Global warming

2009-09-16T03:01:55Z

8bit:

'''Global warming''' is the increase in the [[Instrumental temperature record|average temperature]] of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °[[Celsius|C]] (1.33 ± 0.32 °[[Fahrenheit|F]]) during the last century.<ref name="grida7">{{cite web|url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_SPM.pdf|author=IPCC|format=PDF|title=Summary for Policymakers|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|date=2007-05-04|accessdate=2009-07-03}}</ref>{{Cref2|A}} The [[Intergovernmental Panel on Climate Change]] (IPCC) concludes that increasing [[greenhouse gas]] concentrations resulting from [[anthropogenic|human activity]] such as [[fossil fuel]] burning and [[deforestation]] [[attribution of recent climate change|caused]] most of the observed temperature increase since the middle of the 20th century.<ref name="grida7"/> The IPCC also concludes that variations in natural phenomena such as [[solar variation|solar radiation]] and [[volcano]]es produced most of the warming from [[pre-industrial]] times to 1950 and had a small cooling effect afterward.<ref name=Hegerl-2001>{{cite web|url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch09.pdf|format=PDF|title=Understanding and Attributing Climate Change|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|date=2007|publisher=IPCC|last=Hegerl|first=Gabriele C.|coauthors=''et al.''|quote=Recent estimates indicate a relatively small combined effect of natural forcings on the global mean temperature evolution of the second half of the 20th century, with a small net cooling from the combined effects of solar and volcanic forcings.}}</ref><ref>{{cite journal|last=Ammann|first=Caspar|coauthors=''et al.''|date=2007|title=Solar influence on climate during the past millennium: Results from transient simulations with the NCAR Climate Simulation Model|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=104|issue=10|pages=3713–3718|url=http://www.pnas.org/cgi/reprint/104/10/3713.pdf|format=PDF|quote=Simulations with only natural forcing components included yield an early 20th century peak warming of ≈0.2 °C (≈1950 AD), which is reduced to about half by the end of the century because of increased volcanism.|doi=10.1073/pnas.0605064103|pmid=17360418}}</ref> 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>{{cite web|title=Joint science academies' statement: Global response to climate change|author=Royal Society|url=http://royalsociety.org/displaypagedoc.asp?id=20742|accessdate=19 April 2009|date=2005}}</ref> A small number of [[List of scientists opposing the mainstream scientific assessment of global warming|scientists dispute the consensus view]].

[[Climate model]] projections summarized in the latest IPCC report indicate that the [[global]] surface temperature will probably rise a further {{nowrap|1.1 to 6.4 °C}} {{nowrap|(2.0 to 11.5 °F)}} during the twenty-first 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]]. Some other [[uncertainty|uncertainties]] include how warming and related changes will vary from region to region around the globe. Most studies focus on the period 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 [[CO2|carbon dioxide]] in the atmosphere.<ref>{{cite journal|url=http://geosci.uchicago.edu/~archer/reprints/archer.2005.fate_co2.pdf|format=PDF|first=David|last=Archer|title=Fate of fossil fuel {{chem|CO|2}} in geologic time|journal=Journal of Geophysical Research|volume=110|issue=C9|pages=C09S05.1–C09S05.6|year=2005|doi=10.1029/2004JC002625}}</ref><ref name=solomon-2009>{{cite journal|author=Solomon, S., ''et al.''|year=2009|title=Irreversible climate change due to carbon dioxide emissions|journal=Proceedings of the National Academy of Sciences|volume=106|number=6|pages=1704–1709|doi=10.1073/pnas.0812721106}}</ref>

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 subtropical [[desert]]s.<ref>{{cite journal|last=Lu|first=Jian|xauthorlink=Jian Lu|coauthors=Vecchi, Gabriel A.; Reichler, Thomas|title=Expansion of the Hadley cell under global warming|journal=Geophysical Research Letters|volume=34|date=2007|url=http://www.atmos.berkeley.edu/~jchiang/Class/Spr07/Geog257/Week10/Lu_Hadley06.pdf|doi=10.1029/2006GL028443|pages=L06805}}</ref> The continuing retreat of [[retreat of glaciers since 1850|glaciers]], [[permafrost]] and [[sea ice]] is expected, with warming being [[Arctic shrinkage|strongest in the Arctic]]. Other likely effects include increases in the intensity of [[extreme weather]] events, [[extinction risk from climate change|species extinctions]], and changes in [[agricultural yield]]s.

[[Politics of global warming|Political]] and [[global warming controversy|public debate]] continues regarding climate change, and what actions (if any) 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.

==Temperature changes==

{{Main|Temperature record}}

[[File:2000 Year Temperature Comparison.png|thumb|Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.]]

The most commonly discussed 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°C ±0.18°C over the period 1906-2005. The rate of warming over the last 50 years of that period was almost double that for the period as a whole (0.13°C ±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.<ref>{{Cite book|last=Trenberth|first=Kevin E.|coauthors=''et al.''|title=IPCC Fourth Assessment Report|url=http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf|format=PDF|year=2007|publisher=Cambridge University Press|location=Cambridge, United Kingdom and New York, NY, USA|page=244|chapter=Chapter 3: Observations: Surface and Atmospheric Climate Change|chapterurl=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter3.pdf}}</ref> 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]]. Temperature is believed to have been relatively stable over the [[Temperature record of the past 1000 years|one or two thousand years]] before 1850, with regionally-varying 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, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.<ref>{{cite web|url=http://data.giss.nasa.gov/gistemp/2005/|last=Hansen|first=James E.|authorlink=James Hansen|coauthors=''et al.''|title=Goddard Institute for Space Studies, GISS Surface Temperature Analysis|accessdate=2007-01-17|date=2006-01-12|publisher=NASA Goddard Institute for Space Studies}}</ref> Estimates prepared by the [[World Meteorological Organization]] and the [[Climatic Research Unit]] concluded that 2005 was the second warmest year, behind 1998.<ref>{{cite web|url=http://www.cru.uea.ac.uk/cru/press/2005-12-WMO.pdf|title=Global Temperature for 2005: second warmest year on record|accessdate=2007-04-13|date=2005-12-15|publisher=Climatic Research Unit, School of Environmental Sciences, University of East Anglia|format=PDF}}</ref><ref>{{cite web|url=http://www.wmo.int/pages/prog/wcp/wcdmp/statement/documents/WMO998_E.pdf|format=PDF|title=WMO statement on the status of the global climate in 2005|accessdate=2009-04-24|date=2005-12-15|publisher=World Meteorological Organization}}</ref> Temperatures in 1998 were unusually warm because the strongest [[El Niño-Southern Oscillation|El Niño]] in the past century occurred during that year.<ref name="Changnon2000">{{cite book|title=El Niño, 1997-1998: The Climate Event of the Century|last=Changnon|first=Stanley A.|coauthors=Bell, Gerald D.|year=2000|publisher=Oxford University Press|location=London|isbn=0195135520}}</ref>

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).<ref>{{cite web|url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter3.pdf|title=IPCC Fourth Assessment Report, Chapter 3|accessdate=2009-03-14|date=2007-02-05|pages=237}}</ref> 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.<ref>{{cite journal|title=Land/sea warming ratio in response to climate change: IPCC AR4 model results and comparison with observations|author=Rowan T. Sutton, Buwen Dong, Jonathan M. Gregory|journal=Geophysical Research Letters|volume=34|url=http://www.agu.org/pubs/crossref/2007/2006GL028164.shtml|doi=10.1029/2006GL028164|year=2007|accessdate=2007-09-19|pages=L02701}}</ref> 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 the [[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.<ref>{{cite book|chapterurl=http://www.grida.no/publications/other/ipcc_tar/?src=/CLIMATE/IPCC_TAR/WG1/127.htm|title=Climate Change 2001: The Scientific Basis|chapter=Atmospheric Chemistry and Greenhouse Gases|url=http://www.grida.no/publications/other/ipcc_tar/?src=/CLIMATE/IPCC_TAR/WG1/|author=Intergovernmental Panel on Climate Change|authorlink=Intergovernmental Panel on Climate Change|publisher=Cambridge University Press|location=Cambridge, UK|year=2001}}</ref>

