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Global mean surface temperature difference relative to the 1961–1990 average

Comparison of ground based (blue) and satellite based (red: UAH; green: RSS) records of temperature variations since 1979. Trends plotted since January 1982.

Mean surface temperature change for the period 2000 to 2009 relative to the average temperatures from 1951 to 1980.^[1]

Global warming is the hypothesized increase in the average temperature of Earth's near-surface air and oceans since the mid-20th century and its projected continuation. According to a report by the Intergovernmental Panel on Climate Change (IPCC) global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) between the start and the end of the 20th century.^[2]^[A] Others dispute whether the temperature has in fact increased, or note errors in the observing network.^[3]^[4] The (IPCC) concludes that most of the observed temperature increase since the middle of the 20th century was very likely caused by increasing concentrations of greenhouse gases which they believe result from human activity such as fossil fuel burning and deforestation.

Climate model projections summarized in the latest IPCC report indicate that the global surface temperature is likely to rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.^[2] The uncertainty in this estimate arises from the use of models with differing sensitivity to greenhouse gas concentrations and the use of differing estimates of future greenhouse gas emissions. Most studies focus on the period leading up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere.^[5]^[6]

An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts.^[7] Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects include changes in the frequency and intensity of extreme weather events, species extinctions, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional variations is uncertain.^[8]

Political, public, and scientific debate continues regarding global warming, its causes and what, if any, actions to take in response. The available options include mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions. Some argue that no action should be taken because of the scientific uncertainty surrounding the issue.^[9]

Temperature changes

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.^[10]^[11]^[12]^[13]^[14] The most common measure of global warming is the trend in globally averaged temperature near the Earth's surface. Expressed as a linear trend, this temperature 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.^[15] Temperatures in the lower troposphere have increased between 0.13 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.^[16]

Estimates by NASA's Goddard Institute for Space Studies and the National Climatic Data Center show that 2005 was 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.^[17]^[18] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit show 2005 as the second warmest year, behind 1998.^[19]^[20] Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.^[21] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.^[22]^[23]

Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).^[24] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.^[25] The Northern Hemisphere warms faster than the Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.^[26]

The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. 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.^[27]

External forcings

External forcing refers to processes external to the climate system (though not necessarily external to Earth) that influence climate. Climate responds to several types of external forcing, such as radiative forcing due to changes in atmospheric composition (mainly greenhouse gas concentrations), changes in solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.^[28] Attribution of recent climate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens of thousands of years and thus are too gradual to have caused the temperature changes observed in the past century.

Greenhouse gases

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 proposed by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.^[29] Some scientists dispute the existence of the greenhouse effect; for example, Khabibullo Abdusamatov, head of the space research laboratory at the Saint Petersburg-based Pulkovo Observatory^[30]^[31] has stated that "ascribing 'greenhouse' effect properties to the Earth's atmosphere is not scientifically substantiated."^[32] The question in terms of global warming is how the strength of the presumed greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.

Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F).^[33]^[C] The major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect; carbon dioxide (CO₂), which causes 9–26 percent; methane (CH₄), which causes 4–9 percent; and ozone (O₃), which causes 3–7 percent.^[34]^[35]^[36] Clouds also affect the radiation balance, but they are composed of liquid water or ice and so have different effects on radiation from water vapor.

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. The concentrations of CO₂ and methane have increased by 36% and 148% respectively since 1750.^[37] These levels are much higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores.^[38]^[39]^[40] Less direct geological evidence indicates that CO₂ values higher than this were last seen about 20 million years ago.^[41] Fossil fuel burning has produced about three-quarters of the increase in CO₂ from human activity over the past 20 years. Most of the rest is due to land-use change, particularly deforestation.^[42]

CO₂ concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic, sociological, technological, and natural developments. Accordingly, 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 (an increase by 90-250% since 1750).^[43] Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.^[44]

The destruction of stratospheric ozone by chlorofluorocarbons is sometimes mentioned in relation to global warming. Although there are a few areas of linkage, the relationship between the two is not strong. Reduction of stratospheric ozone has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.^[45] Ozone in the troposphere (the lowest part of the Earth's atmosphere) does contribute to surface warming.^[46]

Aerosols and soot

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.^[47] The main cause of this dimming is aerosols produced by volcanoes and pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. James E. Hansen and colleagues have proposed that 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 driven mainly by non-CO₂ greenhouse gases.^[48]

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.^[49] 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.^[50] 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.^[51]

Soot may cool or warm, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated 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.^[52] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.^[53] The influences of aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.^[54]

Solar variation

Variations in solar output have been the cause of past climate changes.^[55] The consensus among climate scientists is that changes in solar forcing probably had a slight cooling effect in recent decades. This result is less certain than some others, with a few papers suggesting a warming effect.^[28]^[56]^[57]^[58]

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.^[28] Observations show that temperatures in the stratosphere have been 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.^[59]

A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.^[60] Other research has found no relation between warming in recent decades and cosmic rays.^[61]^[62] A recent study 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.^[63]

Feedback

Feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback amplifies the change in the first quantity while negative feedback reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, 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. Imperfect understanding of feedbacks is a major cause of uncertainty and concern about global warming.

