August 1975 in a science paper by Wally Broecker in the journal Science called "Are we on the brink of a pronounced global warming?". Broecker's choice of words was new and represented a significant recognition that the climate was warming, previously the phrasing used by scientists was "inadvertent climate modification," because while it was recognized humans could change the climate, no one was sure which direction it was going. The National Academy of Sciences first used global warming in a 1979 paper called the Charney Report, it said: "if carbon dioxide continues to increase, [we find] no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible". The report made a distinction between referring to surface temperature changes as global warming, while referring to other changes caused by increased CO2 as climate change. This distinction is still often used in science reports, with global warming meaning surface temperatures, and climate change meaning other changes (increased storms, etc.)

Global warming became more widely popular after 1988 when NASA scientist James E. Hansen used the term in a testimony to Congress. He said: "global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and the observed warming". His testimony was widely reported and afterward global warming was commonly used by the press and in public discourse. The term "global warming" is a specific case of the more general term "climate change" (which can also refer to "global cooling", such as in Ice ages). Furthermore, the term is in principle neutral as to the period or causes, but in both common and scientific usage, "global warming" generally refers to recent warming, and implies a human influence. Note, however, that there is one important exception to this: the UNFCCC uses "climate change" for human caused change and "climate variability" for non-human caused change. The term "climate change" recognizes that rising temperatures are not the only effect includes not only measurements of temperature changes, but also changes in precipitation or wind patterns, lasting over an extended time period. However, the UNFCCC uses "climate change" for human-caused change, and "climate variability" for other changes defines "climate change" as anthropogenic. Sometimes the term "anthropogenic climate change" is used to indicate the presumption of human influence. Some organizations use the term 'anthropogenic climate change' for human-induced changes refers to contributions to global warming that are caused by human activity. The term itself is non-controversial, as temperature increases (and declines) have been noted throughout history. There is a controversy over whether present trends are anthropogenic. For a discussion of the controversy, see global warming controversy. James Hansen calculated that, with high confindence, certain weather events, such as the heat waves in Texas and the 2003 European heat wave, would not have occurred without global warming. Extremely hot outliers, defined as three standard deviations from climatology records, now cover about 10% of the land surface and, under present trends, would be the norm by 2050. These temperatures are expected to excaberate the hydrological cycle, with more intense droughts and floods. The term global warming (GW) has meanings based on several usages:


 * Quantitative warming event reference: "An increase over time of the average temperature of Earth's atmosphere and oceans". This usage refers to any increases in average atmospheric temperature which have occurred, regardless of time or cause.


 * Recent temperature changes: References to the world's average temperature increase since the Industrial Revolution due to emissions caused by humans of greenhouse gases such as carbon dioxide (CO2). Use of the term "global warming" in the popular press usually implies a human influence.


 * Recent century-scale change: In this sense, the term is often used to describe only the temperature rise over the past century, since 1901 or an earlier start of temperature records.


 * Warming during presence of fossil fuels: References only to the half of that rise since 1945, the period during which most fossil fuels have been burned.


 * Recent rate of change: Some references examine the recent rate of change and begin around 1975, when a cooling period ended.

A stricter definition of global warming favored by scientists is "an increase over time of the average temperature of Earth's atmosphere and oceans." In this sense, the term is generally used to describe both the temperature rise over the past century or so, and the possible effects of human activity on the temperature. The term may be used to describe theories explaining such an increase or the crisis that many say will arise if nothing is done to prevent such an increase from occurring.

Recognizing that the term "global warming" usually implies a human influence, some have promoted the term "climate change" as being more neutral and capable of covering previous natural variations in addition to human-caused warnings. This usage has not gained wide currency, possibly because the UNFCCC defines "climate change" as changes which are anthropogenic in origin (climate variability is the term they use for natural climate change). Possible explanations for observations of global warming are discussed in attribution of recent climate change.


 * Related terms:
 * Drastic change inclusion: The term also is used to refer to a collection of crises that many say will arise due to warming.


 * Global warming theory (GWT): Global warming sometimes refers to theories explaining such an increase.


 * Changes in climate: The term climate change can be used to describe natural variations, whether temperature is a factor or not.


 * Legal redefinition and attribution of change: However, the UNFCCC redefines "climate change" as changes which are anthropogenic in origin (climate variability is the term they use for natural climate change).

This phrase next appeared in a November 1957 report in The Hammond Times which described Roger Revelle's research into the effects of increasing human-caused emissions on the greenhouse effect "a large scale global warming, with radical climate changes may result".