The [[Volumetric heat capacity|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 {{nowrap|0.5 °C}} {{nowrap|(0.9 °F)}} would still occur.<ref>{{cite journal|last=Meehl|first=Gerald A.|coauthors=''et al.''|date=2005-03-18|title=How Much More Global Warming and Sea Level Rise|journal=Science|volume=307|issue=5716|pages=1769–1772|doi=10.1126/science.1106663|url=http://www.sciencemag.org/cgi/reprint/307/5716/1769.pdf|format=PDF|accessdate=2007-02-11|pmid=15774757}}</ref>

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==Radiative forcing==

{{Main|Radiative forcing}}

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 gases|greenhouse gas]] concentrations, changes in [[solar luminosity]], [[volcano|volcanic]] eruptions, and [[orbital forcing|variations in Earth's orbit]] around the Sun.<ref name=Hegerl-2001/> 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===

{{Main|Greenhouse gas|Greenhouse effect}}

{{Double image stack|right|Greenhouse Effect.svg|Mauna Loa Carbon Dioxide-en.svg|180|Greenhouse effect schematic showing energy flows between the atmosphere, space, and earth's surface. Energy exchanges are expressed in watts per square meter (W/m2).|Recent atmospheric [[carbon dioxide]] (CO2) increases. Monthly CO2 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 CO2.}}

The greenhouse effect is the process by which [[Absorption (electromagnetic radiation)|absorption]] and 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|author=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.

Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °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 °C. 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.|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]] (CO2), which causes 9–26 percent; [[methane]] (CH4), which causes 4–9 percent{{Failed verification|date=June 2009}}; and [[ozone]] (O3), 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|author=Kiehl, J.T. and K.E. Trenberth|format=PDF|journal=Bulletin of the American Meteorological Society|pages=197–208|volume=78|issue=2|date=1997|accessdate=21 April 2009|doi=10.1175/1520-0477(1997)078<0197:EAGMEB>2.0.CO;2|year=1997}}</ref><ref>{{cite web|url=http://www.realclimate.org/index.php?p=142|year=6 Apr 2005|title=Water vapour: feedback or forcing?|author=Gavin Schmidt|publisher=RealClimate|accessdate=21 April 2009}}</ref> Clouds also affect the radiation balance, but they are composed of liquid water or ice and so are [[Cloud forcing|considered separately]] from water vapor and other gases.

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]], [[CFC]]s and [[nitrous oxide]]. The [[Greenhouse gas#Increase of greenhouse gases|concentrations]] of CO2 and methane have increased by 36% and 148% respectively since the mid-1700s.<ref>{{cite web|author=EPA|date=2008|title=Recent Climate Change: Atmosphere Changes|work=Climate Change Science Program|url=http://www.epa.gov/climatechange/science/recentac.html|accessdate=21 April 2009|year=2007|publisher=United States Environmental Protection Agency}}</ref> 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 core]]s.<ref>{{cite journal|author=Neftel, A., E. Moor, H. Oeschger, and B. Stauffer|date=1985|title=Evidence from polar ice cores for the increase in atmospheric CO2 in the past two centuries|journal=Nature|volume=315|pages=45–47|doi=10.1038/315045a0}}</ref> Less direct geological evidence indicates that CO2 values this high were last seen approximately 20 million years ago.<ref>{{cite journal|author=Pearson, P.N. and M.R. Palmer|journal=Nature|title=Atmospheric carbon dioxide concentrations over the past 60 million years|date=2000|volume=406|issue=6797|pages=695–699|doi=10.1038/35021000}}</ref> [[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]].<ref name=spm-2001>{{cite web|url=http://www.grida.no/climate/ipcc_tar/wg1/pdf/WG1_TAR-FRONT.pdf|format=PDF|title=Summary for Policymakers|author=IPCC|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=21 April 2009|date=2001|publisher=IPCC}}</ref>

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, [[sociology|sociological]], [[technology|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.<ref>{{cite web|url=http://www.grida.no/climate/ipcc_tar/wg1/123.htm|author=Prentice, I.C., ''et al.''|title=The Carbon Cycle and Atmospheric Carbon Dioxide: SRES scenarios and their implications for future CO2 concentration|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=21 April 2009|date=2001|publisher=IPCC}}</ref> Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if [[coal]], [[tar sands]] or [[methane clathrate]]s are extensively exploited.<ref>{{cite web|url=http://www.grida.no/climate/ipcc/emission/104.htm|author=Nakicenovic., N., ''et al.''|title=An Overview of Scenarios: Resource Availability|work=IPCC Special Report on Emissions Scenarios|accessdate=21 April 2009|publisher=IPCC|date=2001}}</ref>

The destruction of [[stratosphere|stratospheric]] ozone by [[chlorofluorocarbons]] is sometimes mentioned in relation to global warming. Although there are a few [[Ozone depletion#Ozone depletion and global warming|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.<ref>{{cite web|url=http://www.nas.nasa.gov/About/Education/Ozone/history.html|title=Ozone Depletion, History and politics|last=Sparling|first=Brien|date=May 30, 2001|publisher=NASA|accessdate=2009-02-15}}</ref> [[Tropospheric ozone]] contributes to surface warming.<ref>{{cite journal|doi=10.1029/2005JD006348|title=Role of tropospheric ozone increases in 20th-century climate change|year=2006|author=Shindell, Drew|journal=Journal of Geophysical Research|volume=111|pages=D08302}}</ref>

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===Aerosols and soot===

[[File:ShipTracks MODIS 2005may11.jpg|thumb|[[Ship tracks]] over the [[Atlantic Ocean]] on the east coast of the United States. The climatic impacts from aerosol forcing could have a large effect on climate through the indirect effect.]]

[[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.<ref>{{cite web|url=http://www.grida.no/climate/ipcc_tar/wg1/462.htm|author=Mitchell, J.F.B., ''et al.''|title=Detection of Climate Change and Attribution of Causes: Space-time studies|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=21 April 2009|date=2001|publisher=IPCC}}</ref> The main cause of this dimming is [[aerosols]] produced by volcanoes and [[pollutant]]s. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. [[James 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.<ref name="pmid10944197">{{cite journal|author=Hansen J., Sato M., Ruedy R., Lacis A., and Oinas V.|title=Global warming in the twenty-first century: an alternative scenario|journal=Proc. Natl. Acad. Sci. U.S.A.|volume=97|issue=18|pages=9875–80|year=2000|doi=10.1073/pnas.170278997}}</ref>

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.<ref>{{cite journal|author=Lohmann, U. & J. Feichter|date=2005|title=Global indirect aerosol effects: a review|journal=Atmos. Chem. Phys.|volume=5|pages=715–737|url=http://www.atmos-chem-phys.net/5/715/2005/acp-5-715-2005.html}}</ref> Sulfate aerosols act as [[cloud condensation nuclei]] and thus lead to clouds that have more and smaller cloud droplets. These clouds [[Twomey effect|reflect solar radiation]] more efficiently than clouds with fewer and larger droplets.<ref>{{cite journal|author=Twomey, S.|date=1977|title=Influence of pollution on shortwave albedo of clouds|journal=J. Atmos. Sci.|volume=34|pages=1149–1152|doi=10.1175/1520-0469(1977)034<1149:TIOPOT>2.0.CO;2|year=1977}}</ref> This effect also causes droplets to be of more uniform size, which reduces growth of raindrops by [[Cloud_physics#Collision-Coalescence|collision-coalescence]]. Clouds modified by pollution have been shown to produce less [[drizzle]], making the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum.<ref>{{cite journal|author=Albrecht, B.|date=1989|title=Aerosols, cloud microphysics, and fractional cloudiness|journal=Science|volume=245|pages=1227–1239|doi=10.1126/science.245.4923.1227|pmid=17747885}}</ref>

[[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. Regionally (but not globally), as much as 50% of surface warming due to greenhouse gases may be masked by [[atmospheric brown cloud]]s.<ref>{{cite journal|last=Ramanathan|first=V.|coauthors=''et al.''|year=2005|url=http://www.pnas.org/content/102/15/5326.abstract|title=Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle|journal=Proc. Natl. Acad. Sci.|volume=102|pages=5326–5333|doi=10.1073/pnas.0500656102}}</ref> When deposited, especially on glaciers or on ice in arctic regions, the lower surface [[albedo]] can also directly heat the surface.<ref>{{cite web|author=Ramanathan, V., et al.|date=2008|work=Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia|publisher=United Nations Environment Programme|url=http://www.rrcap.unep.org/abc/impact/files/ABC_Report_Summary_Final.pdf|title=Report Summary}}</ref> 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.<ref>{{cite web|author=Ramanathan, V., et al.|date=2008|work=Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia|publisher=United Nations Environment Programme|url=http://www.rrcap.unep.org/abc/publication/Part%20III.pdf|title=Part III: Global and Future Implications}}</ref>

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===Solar variation===

{{Main|Solar variation}}

[[File:Solar-cycle-data.png|thumb|Solar variation over the last thirty years.]]