Climate models

The main tools for projecting future climate changes are mathematical models 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. For exampleTim Ball, former professor at the University of Winnipeg, has stated that climate models essentially ignore the role of water vapor as a greenhouse gas.^[64]

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.^[65] Warming due to increasing levels of greenhouse gases is not an assumption of the models; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes.^[66] 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.^[67]

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). In addition to human-caused emissions, some models also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.^[68]^[69]^[70] Including uncertainties in future greenhouse gas concentrations and climate sensitivity, 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.^[2]

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.^[28]

The physical realism of models is tested by examining their ability to simulate current or past climates.^[71] 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.^[42] 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.^[72]

Attributed and expected effects

Global warming may be detected in natural, ecological or social systems as a change having statistical significance.^[73] Attribution of these changes e.g., to natural or human activities, is the next step following detection.^[74]

Natural systems

Global warming has been detected in a number of systems. 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.^[14] Most of the increase in global average temperature since the mid-20^th century is, with high probability,^[D] atttributable to human-induced changes in greenhouse gas concentrations.^[75]

Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades.^[76] Over the course of the 21^st century, increases in emissions at or above their current rate would very likely induce changes in the climate system larger than those observed in the 20^th 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 21^st 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. Over the course of centuries to millennia, the melting of ice sheets could result in sea level rise of 4–6 m or more.^[77]

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.^[76] Snow cover area and sea ice extent are expected to decrease. The frequency of hot extremes, heat waves and heavy precipitation will very likely increase.

Ecological systems

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.^[14] Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs.^[76] It is expected that most ecosystems will be affected by higher atmospheric CO₂ levels, combined with higher global temperatures.^[78] Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.^[79]

Social systems

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.^[14] Future climate change is expected to particularly affect some sectors and systems related to human activities.^[76] These include:

Water resources in some dry regions at mid-latitudes, the dry tropics, and areas that depend on snow and ice melt
Agriculture in low latitudes
Low-lying coastal systems
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.

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.^[80] Many countries, both developing and developed, are aiming to use cleaner, less polluting, technologies.^[81] 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.^[82]

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.^[83] The ability to adapt (called "adaptive capacity") is closely linked to social and economic development.^[82] 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.

Another policy response is engineering of the climate (geoengineering). This policy response is sometimes grouped together with mitigation.^[84] Geoengineering is largely unproven, and reliable cost estimates for it have not yet been published.^[85]

UNFCCC

Most countries are Parties to the United Nations Framework Convention on Climate Change (UNFCCC).^[86] The ultimate objective of Convention is the prevent "dangerous" human interference of the climate system.^[87] 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;
economic development can proceed in a sustainable fashion.

The UNFCCC recognizes differences among countries in their responsibility to act on climate change.^[88] 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.^[89] Policy measures taken in response to these commitments have reduced emissions.^[90] For many developing (non-Annex I) countries, reducing poverty is their overriding aim.^[91]

At the 15^th UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC Parties produced the Copenhagen Accord.^[92] Parties agreeing with the Accord aim to limit the future increase in global mean temperature to below 2 °C.^[93]

Views on global warming

There are different views over what the appropriate policy response to climate change should be.^[94]^[95] These competing views weigh the benefits of limiting emissions of greenhouse gases against the costs. In general, it seems likely that climate change will impose greater damages and risks in poorer regions.^[96]

Politics

Developing and developed countries have made different arguments over who should bear the burden of costs for cutting emissions. Developing countries often concentrate on per capita emissions, that is, the total emissions of a country divided by its population.^[97] Per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries.^[98] 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.^[96] On the other hand, commentators from developed countries more often point out that it is total emissions that matter.^[96] In 2008, developing countries made up around half of the world's total emissions of CO₂ from cement production and fossil fuel use.^[99]

The Kyoto Protocol, which came into force in 2005, sets legally binding emission limitations for most developed countries.^[89] Developing countries are not subject to limitations. This exemption led the U.S. (under President George W. Bush) and a previous Australian Government to decide not to ratify the treaty.^[100]^[101] At the time, almost all world leaders expressed their disappointment over President Bush's decision.^[101] Australia has since ratified the Kyoto protocol.^[102]