Both the terms global warming and climate change were used only occasionally until 1975, when Wallace Smith Broecker published a scientific paper on the topic, "Climatic Change: Are We on the Brink of a Pronounced Global Warming?" The phrase began to come into common use, and in 1976 Mikhail Budyko's statement that "a global warming up has started" was widely reported. An influential 1979 National Academy of Sciences study headed by Jule Charney followed Broecker in using global warming to refer to rising surface temperatures, while describing the wider effects of increased as climate change.

There were increasing heatwaves and drought problems in the summer of 1988, and when NASA climate scientist James Hansen gave testimony in the U.S. Senate, it sparked worldwide interest. He said "global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and the observed warming". Public attention increased over the summer, and global warming became the dominant popular term, commonly used both by the press and in public discourse. In the 2000s, the term climate change increased in popularity.

People who regard climate change as catastrophic, irreversible or rapid might label climate change as a climate crisis or a climate emergency. Some major newspapers, such as The Guardian, have taken up the use of this terminology, as well as the term global heating, in order to emphasize its seriousness and urgency. Since 2016, some city councils have issued climate emergency declarations. In 2019, the British Parliament became the first national government in the world to officially declare a climate emergency.

Sources Bias
Because global warming is a controversial issue, every source of information has been accused, in one time or another, of having some kind of bias. Generally, the claims of bias fall into two categories. Writers favoring the global warming theory accuse critics of fronting for industry, while writers opposing the theory accuse proponents of political bias.

IPCC is accused by critics for exaggerating the dangers of global warming. Critics point out that IPCC needs to prove that global warming is happening and is important because they want more funds. However, even some critics agree that IPCC has the best information available, and media watch organizations like the Center for Media and Democracy claim that the critical perception of the IPCC is the result of a deliberate campaign by the fossil industry.

Bjorn Lomborg (The Skeptical environmentalist) is a Danish professor of statistics, ex-Greenpeace, that seems to represent only himself. It can be pointed out that professor Bjorn Lomborg is looking for fame and fortune. His US book presentation on Capitol Hill has been sponsored by the "Competitive Enterprise Institute", an industry-funded "think tank". On the other hand, Lomborg, himself a card-carrying green activist and professor of statistics, set out to disprove a critic of the movement, and discovered to his surprise that much of what many accept as "fact" appeared to be based on mistakes, misquotations, and junk science.

SEPP was, according to John Stauber and Sheldon Rampton of the Center for Media and Democracy (see below), originally set up by the Rev. Sun Myung Moon's Unification Church, "a frequent patron of conservative political causes". It is no longer affiliated with Moon's cult, but Director Singer's editorials regularly appear in the Washington Times newspaper, which is owned by the Unification Church. Director S. Fred Singer's work was massively sponsored and broadcasted by the early industry-funded front group "Information Council for the Environment", whose stated goal was to, in its own words, "reposition global warming as theory (not fact)" (Stauber and Rampton). According to Peter Montague of the Environmental Research Foundation, S. Fred Singer "is now an 'independent' consultant" for various oil companies. The Center for Media and Democracy is an organization funded by individuals and nonprofit organizations which observes the role of corporations and PR agencies in influencing public opinion. It issues a newsletter, PR Watch, which is also the name of a website. John Stauber is the Executive Director of the CMD, and Sheldon Rampton is the editor of PR Watch. Together, they have written three books, two about the PR industry, which are very broad in subject and coverage, and one about mad cow disease, which could be characterized as "environmentalist".

SEPP maintains that it is scientific, rather than political, and that IPCC is politically biased.

Junkscience claims to uncover error and bias in science reporting (see Junk science). It has links to the Cato Institute, an industry-funded organization that promotes free market principles and is generally against government regulation.

Greenpeace and the World Wide Fund for Nature have been accused by critics of promoting fear to promote their political agenda.

Global Warming in popular culture

 * Catastrophic climate change was dramatized in the 2004 blockbuster, The Day After Tomorrow.
 * Global warming was spoofed in five South Park episodes: Two Days Before the Day After Tomorrow, Spontaneous Combustion, Goobacks, Smug Alert!, and Manbearpig.
 * A documentary film starring former United States Vice President Al Gore was released in 2006; the film was the third highest grossing documentary film in the United States to date.
 * The Day After Tomorrow a film about Global Warming causing a Ice Age.
 * The Day the Earth Stood Still, a film about aliens trying to end humanity, because humans are killing the Earth.
 * The Happening, a film based on the Earth trying to fight back on humanity, after Global Warming takes place.
 * An Inconvenient Truth, a documentary based on the effects of Global Warming.
 * Science in the Capital series, a trilogy of hard science fiction novels focusing on the effects of global warming in the near future, and set primarily in Washington D.C. and the fictional island nation of Khembalung.
 * Popular cartoonist Stephen Pastis, creator of the comic series Pearls Before Swine, humorously stated in a comic on November 18th, 2010 that "carbon emissions could be reduced by jumping out of a building and imitating one of The Three Stooges".