Variations in solar output have been the cause of past [[climate change]]s.<ref>{{Cite book|last=National Research Council|title=Solar Influences On Global Change|url=http://books.nap.edu/openbook.php?record_id=4778&page=R1|year=1994|publisher=National Academy Press|location=Washington, D.C.|isbn=0-309-05148-7|page=36}}</ref> Although solar forcing is generally thought to be too small to account for a significant part of global warming in recent decades,<ref>{{Cite journal|title=Climate forcings in Goddard Institute for Space Studies SI2000 simulations|year=2002|author=Hansen, J.|journal=Journal of Geophysical Research|volume=107|pages=4347|doi=10.1029/2001JD001143|title=Climate|doi_brokendate=2009-06-24|unused_data=estimated by global circulation models isdoi=10.1029/2001JD001143}}</ref><ref>{{Cite journal|doi=10.1029/2005JD005776|title=Efficacy of climate forcings|year=2005|author=Hansen, J.|journal=Journal of Geophysical Research|volume=110|pages=D18104}}</ref> a few studies disagree, such as a recent phenomenological analysis that indicates the contribution of solar forcing may be underestimated.<ref>{{Cite journal|doi=10.1029/2007JD008437|title=Phenomenological reconstructions of the solar signature in the Northern Hemisphere surface temperature records since 1600|year=2007|author=Scafetta, N.|journal=Journal of Geophysical Research|volume=112|pages=D24S03|url=http://www.fel.duke.edu/~scafetta/pdf/2007JD008437.pdf}}</ref>

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.<ref name=Hegerl-2001/> 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.<ref name=Randel2009>{{cite journal|doi=10.1029/2008JD010421|title=An update of observed stratospheric temperature trends|year=2009|author=Randel, William J.|journal=Journal of Geophysical Research|volume=114|pages=D02107}}</ref>

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.<ref>{{cite journal|first=Nigel|last=Marsh|coauthors=Henrik, Svensmark|title=Cosmic Rays, Clouds, and Climate|journal=Space Science Reviews|volume=94|number=1–2|pages=215–230|year=2000|month=November|url=http://www.dsri.dk/~hsv/SSR_Paper.pdf|format=PDF|doi=10.1023/A:1026723423896|accessdate=2007-04-17}}</ref> Other research has found no relation between warming in recent decades and [[cosmic ray]]s.<ref>{{cite journal|last=Lockwood|first=Mike|coauthors=Claus Fröhlich|title=Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature|journal=Proceedings of the Royal Society A|volume=463|pages=2447|quote=Our results show 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.|url=http://www.pubs.royalsoc.ac.uk/media/proceedings_a/rspa20071880.pdf|doi=10.1098/rspa.2007.1880|accessdate=2007-07-21|format=PDF|year=2007}}</ref><ref>{{cite journal|doi=10.1088/1748-9326/3/2/024001|title=Testing the proposed causal link between cosmic rays and cloud cover|author=T Sloan and A W Wolfendale|journal=Environ. Res. Lett.|volume=3|page=024001|year=2008|pages=024001|url=http://www.iop.org/EJ/abstract/1748-9326/3/2/024001/}}</ref> 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.<ref>{{cite journal|journal=Geophysical Research Letters|author=Pierce, J.R. and P.J. Adams|date=2009|title=Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates?|volume=36|pages=L09820|doi=10.1029/2009GL037946}}</ref>

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==Feedback==

{{Main|Effects of global warming}}

A positive feedback is a process that amplifies some change. Thus, when a warming trend results in effects that induce further warming, the result is a positive feedback; when the warming results in effects that reduce the original warming, the result is a negative feedback. The main positive feedback in global warming involves the tendency of warming to increase the amount of water vapor in the atmosphere. The main 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 [[Stefan–Boltzmann law|increases with the fourth power]] of its [[absolute temperature]].

; [[Water vapor feedback]] : If the atmosphere is warmed, the [[saturation vapor pressure]] increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a [[positive feedback]]), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes 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.<ref name="soden1">{{cite journal|first=Brian J.|last=Soden|coauthors=Held, Isacc M.|journal=Journal of Climate|title=An Assessment of Climate Feedbacks in Coupled Ocean–Atmosphere Models|date=2005-11-01|volume=19|issue=14|format=PDF|accessdate=2007-04-21|quote=Interestingly, the true feedback is consistently weaker than the constant relative humidity value, implying a small but robust reduction in relative humidity in all models on average" "clouds appear to provide a positive feedback in all models|pages=3354–3360|doi=10.1175/JCLI3799.1}}</ref>
; [[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, 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 [[list of cloud types|type]] and altitude of the cloud, details that are difficult to represent in climate models.<ref name="soden1"/>
; [[Lapse rate]] : The atmosphere's temperature decreases with height in the [[troposphere]]. Since emission of infrared radiation varies with the fourth power of temperature, [[longwave radiation]] escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease 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.<ref>{{cite book|url=http://www.nap.edu/catalog.php?record_id=10850|series=Panel on Climate Change Feedbacks, Climate Research Committee|isbn=0309090725|publisher=National Academies Press|author=National Research Council|date=2004|title=Understanding Climate Change Feedbacks}}</ref>
; [[Ice-albedo feedback]] : [[File:Sea Ice MeltPonds.png|thumb|Aerial photograph showing a section of sea ice. The lighter blue areas are [[melt ponds]] and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to the [[ice-albedo feedback]].]] When 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.<ref>{{cite web|url=http://www.grida.no/climate/ipcc_tar/wg1/295.htm|last=Stocker|first=Thomas F.|coauthors=''et al.''|title=7.5.2 Sea Ice|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=2007-02-11|date=2001-01-20|publisher=IPCC}}</ref>
; [[Arctic methane release]] : Warming is also the triggering variable for the release of methane in the arctic<ref>{{cite doi|10.1029/2009GL039191}}</ref>. Methane released from thawing [[permafrost]] such as the frozen [[peat]] [[bog]]s in [[Siberia]], and from [[methane clathrate]] on the sea floor, creates a [[positive feedback]].<ref>{{cite journal|doi=10.1126/science.1128908|year=2006|month=Jun|author=Zimov, Sa; Schuur, Ea; Chapin, Fs, 3Rd|title=Climate change. Permafrost and the global carbon budget.|volume=312|issue=5780|pages=1612–3|issn=0036-8075|pmid=16778046|journal=Science (New York, N.Y.)}}</ref>
; [[Carbon cycle|Reduced absorption of CO2 by the oceans]] : Ocean ecosystems' ability to sequester carbon is expected to decline as the oceans warm. This is because warming reduces the nutrient levels of the [[mesopelagic zone]] (about 200 to 1000 m deep), which limits the growth of [[diatom]]s in favor of smaller [[phytoplankton]] that are poorer [[biological pump]]s of carbon.<ref>{{cite journal|last=Buesseler|first=Ken O.|coauthors=''et al.''|date=2007-04-27|title=Revisiting Carbon Flux Through the Ocean's Twilight Zone|journal=Science|volume=316|issue=5824|pages=567–570|url=http://www.sciencemag.org/cgi/content/abstract/316/5824/567|accessdate=2007-11-16|format=abstract|doi=10.1126/science.1137959|pmid=17463282}}</ref>
; Gas release : Release of miscellaneous gases of biological origin may be affected by global warming, but research into such effects is at an early stage. Such releases may have direct climate effects, such as [[Nitrous oxide]]<ref>{{cite doi|10.1038/ngeo434}}</ref> released from [[peat]] and indirect effects, such as [[Dimethyl sulfide]]<ref>{{cite doi|10.1029/2001GB001829}}</ref> released from oceans.