Public opinion

In 2007–2008 Gallup Polls surveyed 127 countries. Over a third of the world's population was unaware of global warming, with people in developing countries less aware than those in developed, and those in Africa the least aware. 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 belief.^[103] In the Western world, opinions over the concept and the appropriate responses are divided. Nick Pidgeon of Cardiff University finds that "results show the different stages of engagement^{[clarification needed]} 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.^[104]^[105]^[vague]^{[dubious – discuss]}

Other views

Most scientists accept that humans are contributing to observed climate change.^[106]^[107] National science academies have called on world leaders for policies to cut global emissions.^[108] There are, however, some scientists and non-scientists who question aspects of climate change science.^[109]^[110] For example, meteorologist Prof. Richard Lindzen doubts whether humans are responsible for recently observed warming.^[111]

Organizations such as the libertarian Competitive Enterprise Institute, conservative commentators, and companies such as ExxonMobil 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.^[112]^[113]^[114]^[115] 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.^[116] Some fossil fuel companies have scaled back their efforts in recent years,^[117] or called for policies to reduce global warming.^[118]

Notes

^
Increase is for years 1905 to 2005. Global surface temperature is defined in the IPCC Fourth Assessment Report as the average of near-surface air temperature over land and sea surface temperature. These error bounds are constructed with a 90% confidence interval.
^
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. The 2005 statement added Japan, Russia, and the U.S. The 2007 statement added Mexico and South Africa. The Network of African Science Academies, and the Polish Academy of Sciences have issued separate statements. 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.
^
Note that the greenhouse effect produces an average worldwide temperature increase of about 33 °C (59 °F) compared to black body predictions without the greenhouse effect, not an average surface temperature of 33 °C (91 °F). The average worldwide surface temperature is about 14 °C (57 °F).
^
In the IPCC Fourth Assessment Report, published in 2007, this attribution is given a probability of greater than 90%, based on expert judgement.^[119] According to the US National Research Council Report – Understanding and Responding to Climate Change - published in 2008, "[most] scientists agree that the warming in recent decades has been caused primarily by human activities that have increased the amount of greenhouse gases in the atmosphere."^[106]

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^ IPCC (2007). C. Mitigation in the short and medium term (until 2030). In (book section): Summary for Policymakers. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)). Print version: Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A.. This version: IPCC website. ISBN 9780521880114. Retrieved 2010-05-15.
^ UNFCCC (n.d.). "Essential Background". UNFCCC website. Retrieved 2010-05-18.
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^ ^a ^b Liverman, D.M. (2008). "Conventions of climate change: constructions of danger and the dispossession of the atmosphere" (PDF). Journal of Historical Geography. 35: 12–14. doi:10.1016/j.jhg.2008.08.008. Retrieved 2009-08-08.
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^ UNFCCC (2005). "Sixth compilation and synthesis of initial national communications from Parties not included in Annex I to the Convention. Note by the secretariat. Executive summary. Document code: FCCC/SBI/2005/18". United Nations Office at Geneva, Switzerland. p. 6. Retrieved 2010-05-17. {{cite web}}: Unknown parameter |day= ignored (help); Unknown parameter |month= ignored (help)
^ Müller, B. (2010). Copenhagen 2009: Failure or final wake-up call for our leaders? EV 49 (PDF). Dr Benito Müller's web page on the Oxford Institute for Energy Studies website. p. i. ISBN 978190755046. Retrieved 2010-05-18. {{cite book}}: Check |isbn= value: length (help); Unknown parameter |month= ignored (help)
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Cite error: A list-defined reference named "Ammann-2007" is not used in the content (see the help page).

Cite error: A list-defined reference named "joint-academies" is not used in the content (see the help page).

External links

Template:Wikipedia-Books

Research

Intergovernmental Panel on Climate Change — collection of IPCC reports
Climate Change at the National Academies — repository for reports, workshops, and meetings
Nature Reports Climate Change — free-access web resource
Met Office: Climate change — UK National Weather Service
Global Science and Technology Sources on the Internet — extensive commented list of internet resources
Educational Global Climate Modelling (EdGCM) — research-quality climate change simulator
DISCOVER — satellite-based ocean and climate data since 1979 from NASA
Global Warming Art — collection of figures and images

Educational

What Is Global Warming? — by National Geographic
Global Warming Frequently Asked Questions — from NOAA
Understanding Climate Change – Frequently Asked Questions — from UCAR
Global Climate Change: NASA's Eyes on the Earth — from NASA's JPL and Caltech
OurWorld 2.0 — from the United Nations University
Pew Center on Global Climate Change — business and politics
Best Effort Global Warming Trajectories – Wolfram Demonstrations Project — by Harvey Lam
Koshland Science Museum – Global Warming Facts and Our Future — graphical introduction from National Academy of Sciences
The Discovery of Global Warming – A History — by Spencer R. Weart from The American Institute of Physics
Climate Change: Coral Reefs on the Edge — A video presentation by Prof. Ove Hoegh-Guldberg, University of Auckland
Climate Change Indicators in the United States Report by United States Environmental Protection Agency, 80 pp.
Global Warming (The Warming of the Earth)—A beginner's guide to understanding the issue of global warming—by the Woods Hole Research Center

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[cnote_A] 
Increase is for years 1905 to 2005. Global surface temperature is defined in the IPCC Fourth Assessment Report as the average of near-surface air temperature over land and sea surface temperature. These error bounds are constructed with a 90% confidence interval.