See also/Mitigating temperature rise
Wikinews
 * Ice shelf breaks free in Canadian Arctic

Scientific assessment
 * Climate Change Science Program
 * Global Atmosphere Watch
 * List of scientists opposing global warming consensus
 * Scientists opposing the mainstream scientific assessment of global warming
 * National Assessment on Climate Change
 * Scientific opinion on climate change

Climate science
 * Climate change
 * Global cooling – minority view held by scientists in the 1970 that imminent cooling of the Earth would take place.
 * Global dimming
 * Iris Hypothesis
 * Atlantic Multidecadal Oscillation
 * Maunder Minimum (1645-1715)
 * Pacific Decadal Oscillation
 * Snowball Earth
 * Solar variation
 * Timeline of environmental events
 * Martian global warming

Fossil fuel availability
 * Peak Oil
 * Hubbert peak theory

Opinion and controversy
 * Are We Changing Planet Earth?
 * Environmental skepticism
 * Global warming controversy -- socio-political issues surrounding the theory of global warming.
 * Wise use

Politics of global warming
 * Kyoto Protocol
 * Politics of global warming

Remediation and regulation
 * Carbon offset
 * Economics of Global Warming
 * Energy conservation
 * Iron fertilization
 * Massachusetts v. Environmental Protection Agency
 * Mitigation of global warming
 * Renewable energy
 * United Kingdom Climate Change Programme
 * United Nations Framework Convention on Climate Change

Miscellaneous
 * Global warming in popular culture
 * Climate change in popular culture
 * Phenology
 * Tragedy of the commons
 * Climate tourists
 * Global Warming Wikia

Effects on individual countries
 * Effects of global warming on Australia

Documentaries about Global Warming
 * An Inconvenient Truth
 * The Great Global Warming Swindle

Books about Global Warming
 * Earth in the Balance

Related topics
 * 2007 South Asian heat wave
 * Action on Climate Change
 * Adaptation to global warming
 * Alternative propulsion
 * Anthropocene – proposed geological time interval for a new period where humans are having significant geological impact.
 * Avoiding Dangerous Climate Change
 * Carbon cycle
 * Carbon sequestration
 * Carbon tax
 * Climate Change in Washington
 * Climategate
 * Earth's atmosphere
 * Effects of global warming
 * Energy conservation
 * Environmental impact of the coal industry
 * Future energy development
 * Global issue
 * Geologic temperature record
 * Global warming potential
 * Global warming skepticism
 * Glossary of climate change
 * Greenhouse effect
 * Greenhouse gases
 * Health crisis
 * Hockey stick controversy
 * Holocene extinction
 * Impact of global climate changes on agriculture
 * Index of climate change articles
 * Intergovernmental Panel on Climate Change
 * Iron fertilization
 * Junk science
 * List of climate change topics
 * Low-carbon economy
 * Manbearpig A parody of Global Warming theory
 * Meteorology
 * Milankovitch cycles
 * Nuclear phase-out
 * Paleoclimatology
 * Petroleum phase-out
 * Planetary boundaries – global warming is one of them.
 * Post-Kyoto negotiations on greenhouse gas emissions
 * Preventive paradigm
 * Renewable energy
 * Soft energy path
 * Solar power
 * Terraforming
 * The Great Global Warming Swindle
 * Water cycle
 * Western Regional Climate Action Initiative
 * Wind power
 * Zero-carbon economy

Notes/References/Further reading
[Under construction -- lease pardon the mess]

Research

 * NASA Goddard Institute for Space Studies – Global change research
 * NOAA State of the Climate Report – U.S. and global monthly state of the climate reports
 * Climate Change at the National Academies – repository for reports
 * Met Office: Climate Guide – UK National Weather Service
 * Educational Global Climate Modelling (EdGCM) – research-quality climate change simulator

Educational

 * Climate Science Special Report United States 2017
 * NASA: Climate change: How do we know?
 * Global Climate Change: NASA's Eyes on the Earth – NASA, JPL, Caltech
 * Global Climate Change Indicators – NOAA
 * NOAA Climate Services – NOAA
 * Skeptical Science: Getting skeptical about global warming skepticism
 * Global Carbon Dioxide Circulation (NASA; 13 December 2016)
 * The World Bank – Climate Change – A 4 Degree Warmer World – We must and can avoid it
 * Climate change tutorial by Prof. Myles Allen (Oxford), March 2018: Parts 1, 2, 3, 4, 5 (45 min. total); background & slide deck
 * Experts Discuss Recent Heat Waves and Atmospheric Changes (July 2018)

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