{{Clear}}

==Climate models==
{{Main|Global climate model}}

{{Double image stack|right|Global Warming Predictions.png|Global Warming Predictions Map.jpg|180|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 21st 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 °C (5.4 °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|date=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 in the models.<ref>{{cite web|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|date=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|date=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.''|date=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|date=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 °C to 6.4 °C}} {{nowrap|(2.0 °F to 11.5 °F)}} by the end of the 21st century, relative to 1980–1999.<ref name="grida7"/> A 2008 paper predicts that the global temperature may not increase during the next decade because short-term natural fluctuations may temporarily outweigh greenhouse gas-induced warming.<ref>{{cite journal|url=http://www.nature.com/nature/journal/v453/n7191/abs/nature06921.html|title=Advancing decadal-scale climate prediction in the North Atlantic sector|author=N. S. Keenlyside, M. Latif, J. Jungclaus, L. Kornblueh2, E. Roeckner|date=2008|pages=84–88|journal=Nature|issue=453|accessdate=2008-07-06|doi=10.1038/nature06921|volume=453}}</ref>

Models are also used to help investigate the [[Attribution of recent climate change|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.<ref>http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter9.pdf</ref>

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.<ref name=spm-2001/> The physical realism of models is tested by examining their ability to simulate current or past climates.<ref>{{cite web|url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter8.pdf|author=Randall, D.A., et al.|format=PDF|title=Chapter 8, Climate Models and Their Evaluation|work=Fourth Assessment Report of the Intergovernmental Panel on Climate Change|accessdate=2009-03-21|date=2007|publisher=IPCC}}</ref> While a 2007 study by [[David Douglass]] and colleagues found that the models did not accurately predict observed changes in the tropical [[troposphere]],<ref>{{cite journal|last=Douglass|first=David H.|authorlink=David Douglass|coauthors=''et al.''|date=2007|title=A comparison of tropical temperature trends with model predictions|journal=International Journal of Climatology|volume=9999|issue=9999|doi=10.1002/joc.1651|url=http://icecap.us/images/uploads/DOUGLASPAPER.pdf|format=PDF|accessdate=2008-05-12|pages=1693}}</ref> a 2008 paper published by a 17-member team led by [[Ben Santer]] noted errors in the Douglass study, and found instead that the models and observations were not statistically different.<ref>{{cite journal|last=Santer|first=B.D.|authorlink=Benjamin D. Santer|coauthors=''et al.''|date=2008|title=Consistency of modelled and observed temperature trends in the tropical troposphere|journal=International Journal of Climatology|volume=28|issue=13|doi=10.1002/joc.1756|url=https://publicaffairs.llnl.gov/news/news_releases/2008/NR-08-10-05-article.pdf|format=PDF|accessdate=2008-10-22|pages=1703}}</ref> Not all [[effects of global warming]] are accurately predicted by the [[climate models]] used by the [[Intergovernmental Panel on Climate Change|IPCC]]. For example, observed [[Arctic shrinkage]] has been faster than that predicted.<ref>{{Cite journal|doi=10.1029/2007GL029703|title=Arctic sea ice decline: Faster than forecast|year=2007|author=Stroeve, J., ''et al.''|journal=Geophysical Research Letters|volume=34|pages=L09501}}</ref>

==Attributed and expected effects==
===Environmental===

{{Main|Effects of global warming|Regional effects of global warming}}

[[File:Glacier Mass Balance.png|thumb|Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the [[World Glacier Monitoring Service|WGMS]] and the [[National Snow and Ice Data Center|NSIDC]].]]

It usually is impossible to connect specific weather events to global warming. Instead, global warming is expected to cause changes in the overall distribution and intensity of events, such as changes to the frequency and intensity of heavy precipitation. Broader effects are expected to include [[Glacier mass balance|glacial retreat]], [[Arctic shrinkage]], and worldwide sea level rise. Some effects on both the [[natural environment]] and [[civilization|human life]] are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that [[Retreat of glaciers since 1850|glacier retreat]], [[Ice shelf#Ice shelf disruption|ice shelf disruption]] such as that of the [[Larsen Ice Shelf]], sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events are attributable in part to global warming.<ref name="tar_wg2">{{cite web|title=Climate Change 2001: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change|url=http://www.grida.no/climate/ipcc_tar/wg2/index.htm|publisher=IPCC|date=2001-02-16|accessdate=2007-03-14}}</ref> Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and some adverse health effects from warmer temperatures.<ref>{{cite journal|author=McMichael AJ, Woodruff RE, Hales S|title=Climate change and human health: present and future risks|journal=Lancet|volume=367|issue=9513|pages=859–69|year=2006|pmid=16530580|doi=10.1016/S0140-6736(06)68079-3}}</ref>

Social and economic effects of global warming may be exacerbated by [[Population growth|growing population]] densities in affected areas. Temperate regions are projected to experience some benefits, such as fewer cold-related deaths.<ref name="WGII SPM AR4">{{cite web|title=Summary for Policymakers|work=Climate Change 2007: Impacts, Adaptation and Vulnerability. Working Group II Contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report|url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-spm.pdf|format=PDF|publisher=IPCC|date=2007-04-13|accessdate=2007-04-28}}</ref> A summary of probable effects and recent understanding can be found in the report made for the [[IPCC Third Assessment Report]] by Working Group II.<ref name="tar_wg2"/> 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.<ref name="grida7"/>