[cnote_B] 
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. The 2005 statement added Japan, Russia, and the U.S. The 2007 statement added Mexico and South Africa. The Network of African Science Academies, and the Polish Academy of Sciences have issued separate statements. 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.

[cnote_C] 
Note that the greenhouse effect produces an average worldwide temperature increase of about 33 °C (59 °F) compared to black body predictions without the greenhouse effect, not an average surface temperature of 33 °C (91 °F). The average worldwide surface temperature is about 14 °C (57 °F).

[cnote_D] 
In the IPCC Fourth Assessment Report, published in 2007, this attribution is given a probability of greater than 90%, based on expert judgement.^[119] According to the US National Research Council Report – Understanding and Responding to Climate Change - published in 2008, "[most] scientists agree that the warming in recent decades has been caused primarily by human activities that have increased the amount of greenhouse gases in the atmosphere."^[106]

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[95] Banuri, T.; et al. (1996). Equity and Social Considerations. In: Climate Change 1995: Economic and Social Dimensions of Climate Change. Contribution of Working Group III to the Second Assessment Report of the Intergovernmental Panel on Climate Change (J.P. Bruce et al. Eds.). Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A. p. 87. doi:10.2277/0521568544. ISBN 9780521568548. {{cite book}}: Explicit use of et al. in: |author= (help)

[banuri-96] Banuri et al., 1996, p. 83

[97] Banuri et al., 1996, pp. 94-95

[98] Grubb, M. (July–September 2003). "The Economics of the Kyoto Protocol" (PDF). World Economics. 4 (3): 144. Retrieved 2010-03-25.{{cite journal}}: CS1 maint: date format (link)

[99] PBL (2009). "Global CO₂ emissions: annual increase halves in 2008". Netherlands Environmental Assessment Agency (PBL) website. Retrieved 2010-05-05. {{cite web}}: Unknown parameter |day= ignored (help); Unknown parameter |month= ignored (help)

[100] IEA (2005). "Energy Policies of IEA Countries - Australia- 2005 Review". International Energy Agency (IEA), Head of Publications Service, 9 rue de la Fédération, 75739 Paris Cedex 15, France. p. 51. Retrieved 2010-04-29. {{cite web}}: line feed character in |publisher= at position 65 (help)

[dessai-101] Dessai, S. (2001). "The climate regime from The Hague to Marrakech: Saving or sinking the Kyoto Protocol? Tyndall Centre Working Paper 12". Tyndall Centre website. pp. 5–6. Retrieved 2010-05-05.

[102] UNFCCC (2009). "Report of the in-depth review of the fourth national assessment communication of Australia". United Nations Office at Geneva, Switzerland. p. 3. Retrieved 2010-05-05. {{cite web}}: Unknown parameter |day= ignored (help); Unknown parameter |month= ignored (help)

[103] Pelham, Brett (2009-04-22). "Awareness, Opinions About Global Warming Vary Worldwide". Gallup. Retrieved 2009-07-14.

[pewpoll-104] "Summary of Findings". Little Consensus on Global Warming. Partisanship Drives Opinion. Pew Research Center. 2006-07-12. Retrieved 2007-04-14.

[105] More in Europe worry about climate than in U.S., poll shows - Health & Science - International Herald Tribune

[nrc2008-106] NRC, 2008, p. 2

[107] Wallace, D. and J. Houghton (2005). "A guide to facts and fictions about climate change". UK Royal Society website. pp. 3–4. Retrieved 2010-05-05. {{cite web}}: Unknown parameter |month= ignored (help)

[108] Academia Brasileira de Ciéncias (Brazil), Royal Society of Canada, Chinese Academy of Sciences, Académie des Sciences (France), Deutsche Akademie der Naturforscher Leopoldina (Germany), Indian National Science Academy, Accademia Nazionale dei Lincei (Italy), Science Council of Japan, Academia Mexicana de Ciencias, Russian Academy of Sciences, Academy of Science of South Africa, Royal Society (United Kingdom), National Academy of Sciences (United States of America) (2009). "G8+5 Academies' joint statement: Climate change and the transformation of energy technologies for a low carbon future" (PDF). US National Academies website. Retrieved 2010-05-05. {{cite web}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[109] Weart, S. (2009). "The Public and Climate Change (cont. – since 1980). Section: After 1988". American Institute of Physics website. Retrieved 2010-05-05. {{cite web}}: Unknown parameter |month= ignored (help)

[110] SEPP (n.d.). "Frequently Asked Questions About Climate Change". Science & Environmental Policy Project (SEPP) website. Retrieved 2010-05-05.