Additional anticipated effects include [[sea level rise]] of {{nowrap|0.18 to 0.59 meters}} {{nowrap|(0.59 to 1.9 ft)}} in 2090-2100 relative to 1980-1999,<ref name="grida7"/> [[Northwest Passage|new trade routes]] resulting from arctic shrinkage,<ref>{{cite news|first=Jennifer|last=Macey|title=Global warming opens up Northwest Passage|url=http://www.abc.net.au/news/stories/2007/09/19/2037198.htm?section=business|publisher=ABC News|date=September 19, 2007|accessdate=2007-12-11}}</ref> [[Shutdown of thermohaline circulation|possible thermohaline circulation slowing]], increasingly intense (but less frequent) hurricanes and extreme weather events,<ref>{{cite journal|doi=10.1038/ngeo202|title=Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions|year=2008|author=Knutson, Thomas R.|journal=Nature Geoscience|volume=1|pages=359}}</ref> reductions in the [[ozone layer]], [[Climate change and agriculture|changes in agriculture yields]], changes in the range of climate-dependent [[Vector (epidemiology)|disease vectors]],<ref name="KingEtAlASM">{{Cite book|last=King|first=Gary M.|coauthors=''et al.''|title=Global Environmental Change Microbial Contributions Microbial Solutions|url=http://www.asm.org/ASM/files/CCPAGECONTENT/DOCFILENAME/0000006005/globalwarming%5B1%5D.pdf|format=PDF|accessdate=2009-05-23|publisher=American Society for Microbiology|pages=7}}</ref> which has been linked to increases in the prevalence of [[malaria]] and [[dengue fever]],<ref name="AR4WG2CH8">{{Cite book|chapter=Chapter 8: Human Health|chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter8.pdf|url=http://www.ipcc.ch/ipccreports/ar4-wg2.htm|editor1-first=M.L.|editor1-last=Parry|editor2-first=O.F.|editor2-last=Canziani|editor3-first=J.P.|editor3-last=Palutikof|editor4-first=P.J.|editor4-last=van der Linden|editor5-first=C.E.|editor5-last=Hanson|publisher=Cambridge University Press|author1-first=U.|author1-last=Confalonieri|author2-first=B.|author2-last=Menne|author3-first=R.|author3-last=Ebi|author4-first=K.L.|author5-first=M.|author5-last=Hauengue|author6-first=R.S.|author6-last=Kovats|author7-first=B.|author7-last=Revich|author8-first=A.|author8-last=Woodward|year=2007|title=Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change|isbn=978-0521-88010-7}}</ref> and ocean oxygen depletion.<ref>{{cite journal|author=Shaffer, G., S.M. Olsen and G.O.P Pederson|year=2009|title=Long-term ocean oxygen depletion in response to carbon dioxide emissions from fossil fuels|journal=Nature Geoscience|volume=2|pages=105–109|doi=10.1038/ngeo420}}</ref> Increased atmospheric CO2 increases the amount of CO2 dissolved in the [[ocean]]s.<ref>{{cite web|url=http://nasascience.nasa.gov/earth-science/oceanography/ocean-earth-system/ocean-carbon-cycle|title=Carbon Cycle|accessdate=2009-06-24|work=NASA}}</ref> CO2 dissolved in the ocean reacts with water to form [[carbonic acid]], resulting in [[ocean acidification]]. Ocean surface [[pH]] is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,<ref>{{cite journal|last=Jacobson|first=Mark Z.|date=2005-04-02|title=Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry|journal=Journal of Geophysical Research|volume=110|issue=D7|id=D07302|url=http://www.stanford.edu/group/efmh/jacobson/2004JD005220.pdf|format=PDF|doi=10.1029/2004JD005220|accessdate=2007-04-28|pages=D07302}}</ref> and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.<ref name="grida7"/><ref>{{cite journal|last=Caldeira|first=Ken|coauthors=Wickett, Michael E.|title=Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean|journal=Journal of Geophysical Research|volume=110|issue=C09S04|doi=10.1029/2004JC002671|pages=1–12|url=http://www.agu.org/pubs/crossref/2005/2004JC002671.shtml|date=2005-09-21|accessdate=2006-02-14}}</ref> 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.<ref name=solomon-2009/> Since [[organism]]s and [[ecosystems]] are adapted to a narrow range of [[pH]], this raises [[extinction]] concerns and disruptions in [[food webs]].<ref>{{cite paper|author=Raven, John A.; ''et al.''|title=Ocean acidification due to increasing atmospheric carbon dioxide|publisher=Royal Society|date=2005-06-30|url=http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13314|format=ASP|accessdate=2007-05-04}}</ref> One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.<ref>{{cite journal|last=Thomas|first=Chris D.|coauthors=''et al.''|date=2004-01-08|title=Extinction risk from climate change|journal=Nature|volume=427|issue=6970|pages=145–138|doi=10.1038/nature02121|url=http://www.geog.umd.edu/resac/outgoing/GEOG442%20Fall%202005/Lecture%20materials/extinctions%20and%20climate%20change.pdf|format=PDF|accessdate=2007-03-18}}</ref> However, few mechanistic studies have documented extinctions due to recent climate change,<ref>{{cite journal|last=McLaughlin|first=John F.|coauthors=''et al.''|date=2002-04-30|title=Climate change hastens population extinctions|journal=PNAS|volume=99|issue=9|pages=6070–6074|doi=10.1073/pnas.052131199|url=http://www.nd.edu/~hellmann/pnas.pdf|format=PDF|accessdate=2007-03-29|pmid=11972020}}</ref> and one study suggests that projected rates of [[Extinction risk from climate change|extinction]] are uncertain.<ref>{{cite journal|last=Botkin|first=Daniel B.|coauthors=''et al.''|year=2007|month=March|title=Forecasting the Effects of Global Warming on Biodiversity|journal=BioScience|volume=57|issue=3|pages=227–236|doi=10.1641/B570306|url=http://www.imv.dk/Admin/Public/DWSDownload.aspx?File=%2FFiles%2FFiler%2FIMV%2FPublikationer%2FFagartikler%2F2007%2F050307_Botkin_et_al.pdf|accessdate=2007-11-30|format=PDF}}</ref>

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:

<blockquote>"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."<ref>[http://www.google.com/hostednews/afp/article/ALeqM5g1eE4Xw3njaW1MKpJRYOch4hOdLQ Global warming benefits to Tibet: Chinese official. Reported 18/Aug/2009.]</ref></blockquote>

{{clear}}

===Economic===

{{Main|Economics of global warming|Low-carbon economy}}

[[File:IPCC AR4 WGIII GHG concentration stabilization levels.png|thumb|Projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From [[IPCC Fourth Assessment Report|IPCC AR4]].]]

The IPCC reports the [[aggregate data|aggregate]] net economic costs of damages from climate change globally ([[social discount rate|discounted]] to the specified year). In 2005, the average [[social cost of carbon]] from 100 [[peer-review]]ed estimates is US$12 per tonne of CO2, but range -$3 to $95/tCO2. The IPCC's gives these cost estimates with the caveats, "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."<ref name=ipccsr>{{cite web|date=2007|title=Summary for Policymakers|work=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|author=Core Writing Team, Pachauri, R.K and Reisinger, A. (eds.)|url=http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_frontmatter.pdf|accessdate=2009-05-20|page=22}}</ref>

One widely publicized report on potential economic impact is the [[Stern Review]], written by Sir [[Nicholas Stern]]. It suggests that [[extreme weather]] might reduce global [[gross domestic product]] by up to one percent, and that in a worst-case scenario global per capita consumption could fall by the equivalent of 20 percent.<ref>{{cite web|url=http://news.bbc.co.uk/2/hi/business/6098362.stm|title=At-a-glance: The Stern Review|publisher=BBC|accessdate=2007-04-29|date=2006-10-30}}</ref> The response to the Stern Review was mixed. The Review's methodology, advocacy and conclusions were criticized by several economists, including [[Richard Tol]], [[Gary Yohe]],<ref>{{cite journal|author=Tol, R. and G. Yohe|date=2006|title=A Review of the Stern Review|journal=World Economics|volume=7|issue=4|pages=233–250|url=http://www.fnu.zmaw.de/fileadmin/fnu-files/publication/tol/RM551.pdf}}</ref> [[Robert O. Mendelsohn|Robert Mendelsohn]]<ref name=mendelsohnr>{{cite web|date=2006-2007|title=A Critique of the Stern Report|publisher=Regulation|author=Mendelsohn, R.|url=http://www.cato.org/pubs/regulation/regv29n4/v29n4-5.pdf|accessdate=2009-05-20}}</ref> and [[William Nordhaus]].<ref name=yale>{{cite web|date=2005|title=The Economics of Climate Change, Part Two: Comments on the Stern Review. Chapter 5: William Nordhaus, Yale University, 'Opposite Ends of the Globe'|publisher=[[Yale University|Yale]] Center for the Study of Globalization|author=Nordhaus, W.|url=http://www.ycsg.yale.edu/climate/|accessdate=2009-05-20}}</ref> Economists that have generally supported the Review include Terry Barker,<ref name=barkert>{{cite journal|date=August 2008|title=The economics of avoiding dangerous climate change. An editorial essay on The Stern Review|author=Barker, T.|journal=Climatic Change|volume=89|doi=10.1007/s10584-008-9433-x|issue=Volume 89, Numbers 3-4 / August, 2008|pages=173-194|url=http://www.springerlink.com/content/612k4k5v68r2577m/?p=20cde4d2d12e41939a4faac4082d8512&pi=0|accessdate=2009-05-20}}</ref> William Cline,<ref name=clinew>{{cite web|date=January 5, 2008|title=Comments on the Stern Review|publisher=Peter G. Peterson Institute for International Economics|author=Cline, W.|url=http://www.iie.com/publications/papers/paper.cfm?ResearchID=874|accessdate=2009-05-20}}</ref> and [[Frank Ackerman]].<ref name=ackermanf>{{cite web|date=July 2007|title=Debating Climate Economics: The Stern Review vs. Its Critics|publisher=Report to [[Friends of the Earth]]-UK|author=Ackerman, F.|url=http://ase.tufts.edu/gdae/Pubs/rp/SternDebateReport.pdf|accessdate=2009-05-20}}</ref> According to Barker, the costs of mitigating climate change are 'insignificant' relative to the risks of unmitigated climate change.<ref>{{web cite|url=http://www.ft.com/cms/s/0/38c1bfa0-09bd-11dd-81bf-0000779fd2ac.html|title=Full quote from IPCC on costs of climate change|author=Terry Barker|date=April 14, 2008|accessdate=2008-04-14|publisher=FT.com}}</ref>