[111] Lindzen, R.S. (2005). "Memorandum by Professor Richard S Lindzen, Massachusetts Institute of Technology. Minutes of evidence, Tuesday, 25 January 2005. In: The Economics of Climate Change. A report by the UK Parliament House of Lords Select Committee on Economic Affairs. Second report of the 2005-06 session". UK Parliament website. Retrieved 2010-05-05. {{cite web}}: Unknown parameter |day= ignored (help); Unknown parameter |month= ignored (help)

[112] Begley, Sharon (2007-08-13). "The Truth About Denial". Newsweek. Retrieved 2007-08-13.

[113] Adams, David (2006-09-20). "Royal Society tells Exxon: stop funding climate change denial". The Guardian. Retrieved 2007-08-09.

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[115] Sandell, Clayton (2007-01-03). "Report: Big Money Confusing Public on Global Warming". ABC. Retrieved 2007-04-27.

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[118] "Global Warming Resolutions at U.S. Oil Companies Bring Policy Commitments from Leaders, and Record High Votes at Laggards" (Press release). Ceres. May 13, 2004. Retrieved 2010-03-04.

[119] IPCC (2007d). "Introduction. In (section): Synthesis Report. In (book): Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Core Writing Team, Pachauri, R.K and Reisinger, A. (eds.))". Book version: IPCC, Geneva, Switzerland. This version: IPCC website. Retrieved 2010-04-26.