According to [[United Nations Environment Programme]] (UNEP), economic sectors likely to face difficulties related to climate change include [[bank]]s, [[Climate change and agriculture|agriculture]], transport and others.<ref>{{cite web|url=http://www.unepfi.org/fileadmin/documents/CEO_briefing_climate_change_2002_en.pdf|format=PDF|title=Climate Risk to Global Economy|last=Dlugolecki|first=Andrew|coauthors=''et al.''|work=CEO Briefing: UNEP FI Climate Change Working Group|publisher=United Nations Environment Programme|accessdate=2007-04-29|year=2002}}</ref> Developing countries dependent upon agriculture will be particularly harmed by global warming.<ref>{{cite web|url=http://www.rff.org/Publications/Resources/Documents/164/RFF-Resources-164_Thomas%20Schelling.pdf|title=Thomas Schelling: Developing Countries Will Suffer Most from Global Warming|accessdate=2008-03-01|work=Resources 164|format=PDF}}</ref>

{{Clear}}

==Responses to global warming==

The [[Scientific opinion on climate change|broad agreement]] among climate scientists that global temperatures will continue to increase has led some [[nation]]s, [[State (law)|states]], [[corporation]]s and individuals to implement responses. These responses to global warming can be divided into [[Mitigation of global warming|mitigation]] of the causes and effects of global warming, [[Adaptation to global warming|adaptation]] to the changing global environment, and [[geoengineering]] to reverse global warming.

===Mitigation===

{{Main|Mitigation of global warming}}

[[File:Carbon sequestration-2009-10-07.svg|thumb|[[Carbon capture and storage]] (CCS) is an approach to mitigation. Emissions may be [[Carbon sequestration|sequestered]] from fossil fuel power plants, or removed during processing in hydrogen production. When used on plants, it is known as [[bio-energy with carbon capture and storage]].]]

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.<ref>{{cite doi|10.1088/1748-9326/4/1/014012}}</ref> The world's primary international agreement on reducing greenhouse gas emissions is the [[Kyoto Protocol]], an amendment to the [[United Nations Framework Convention on Climate Change|UNFCCC]] negotiated in 1997. The Protocol now covers more than 160 countries and over 55 percent of global greenhouse gas emissions.<ref>{{cite web|url=http://unfccc.int/files/essential_background/kyoto_protocol/application/pdf/kpstats.pdf|format=PDF|title=Kyoto Protocol Status of Ratification|publisher=United Nations Framework Convention on Climate Change|date=2006-07-10|accessdate=2007-04-27}}</ref> As of June 2009, only the [[United States]], historically the world's [[greenhouse gas emissions by the United States|largest emitter]] of greenhouse gases, has [[List of Kyoto Protocol signatories|refused to ratify the treaty]]. The treaty expires in 2012. International talks began in May 2007 on a future treaty to succeed the current one.<ref>{{cite web|title=Twenty-sixth session and Ad Hoc Working Group on Further Commitments for Annex I Parties under the Kyoto Protocol (AWG), Third session|url=http://unfccc.int/meetings/sb26/items/3919.php|publisher=United Nations Framework Convention on Climate Change|accessdate=2009-06-21}}</ref> UN negotiations are now gathering pace in advance of a [[2009 United Nations Climate Change Conference|meeting in Copenhagen]] in December 2009.<ref>{{cite web|url=http://www.guardian.co.uk/environment/2009/apr/14/global-warming-target-2c|quote=The poll comes as UN negotiations to agree a new global treaty to regulate carbon pollution gather pace in advance of a key meeting in Copenhagen in December. Officials will try to agree a successor to the Kyoto protocol, the first phase of which expires in 2012.|title=World will not meet 2C warming target, climate change experts agree|publisher=Guardian News and Media Limited|last=Adam|first=David|date=14 April 2009|accessdate=2009-04-14}}</ref>

Many environmental groups encourage [[Individual and political action on climate change|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.<ref>{{cite web|title=Climate Control: a proposal for controlling global greenhouse gas emissions|publisher=Sustento Institute|url=http://sustento.org.nz/wp-content/uploads/2007/06/climate-control.pdf|format=PDF|accessdate=2007-12-10}}</ref><ref>{{cite web|title=Rigged - The climate talks are a stitch-up, as no one is talking about supply.|first=George|last=Monbiot|url=http://www.monbiot.com/archives/2007/12/11/rigged/|accessdate=2007-12-22}}</ref>

There has also been [[business action on climate change]], including efforts to improve energy efficiency and limited moves towards use of [[alternative fuels]]. In January 2005 the European Union introduced its [[European Union Emission 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. United States President [[Barack Obama]] has announced plans to introduce an economy-wide [[cap and trade]] scheme.<ref>{{cite web|url=http://my.barackobama.com/page/content/newenergy|title=Barack Obama and Joe Biden: New Energy for America|accessdate=2008-12-19}}</ref>

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]], [[transport]]ation, [[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]].<ref name="IPCCAR4WG3SPM">{{cite web|url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf|format=PDF|title=Summary for Policymakers|work=Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change|date=2007-05-04|accessdate=2007-12-09}}</ref>

===Adaptation===

{{Main|Adaptation to global warming}}

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]],<ref>{{cite journal|author=Boland, John J.|year=1997|title=Assessing Urban Water Use and the Role of Water Conservation Measures under Climate Uncertainty|journal=Climatic Change|volume=37|issue=1|pages=157–176|doi=10.1023/A:1005324621274}}</ref> [[Outdoor water-use restriction|water rationing]], adaptive agricultural practices,<ref>{{cite journal|author=Adams, R.M., ''et al.''|year=1990|title=Global climate change and US agriculture|journal=Nature|volume=345|pages=219|doi=10.1038/345219a0}}</ref> construction of [[flood defences]],<ref>{{Cite journal|doi=10.1016/j.gloenvcha.2003.10.007|title=Coastal flooding and wetland loss in the 21st century: changes under the SRES climate and socio-economic scenarios|year=2004|author=Nicholls, R|journal=Global Environmental Change|volume=14|pages=69}}</ref> [[Colonization of Mars|Martian colonization]],<ref>Lovelock, James and Allaby, Michael, "''The Greening of Mars''" 1984</ref> changes to medical care,<ref>{{Cite journal|doi=10.1016/j.gloenvcha.2003.10.009|title=Climate change and malaria: analysis of the SRES climate and socio-economic scenarios|year=2004|author=Vanlieshout, M, R.S. Kovats, M.T.J. Livermore and P. Martens|journal=Global Environmental Change|volume=14|pages=87}}</ref> and interventions to protect [[Extinction risk from climate change|threatened species]]<ref>{{cite journal|author=Hulme, P.E.|year=2005|title=Adapting to climate change: is there scope for ecological management in the face of a global threat?|journal=Journal of Applied Ecology|volume=42|issue=5|pages=784|doi=10.1111/j.1365-2664.2005.01082.x}}</ref> 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]].<ref>{{cite web|url=http://www.imeche.org/NR/rdonlyres/FA401F02-3193-4A19-826A-3FEEFB89DEDE/0/ClimateChangeAdaptationReportIMechE.pdf|title=Climate Change: Adapting to the inevitable|publisher=IMechE|accessdate=2009-03-07}}</ref>

===Geoengineering===

{{Main|Geoengineering}}

Geoengineering is the deliberate modification of Earth's [[natural environment]] on a large scale to suit human needs.<ref>{{cite web|quote=...a controversial field known as geoengineering, which means rearranging the Earth's environment on a large scale to suit human needs and promote habitability|url=http://www.nytimes.com/2006/06/27/science/earth/27cool.html?ex=1151985600&en=ca9e39a26d7e4ece&ei=5065&partner=MYWAY|title=How to Cool a Planet (Maybe)|work=New York Times|date=27 June 2006|accessdate=10 March 2009|author=William J. Broad}}</ref> An example is [[greenhouse gas remediation]], which removes greenhouse gases from the atmosphere, usually through [[carbon sequestration]] techniques such as [[carbon dioxide air capture]].<ref>{{Cite journal|doi=10.1007/s10584-005-9026-x|title=Climate Strategy with CO2 Capture from the Air|year=2006|author=Keith, D.W., M. Ha-Duong and J.K. Stolaroff|journal=Climatic Change|volume=74|pages=17}}</ref> [[Solar radiation management]] reduces [[insolation]], such as by the addition of [[stratospheric sulfur aerosols (geoengineering)|stratospheric sulfur aerosols]].<ref>{{Cite journal|doi=10.1007/s10584-006-9101-y|title=Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?|year=2006|author=Crutzen, Paul J.|journal=Climatic Change|volume=77|pages=211}}</ref> No large-scale geoengineering projects have yet been undertaken.