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@@ Line 5: / Line 5: @@
 {{multiple image|align=right|direction=vertical|width=220|image1=Instrumental Temperature Record.png|caption1=Global mean surface temperature difference relative to the 1961–1990 average|image2=Satellite Temperatures.png|caption2=Comparison of ground based (blue) and satellite based (red: [[University of Alabama in Huntsville|UAH]]; green: [[Remote Sensing Systems|RSS]]) records of temperature variations since 1979. Trends plotted since January 1982.|image3=GISS temperature 2000-09 lrg.png|image4=GISS temperature palette.png|caption4=Mean surface temperature change for the period 2000 to 2009 relative to the average temperatures from 1951 to 1980.<ref>[http://earthobservatory.nasa.gov/IOTD/view.php?id=42392 2009 Ends Warmest Decade on Record]. [[NASA]] Earth Observatory Image of the Day, January 22, 2010.</ref>}}
-'''Global warming''' is the increase in the [[Instrumental temperature record|average temperature]] of [[Earth]]'s near-surface air and oceans since the mid-[[20th century]] and its projected continuation. Global surface temperature increased 0.74&nbsp;±&nbsp;0.18&nbsp;°[[Celsius|C]] (1.33&nbsp;±&nbsp;0.32&nbsp;°[[Fahrenheit|F]]) between the start and the end of the 20th century.<ref name="grida7"/>{{Cref2|A}} The [[Intergovernmental Panel on Climate Change]] (IPCC) concludes that most of the observed temperature increase since the middle of the 20th century was very likely [[attribution of recent climate change|caused]] by increasing concentrations of [[greenhouse gas]]es resulting from [[anthropogenic|human activity]] such as [[fossil fuel]] burning and [[deforestation]].<ref name="grida7"/> The IPCC also concludes that variations in natural phenomena such as [[solar variation|solar radiation]] and [[Volcano#Effects_of_volcanoes|volcanic eruptions]] had a small cooling effect after 1950.<ref name=Hegerl-2001/><ref name="Ammann-2007"/> These basic conclusions have been [[Scientific opinion on climate change|endorsed by more than 40 scientific societies and academies of science]],{{Cref2|B}} including all of the [[National academy|national academies of science]] of the [[G8|major industrialized countries]].<ref name="joint-academies"/>
+'''Global warming''' is the hypothesized increase in the [[Instrumental temperature record|average temperature]] of [[Earth]]'s near-surface air and oceans since the mid-[[20th century]] and its projected continuation. According to a report by the [[Intergovernmental Panel on Climate Change]] (IPCC) global surface temperature increased 0.74&nbsp;±&nbsp;0.18&nbsp;°[[Celsius|C]] (1.33&nbsp;±&nbsp;0.32&nbsp;°[[Fahrenheit|F]]) between the start and the end of the 20th century.<ref name="grida7"/>{{Cref2|A}} Others dispute whether the temperature has in fact increased, or note errors in the observing network.<ref>[http://www.canadafreepress.com/index.php/article/13570 Dangerous Deception?]</ref><ref>[http://www.heartland.org/books/SurfaceStations.html Watts, Anthony, 2009: Is the U.S. Surface Temperature Record Reliable?] The Heartland Institute, Chicago. 29 pp.</ref> The (IPCC) concludes that most of the observed temperature increase since the middle of the 20th century was very likely [[attribution of recent climate change|caused]] by increasing concentrations of [[greenhouse gas]]es which they believe result from [[anthropogenic|human activity]] such as [[fossil fuel]] burning and [[deforestation]].
 [[Climate model]] projections summarized in the latest IPCC report indicate that the global [[surface temperature]] is likely to rise a further {{nowrap|1.1 to 6.4&nbsp;°C}} {{nowrap|(2.0 to 11.5&nbsp;°F)}} during the 21st century.<ref name="grida7"/> The uncertainty in this estimate arises from the use of models with differing [[climate sensitivity|sensitivity to greenhouse gas concentrations]] and the use of differing [[Special Report on Emissions Scenarios|estimates of future greenhouse gas emissions]]. Most studies focus on the period leading up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large [[heat capacity]] of the oceans and the long lifetime of [[carbon dioxide]] in the atmosphere.<ref name="Archer-2005"/><ref name=solomon-2009/>
@@ Line 11: / Line 11: @@
 An increase in global temperature will cause [[sea level rise|sea levels to rise]] and will change the amount and pattern of [[precipitation (meteorology)|precipitation]], probably including expansion of [[Subtropics|subtropical]] [[desert]]s.<ref name="Lu-2007"/> Warming is expected to be [[Arctic shrinkage|strongest in the Arctic]] and would be associated with continuing [[retreat of glaciers since 1850|retreat of glaciers]], [[permafrost]] and [[sea ice]]. Other likely effects include changes in the frequency and intensity of [[extreme weather]] events, [[extinction risk from climate change|species extinctions]], and changes in [[agricultural yield]]s. Warming and related changes will vary from region to region around the globe, though the nature of these regional variations is uncertain.<ref name="IPCCSynthesisReport2007" />
-[[Politics of global warming|Political]] and [[global warming controversy|public debate]] continues regarding global warming, its causes and what actions to take in response. The available options are [[Mitigation of global warming|mitigation]] to reduce further emissions; [[Adaptation to global warming|adaptation]] to reduce the damage caused by warming; and, more speculatively, [[geoengineering]] to reverse global warming. [[List of Kyoto Protocol signatories|Most national governments]] have signed and ratified the [[Kyoto Protocol]] aimed at reducing [[greenhouse gas]] emissions.
+[[Politics of global warming|Political]], [[global warming controversy|public, and scientific debate]] continues regarding global warming, its causes and what, if any, actions to take in response. The available options include [[Mitigation of global warming|mitigation]] to reduce further emissions; [[Adaptation to global warming|adaptation]] to reduce the damage caused by warming; and, more speculatively, [[geoengineering]] to reverse global warming. [[List of Kyoto Protocol signatories|Most national governments]] have signed and ratified the [[Kyoto Protocol]] aimed at reducing [[greenhouse gas]] emissions. Some argue that no action should be taken because of the scientific uncertainty surrounding the issue.<ref>[http://www.washingtontimes.com/news/2009/dec/01/inside-the-beltway-41029681/print/ "Inside the Beltway," The Washington Times, December 1, 2009 ]</ref>
 ==Temperature changes==
@@ Line 74: / Line 74: @@
 {{Double image stack|right|Greenhouse Effect.svg|Mauna Loa Carbon Dioxide-en.svg|200|Greenhouse effect schematic showing energy flows between space, the atmosphere, and earth's surface. Energy exchanges are expressed in watts per square meter (W/m<sup>2</sup>).|Recent atmospheric [[carbon dioxide]] (CO<sub>2</sub>) increases. Monthly CO<sub>2</sub> measurements display seasonal oscillations in overall yearly uptrend; each year's maximum occurs during the [[Northern Hemisphere]]'s late spring, and declines during its growing season as plants remove some atmospheric CO<sub>2</sub>.}}
-The greenhouse effect is the process by which [[Absorption (electromagnetic radiation)|absorption]] and [[Emission (electromagnetic radiation)|emission]] of [[infrared]] radiation by gases in the [[atmosphere]] 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.<ref>{{Cite web|title=The Carbon Dioxide Greenhouse Effect|work=The Discovery of Global Warming|url=http://www.aip.org/history/climate/co2.htm|accessdate=21 April 2009|year=2008|publisher=American Institute of Physics|first=Spencer | last=Weart | authorlink=Spencer Weart}}</ref> Existence of the greenhouse effect as such is not disputed, even by those who do not agree that the recent temperature increase is attributable to human activity. The question is instead how the strength of the greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.
+The greenhouse effect is the process by which [[Absorption (electromagnetic radiation)|absorption]] and [[Emission (electromagnetic radiation)|emission]] of [[infrared]] radiation by gases in the [[atmosphere]] are purported to warm a [[planet]]'s lower atmosphere and surface. It was proposed by [[Joseph Fourier]] in 1824 and was first investigated quantitatively by [[Svante Arrhenius]] in 1896.<ref>{{Cite web|title=The Carbon Dioxide Greenhouse Effect|work=The Discovery of Global Warming|url=http://www.aip.org/history/climate/co2.htm|accessdate=21 April 2009|year=2008|publisher=American Institute of Physics|first=Spencer | last=Weart | authorlink=Spencer Weart}}</ref> Some scientists dispute the existence of the greenhouse effect; for example, [[Khabibullo Abdusamatov]], head of the space research laboratory at the [[Saint Petersburg]]-based [[Pulkovo Observatory]]<ref>[http://www.gao.spb.ru/english/ link]</ref><ref>[http://www.encspb.ru/en/bigimage.php?kod=2803981268 photo]</ref> has stated that "ascribing 'greenhouse' effect properties to the Earth's atmosphere is not scientifically substantiated."<ref>[http://www.canada.com/nationalpost/story.html?id=edae9952-3c3e-47ba-913f-7359a5c7f723&k=0</ref> The question in terms of global warming is how the strength of the presumed greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.
 Naturally occurring greenhouse gases have a mean warming effect of about 33&nbsp;°C (59&nbsp;°F).<ref name="IPCC_WG1_AR4_Ch1">{{Cite web|author=IPCC|work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change|title=Chapter 1: Historical Overview of Climate Change Science|url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch01.pdf|format=PDF|accessdate=21 April 2009|year=2007|publisher=IPCC|quote=To emit 240 W m–2, a surface would have to have a temperature of around −19&nbsp;°C. This is much colder than the conditions that actually exist at the Earth’s surface (the global mean surface temperature is about 14&nbsp;°C). Instead, the necessary −19&nbsp;°C is found at an altitude about 5 km above the surface.|pages=p97 (PDF page 5 of 36)|work=IPCC WG1 AR4 Report}}</ref>{{Cref2|C}} The major greenhouse gases are [[water vapor]], which causes about 36–70 percent of the greenhouse effect; [[carbon dioxide]] (CO<sub>2</sub>), which causes 9–26 percent; [[methane]] (CH<sub>4</sub>), which causes 4–9 percent; and [[ozone]] (O<sub>3</sub>), which causes 3–7 percent.<ref>{{Cite journal|url=http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf|title=Earth's Annual Global Mean Energy Budget|first=J.T. | last=Kiehl | first2=K.E. | last2=Trenberth | author2-link=Kevin Trenberth |format=PDF|journal=Bulletin of the American Meteorological Society|pages=197–208|volume=78|issue=2|year=1997|accessdate=21 April 2009|doi=10.1175/1520-0477(1997)078<0197:EAGMEB>2.0.CO;2}}</ref><ref>{{Cite web|url=http://www.realclimate.org/index.php?p=142|year=6 Apr 2005|title=Water vapour: feedback or forcing?|first=Gavin | last=Schmidt | authorlink=Gavin Schmidt |publisher=[[RealClimate]]|accessdate=21 April 2009}}</ref><ref>{{cite web | first=Randy | last=Russell | publisher=[[University Corporation for Atmospheric Research]] Windows to the Universe | url=http://www.windows.ucar.edu/tour/link=/earth/climate/greenhouse_effect_gases.html&edu=high | title=The Greenhouse Effect & Greenhouse Gases | date=May 16, 2007 | accessdate=Dec 27, 2009}}</ref> Clouds also affect the radiation balance, but they are composed of liquid water or ice and so have [[Cloud forcing|different effects on radiation]] from water vapor.