==Debate and skepticism==

{{Main|Global warming controversy|Politics of global warming}}
{{See also|Scientific opinion on climate change|Climate change denial}}

{{Double image stack|right|GHG per capita 2000.svg|GHG by country 2000.svg|180|Per capita greenhouse gas emissions in 2000, including [[Land use, land-use change and forestry|land-use change]].|Per country [[greenhouse gas]] emissions in 2000, including land-use change.}}

Increased publicity of the scientific findings surrounding global warming has resulted in political and economic debate.<ref>{{Cite book|first=Spencer|last=Weart|author-link=Spencer R. Weart|contribution=The Public and Climate Change|contribution-url=http://www.aip.org/history/climate/Public.htm|title=The Discovery of Global Warming|editor-first=Spencer|editor-last=Weart|editor-link=Spencer R. Weart|url=http://www.aip.org/history/climate/index.html|publisher=American Institute of Physics|year=2006|access-date=2007-04-14}}</ref> 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.<ref>{{cite news|title=Poor Nations to Bear Brunt as World Warms|first=Andrew|last=Revkin|date=2007-04-01|publisher=The New York Times|url=http://www.nytimes.com/2007/04/01/science/earth/01climate.html?ex=1333080000&en=6c687d64add0b7ba&ei=5088&partner=rssnyt&emc=rss|accessdate=2007-05-02}}</ref> The exemption of [[developing country|developing countries]] from [[Kyoto Protocol]] restrictions has been used to rationalize non-ratification by the U.S. and criticism from Australia.<ref>{{cite web|title=China's emissions may surpass the US in 2007|first=Catherine|last=Brahic|date=2006-04-25|publisher=New Scientist|url=http://environment.newscientist.com/article/dn11707-chinas-emissions-to-surpass-the-us-within-months.html|accessdate=2007-05-02}}</ref> Another point of contention is the degree to which [[Newly industrialized country|emerging economies]] such as India and China should be expected to constrain their emissions.<ref>{{citenews|title=US envoy says China wants top line US technology in exchange for reining in CO2 emissions|first=Arthur|last=Max|url=http://www.startribune.com/world/47783247.html?elr=KArks7PYDiaK7DUjyDD:_HP5P:QUiD3aPc:_Yyc:aUU|agency=Associated Press|work=Star Tribune|accessdate=2009-06-26}}</ref> The U.S. contends that if it must bear the cost of reducing emissions, then China should do the same<ref>{{cite news|url=http://news.bbc.co.uk/2/hi/science/nature/6610653.stm|title=Chinese object to climate draft|date=2008-05-01|publisher=BBC|accessdate=2009-05-21}}</ref><ref>{{cite news|url=http://www.washingtonpost.com/wp-dyn/content/article/2007/06/05/AR2007060502546.html|title=In Battle for U.S. Carbon Caps, Eyes and Efforts Focus on China|last=Mufson|first=Steven|date=2007-06-06|publisher=The Washington Post|accessdate=2009-05-21}}</ref> since China's [[List of countries by carbon dioxide emissions|gross national CO2 emissions]] now exceed those of the U.S.<ref>{{cite news|url=http://news.bbc.co.uk/2/hi/asia-pacific/7347638.stm|title=China now top carbon polluter|publisher=BBC News|date=2008-04-14|accessdate=2008-04-22}}</ref><ref>{{cite news|url=http://www.usatoday.com/tech/science/2007-06-20-124188869_x.htm|title=Group: China tops world in CO2 emissions|agency=Associated Press|work=USA Today|date=2007-06-20|accessdate=2007-10-16}}</ref><ref>{{cite news|url=http://www.livemint.com/2007/06/20235536/China-surpassed-US-in-carbon-e.html|title=Group: China surpassed US in carbon emissions in 2006: Dutch report|agency=Reuters|work=livemint.com|date=2007-06-20|accessdate=2007-10-16}}</ref> China has contended that it is less obligated to reduce emissions since its per capita responsibility and [[List of countries by carbon dioxide emissions per capita|per capita emissions]] are less that of the U.S.<ref>{{cite web|last=Casey|first=Michael|title=China Says West Should Deal With Warming|publisher=Newsvine|date=2007-12-07|url=http://www.newsvine.com/_news/2007/12/07/1147788-china-says-west-should-deal-with-warming|accessdate=2009-06-06}}</ref> India, also exempt, has made similar contentions.<ref>{{citeweb|author=IANS|title=India can’t be exempt from mandatory greenhouse gas emission cap: John Kerry|url=http://www.thaindian.com/newsportal/uncategorized/india-cant-be-exempt-from-mandatory-greenhouse-gas-emission-cap-john-kerry_100151668.html|publisher=Thaindian.com|date=2009-02-05|accessdate=2009-06-24}}</ref>

In 2007-2008 the [[Gallup Poll]]s surveyed 127 countries. Over a third of the world's population were unaware of global warming, developing countries less aware than [[Developed country|developed]], and Africa the least aware. 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.<ref>{{cite web|first=Brett|last=Pelham|title=Awareness, Opinions About Global Warming Vary Worldwide|url=http://www.gallup.com/poll/117772/Awareness-Opinions-Global-Warming-Vary-Worldwide.aspx|publisher=Gallup|date=2009-04-22|accessdate=2009-07-14}}</ref> In the western world, the concept and the appropriate responses are contested. Nick Pidgeon of [[Cardiff University]] finds that "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.<ref name="pewpoll">{{cite web|title=Summary of Findings|work=Little Consensus on Global Warming. Partisanship Drives Opinion|publisher=Pew Research Center|date=2006-07-12|accessdate=2007-04-14|url=http://people-press.org/reports/display.php3?ReportID=280}}</ref>