@@ Line 122: / Line 122: @@
 {{Double image stack|right|Global Warming Predictions.png|Global Warming Predictions Map.jpg|220|Calculations of global warming prepared in or before 2001 from a range of [[climate model]]s under the [[Special Report on Emissions Scenarios|SRES]] A2 emissions scenario, which [[Special_Report_on_Emissions_Scenarios#A2|assumes no action is taken to reduce emissions and regionally divided economic development]].|The geographic distribution of surface warming during the 21<sup>st</sup> century calculated by the [[HadCM3]] climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0&nbsp;°C (5.4&nbsp;°F).}}
-The main tools for projecting future climate changes are [[mathematical models]] 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, [[salinity|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.<ref>{{Cite web|url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf|author=Denman, K.L., ''et al.''|format=PDF|title=Chapter 7, Couplings Between Changes in the Climate System and Biogeochemistry|work=Fourth Assessment Report of the Intergovernmental Panel on Climate Change|accessdate=2008-02-21|year=2007|publisher=IPCC}}</ref> Warming due to increasing levels of greenhouse gases is not an assumption of the models; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes.<ref>{{Cite book|url=http://books.google.com/books?id=sx6DFr8rbpIC&dq=robert+lanza&printsec=frontcover&source=web&ots=S7MXYzoDqR&sig=jfUo33FtVZ3PSUS2fcc_EtawEnQ|last=Hansen|first=James|title=Climatic Change: Understanding Global Warming|work=One World: The Health & Survival of the Human Species in the 21st century|accessdate=2007-08-18|year=2000|publisher=Health Press}}</ref> 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.<ref>{{Cite web|url=http://www.grida.no/climate/ipcc_tar/wg1/271.htm|last=Stocker|first=Thomas F.|coauthors=''et al.''|title=7.2.2 Cloud Processes and Feedbacks|work=Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change|accessdate=2007-03-04|year=2001|publisher=IPCC}}</ref>
+The main tools for projecting future climate changes are [[mathematical models]] 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. For example[[Tim Ball]], former professor at the University of Winnipeg, has stated that climate models essentially ignore the role of water vapor as a greenhouse gas.<ref>[http://www.canadafreepress.com/index.php/article/19548 Climategate Necessary to Cover Incorrect Climate Basics of IPCC]</ref>
+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, [[salinity|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.<ref>{{Cite web|url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf|author=Denman, K.L., ''et al.''|format=PDF|title=Chapter 7, Couplings Between Changes in the Climate System and Biogeochemistry|work=Fourth Assessment Report of the Intergovernmental Panel on Climate Change|accessdate=2008-02-21|year=2007|publisher=IPCC}}</ref> Warming due to increasing levels of greenhouse gases is not an assumption of the models; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes.<ref>{{Cite book|url=http://books.google.com/books?id=sx6DFr8rbpIC&dq=robert+lanza&printsec=frontcover&source=web&ots=S7MXYzoDqR&sig=jfUo33FtVZ3PSUS2fcc_EtawEnQ|last=Hansen|first=James|title=Climatic Change: Understanding Global Warming|work=One World: The Health & Survival of the Human Species in the 21st century|accessdate=2007-08-18|year=2000|publisher=Health Press}}</ref> 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.<ref>{{Cite web|url=http://www.grida.no/climate/ipcc_tar/wg1/271.htm|last=Stocker|first=Thomas F.|coauthors=''et al.''|title=7.2.2 Cloud Processes and Feedbacks|work=Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change|accessdate=2007-03-04|year=2001|publisher=IPCC}}</ref>
 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). In addition to human-caused emissions, some models also include a simulation of the [[carbon cycle]]; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.<ref>{{Cite journal|last=Torn|first=Margaret|coauthors=Harte, John|year=2006|title=Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming|journal=Geophysical Research Letters|volume=33|issue=10|id=L10703|url=http://www.agu.org/pubs/crossref/2006/2005GL025540.shtml|accessdate=2007-03-04|doi=10.1029/2005GL025540|pages=L10703}}</ref><ref>{{Cite journal|last=Harte|first=John|coauthors=''et al.''|year=2006|title=Shifts in plant dominance control carbon-cycle responses to experimental warming and widespread drought|journal=Environmental Research Letters|volume=1|issue=1|id=014001|url=http://www.iop.org/EJ/article/1748-9326/1/1/014001/erl6_1_014001.html|accessdate=2007-05-02|doi=10.1088/1748-9326/1/1/014001|pages=014001}}</ref><ref>{{Cite journal|last=Scheffer|first=Marten|coauthors=''et al.''|title=Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change.|journal=Geophysical Research Letters|volume=33|url=http://www.pik-potsdam.de/~victor/recent/scheffer_etal_T_CO2_GRL_in_press.pdf|doi=10.1029/2005gl025044|year=2006|accessdate=2007-05-04|pages=L10702|format=PDF}}</ref> Including uncertainties in future greenhouse gas concentrations and climate sensitivity, the IPCC anticipates a warming of {{nowrap|1.1&nbsp;°C to 6.4&nbsp;°C}} {{nowrap|(2.0&nbsp;°F to 11.5&nbsp;°F)}} by the end of the 21st century, relative to 1980–1999.<ref name="grida7"/>