Debates weigh the benefits of limiting [[industrial process|industrial]] [[Air pollution|emissions]] of greenhouse gases against the [[Economics of global warming|costs]] that such changes would entail.<ref name="IPCCAR4WG3SPM"/> Using [[Emissions trading|economic incentives]], alternative and renewable energy have been promoted to reduce emissions while building infrastructure.<ref>{{cite news|title=Breaking the Climate Deadlock|first=Tony|last=Blair|url=http://www.kosovotimes.net/project-syndicate/708-breaking-the-climate-deadlock.html|publisher=Kosovo Times|date=2009-07-03|accessdate=2009-07-03}}</ref><ref>{{cite news|title=Energy bill causing some tension among U.S. officials|first=Holly|last=Richards|url=http://www.coshoctontribune.com/article/20090702/NEWS01/907020302|publisher=Coshocton Tribune|date=2009-07-02|accessdate=2009-07-03}}</ref> Business-centered organizations such as the [[Competitive Enterprise Institute]], conservative commentators, and companies such as [[ExxonMobil]] have downplayed IPCC climate change scenarios, funded scientists who disagree with the [[Scientific opinion on climate change|scientific consensus]], and provided their own projections of the economic cost of stricter controls.<ref>{{cite news|last=Begley|first=Sharon|title=The Truth About Denial|publisher=Newsweek|date=2007-08-13|url=http://www.newsweek.com/id/32482|accessdate=2007-08-13}}</ref><ref>{{cite web|title=Royal Society tells Exxon: stop funding climate change denial|url=http://www.guardian.co.uk/environment/2006/sep/20/oilandpetrol.business|first=David|last=Adams|publisher=The Guardian|date=2006-09-20|accessdate=2007-08-09}}</ref><ref name="MSNBC 01-12">{{cite news|title=Exxon cuts ties to global warming skeptics|url=http://www.msnbc.msn.com/id/16593606|publisher=MSNBC|date=2007-01-12|accessdate=2007-05-02}}</ref><ref>{{cite news|title=Report: Big Money Confusing Public on Global Warming|url=http://abcnews.go.com/Technology/Business/story?id=2767979&page=1|last=Sandell|first=Clayton|publisher=ABC|date=2007-01-03|accessdate=2007-04-27}}</ref> 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.<ref>{{cite press release|title=New Report Provides Authoritative Assessment of National, Regional Impacts of Global Climate Change|publisher=U.S. Global Change Research Program|date=2009-06-06|format=PDF|url=http://www.globalchange.gov/images/cir/pdf/Climate-Impacts-PR_june-6-2009.pdf|accessdate=2009-06-27}}</ref> Some fossil fuel companies have scaled back their efforts in recent years,<ref>{{cite news|url=http://www.usatoday.com/weather/climate/globalwarming/2007-05-18-greenpeace-exxon_N.htm|title=Greenpeace: Exxon still funding climate skeptics|date=2007-05-18|publisher=USA Today|accessdate=2007-07-09}}</ref> or called for policies to reduce global warming.<ref>{{cite press release|url=http://www.ceres.org/news/news_item.php?nid=56|title=Global Warming Resolutions at U.S. Oil Companies Bring Policy Commitments from Leaders, and Record High Votes at Laggards|date=2004-04-28|publisher=Ceres|accessdate=2007-07-27}}</ref>

Some global warming skeptics in the science or political community [[List of scientists opposing the mainstream scientific assessment of global warming|dispute]] all or some of the global warming scientific consensus objecting to whether global warming is actually occurring, if human activity is truly to blame, and if the threat is as great a threat as has been alleged. Prominent global warming skeptics include [[Frederick Seitz]], [[Richard Lindzen]], [[Fred Singer]], [[Patrick Michaels]], [[John Christy]], and [[Robert Balling]].<ref>{{cite news|title=The Doubters of Global Warming|first=Oriana Zill|last=de Granados|url=http://www.pbs.org/wgbh/pages/frontline/hotpolitics/reports/skeptics.html|publisher=Frontline|location=|date=2007-04-24|accessdate=2009-07-31}}</ref><ref>{{cite news|title=Skeptics Dispute Climate Worries and Each Other|first=Andrew C.|last=Revkin|url=http://www.nytimes.com/2009/03/09/science/earth/09climate.html?pagewanted=1&_r=1&sq=global%20warming%20skeptic&st=cse&scp=1|publisher=New York Times|date=2009-03-08|accessdate=2009-07-31}}</ref><ref>{{citenews|title=Heretical thoughts about science and society|first=Freeman|last=Dyson|author2=Brockman, John (Editor)|url=http://www.edge.org/3rd_culture/dysonf07/dysonf07_index.html|publisher=Edge - the third culture|date=2007-08-08|accessdate=2009-07-31}}</ref>

{{Clear}}

==See also==
{{Portal|Global warming|Global Warming Map.jpg}}
* [[Glossary of climate change]]
* [[Index of climate change articles]]

==Notes==

{{Cnote2 Begin|list-style=upper-alpha|colwidth=40em}}
{{Cnote2|A|Increase is for years 1905 to 2005. Global surface temperature is defined in the [[AR4|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|uncertainty interval]].}}
{{Cnote2|B|The 2001 joint statement was signed by the national academies of science of [[Australia]], [[Belgium]], [[Brazil]], [[Canada]], [[the Caribbean]], [[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]], [http://www.nagt.org/index.html National Association of Geoscience Teachers], [[United States National Research Council|National Research Council (US)]], [[Royal Meteorological Society]], and [[World Meteorological Organization]].}}
{{Cnote2|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).}}
{{Cnote2 End}}

==References==

{{reflist|colwidth=30em}}

==Further reading==
{{refbegin}}
*{{cite book|title=Financial Risks of Climate Change|author=Association of British Insurers|year=2005-06|url=http://www.climatewise.org.uk/storage/610/financial_risks_of_climate_change.pdf|format=PDF}}
*{{cite journal|last=Barnett|first=Tim P.|coauthors=Adam, J. C.; Lettenmaier, D. P.|date=2005-11-17|title=Potential impacts of a warming climate on water availability in snow-dominated regions|journal=Nature|volume=438|issue=7066|pages=303–309|url=http://www.nature.com/nature/journal/v438/n7066/abs/nature04141.html|doi=10.1038/nature04141|Format=abstract}}
*{{cite journal|last=Behrenfeld|first=Michael J.|coauthors=''et al.''|date=2006-12-07|title=Climate-driven trends in contemporary ocean productivity|journal=Nature|volume=444|issue=7120|pages=752–755|url=http://www.icess.ucsb.edu/~davey/MyPapers/Behrenfeld_etal_2006_Nature.pdf|format=PDF|doi=10.1038/nature05317}}
*{{cite journal|first=Onelack|last=Choi|coauthors=Fisher, Ann|month=May|year=2005|title=The Impacts of Socioeconomic Development and Climate Change on Severe Weather Catastrophe Losses: Mid-Atlantic Region (MAR) and the U.S.|journal=Climate Change|volume=58|issue=1–2|pages=149–170|doi=10.1023/A:1023459216609|url=http://www.springerlink.com/content/m6308777613702q0/}}
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{{refend}}

==External links==
{{Wikipedia-Books}}
{{Sisterlinks|wikt=Global warming|b=Climate Change|q=Global warming|s=Global warming|commons=Category:Global warming|n=Category:Climate change|v=Global warming}}
'''Research'''
{{refbegin}}
*[http://www.ipcc.ch Intergovernmental Panel on Climate Change] — collection of IPCC reports
*[http://www.nas.edu/climatechange Climate Change at the National Academies] — repository for reports, workshops, and meetings
*[http://www.nature.com/climate/index.html Nature Reports Climate Change] — free-access web resource
*[http://www.metoffice.gov.uk/climatechange/ Met Office: Climate change] — UK National Weather Service
*[http://www.istl.org/01-fall/internet.html Global Science and Technology Sources on the Internet] — extensive commented list of internet resources
*[http://edgcm.columbia.edu/ Educational Global Climate Modelling] (EdGCM) — research-quality climate change simulator
*[http://discover.itsc.uah.edu/ DISCOVER] — satellite-based ocean and climate data since 1979 from NASA
*[http://www.globalwarmingart.com/ Global Warming Art] — collection of figures and images
{{refend}}

'''Educational'''
{{refbegin}}
*[http://green.nationalgeographic.com/environment/global-warming/gw-overview.html What Is Global Warming?] — by National Geographic
*[http://www.ncdc.noaa.gov/oa/climate/globalwarming.html Global Warming Frequently Asked Questions] — from NOAA
*[http://www.ucar.edu/news/features/climatechange/faqs.jsp Understanding Climate Change - Frequently Asked Questions] — from UCAR
*[http://climate.jpl.nasa.gov/ Global Climate Change: NASA's Eyes on the Earth] — from NASA's JPL and Caltech
*[http://ourworld.unu.edu/en/series/climate/ OurWorld 2.0] — from the United Nations University
*[http://www.pewclimate.org/ Pew Center on Global Climate Change] — business and politics
*[http://demonstrations.wolfram.com/BestEffortGlobalWarmingTrajectories/ Best Effort Global Warming Trajectories - Wolfram Demonstrations Project] — by Harvey Lam
*[http://www.koshland-science-museum.org/exhibitgcc/ Koshland Science Museum - Global Warming Facts and Our Future] — graphical introduction from National Academy of Sciences
*[http://www.aip.org/history/climate The Discovery of Global Warming - A History] — by Spencer R. Weart from The American Institute of Physics
{{refend}}
{{Global warming|state=expanded}}
{{DEFAULTSORT:Global Warming}}

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