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[[Image:Instrumental Temperature Record.png|thumb|280px|Global mean surface temperature anomaly relative to 1961–1990]]
[[Image:Global Warming Map.jpg|thumb|280px|Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980]]
<!-- Please keep the intro as a simple declarative sentence. Details should be placed later. -->
'''Global warming''' is the increase in the [[Instrumental temperature record|average temperature]] of the Earth's near-surface air and [[ocean]]s since the mid-twentieth century and its projected continuation.

The average global air temperature near the Earth's surface increased 0.74 [[Plus-minus sign|±]] 0.18&nbsp;°[[Celsius|C]] (1.33 ± 0.32&nbsp;°[[Fahrenheit|F]]) during the hundred years ending in 2005.<ref name="grida7">{{cite web | url= http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_SPM.pdf | format=[[Portable Document 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 | accessdate=2007-02-02 | date=[[2007-02-05]] | publisher=[[Intergovernmental Panel on Climate Change]]|quote=The updated hundred-year linear trend (1906 to 2005) of 0.74 °C [0.56 °C to 0.92 °C] is therefore larger than the corresponding trend for 1901 to 2000 given in the TAR of 0.6 °C [0.4 °C to 0.8 °C].}}</ref> The [[Intergovernmental Panel on Climate Change]] (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-twentieth century is very likely [[Attribution of recent climate change|due to]] the observed increase in [[anthropogenic]] greenhouse gas concentrations"<ref name="grida7" /> via the [[greenhouse effect]]. Natural phenomena such as [[solar variation]] combined with [[volcano]]es probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.<ref>
{{cite web | url= http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch09.pdf | format=[[Portable Document 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 | accessdate=2007-05-20 | date=[[2007-05-07]] | publisher=[[Intergovernmental Panel on Climate Change]] | last=Hegerl | first=Gabriele C. | coauthors=''et al.'' | pages=690
| quote=Recent estimates (Figure 9.9) indicate a relatively small combined effect of natural forcings on the global mean temperature evolution of the seconds half of the twentieth 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=[[2007-04-06]] | title=Solar influence on climate during the past millennium: Results from ransient 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=[[Portable Document Format|PDF]] | quote=However, because of a lack of interactive ozone, the model cannot fully simulate features discussed in (44)." "While the NH temperatures of the high-scaled experiment are often colder than the lower bound from proxy data, the modeled decadal-scale NH surface temperature for the medium-scaled case falls within the uncertainty range of the available temperature reconstructions. The medium-scaled simulation also broadly reproduces the main features seen in the proxy records." "Without anthropogenic forcing, the 20th century warming is small. The simulations with only natural forcing components included yield an early 20th century peak warming of ≈0.2&nbsp;°C (≈1950 AD), which is reduced to about half by the end of the century because of increased volcanism.}}

</ref>
These basic conclusions have been endorsed by at least thirty [[Scientific opinion on climate change|scientific societies and academies of science]],<ref>The 2001 joint statement was signed by the scientific academies 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. Professional societies include American Meteorological Society, American Geophysical Union, American Institute of Physics, American Astronomical Society, American Association for the Advancement of Science, Stratigraphy Commission of the Geological Society of London, Geological Society of America, American Chemical Society, and Engineers Australia.</ref> including all of the national academies of science of the [[G8|major industrialized countries]].<ref>{{cite web | title = The Science Of Climate Change | publisher = [[Royal Society]] | url = http://royalsociety.org/displaypagedoc.asp?id=13619 | accessdate = 2008-01-04 |date=May 2001}}</ref><ref>{{cite web | title = Joint science academies' statement: Global response to climate change | publisher = [[Royal Society]] | url = http://royalsociety.org/displaypagedoc.asp?id=20742 | accessdate = 2008-01-04 |date=June 2005}}</ref><ref>{{cite web | title = Joint science academies' statement on growth and responsibility: sustainability, energy efficiency and climate protection | publisher = [http://www.pik-potsdam.de/index_html Potsdam Institute for Climate Impact Research] | url = http://www.pik-potsdam.de/news-1/G8_Academies%20Declaration.pdf | accessdate = 2008-01-04 |date=May 2007}}</ref> While [[List of scientists opposing the mainstream scientific assessment of global warming|individual scientists]] have voiced disagreement with some findings of the IPCC,<ref>
{{cite web | title = Don't fight, adapt | publisher = [[National Post]] | url = http://www.nationalpost.com/news/story.html?id=164002 | accessdate = 2007-11-18 |date=December 2007}}
</ref> the overwhelming majority of scientists working on climate change agree with the IPCC's main conclusions.<ref>{{cite web | title = A guide to facts and fictions about climate change | publisher = [[Royal Society]] | url = http://www.royalsoc.ac.uk/downloaddoc.asp?id=1630 | accessdate = 2007-11-18 | date = March 2005 | quote = "However, the overwhelming majority of scientists who work on climate change agree on the main points"}}</ref><ref>{{cite web | title = Beyond the Ivory Tower: The Scientific Consensus on Climate Change | publisher = [[Science Magazine]] | url = http://www.sciencemag.org/cgi/content/full/306/5702/1686 | accessdate = 2008-01-04 |date=December 2004}}</ref>

[[Climate model]] projections summarized by the IPCC indicate that average global surface temperature will likely rise a further {{nowrap|1.1 to 6.4&nbsp;°C}} {{nowrap|(2.0 to 11.5&nbsp;°F)}} during the twenty-first century.<ref name="grida7" /> The range of values results from the use of differing [[Special Report on Emissions Scenarios|scenarios]] of future [[greenhouse gas]] emissions as well as models with differing [[climate sensitivity]]. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a thousand years even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.<ref name="grida7" />

Increasing global temperature will cause [[sea level rise|sea level to rise]], and is expected to increase the intensity of [[extreme weather]] events and to change the amount and pattern of [[precipitation (meteorology)|precipitation]]. Other [[effects of global warming]] include changes in [[agricultural yield]]s, trade routes, [[retreat of glaciers since 1850|glacier retreat]], species [[extinction risk from climate change|extinctions]] and increases in the ranges of [[Vector (biology)|disease vectors]].

Remaining scientific [[uncertainty|uncertainties]] include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. [[List of Kyoto Protocol signatories|Most national governments]] have signed and ratified the [[Kyoto Protocol]] aimed at reducing greenhouse gas emissions, but there is ongoing [[Politics of global warming|political]] and [[global warming controversy|public debate]] worldwide regarding what, if any, action should be taken to [[Mitigation of global warming|reduce or reverse]] future warming or to [[Adaptation to global warming|adapt]] to its expected consequences.

==Terminology==
The term "global warming" is a specific example of global [[climate change]]. The term "climate change" can also refer to other periods of overall temperature change such as [[global cooling]]. In common usage, the term "global warming" refers to the warming in recent decades and its projected continuation, and implies a human influence.<ref>http://www.merriam-webster.com/dictionary/global+warming</ref><ref>{{cite web | title = Climate Change: Basic Information | publisher = [[United States Environmental Protection Agency]] | url = http://epa.gov/climatechange/basicinfo.html | accessdate = 2007-02-09 | date = [[2006-12-14]] | quote = In common usage, 'global warming' often refers to the warming that can occur as a result of increased emissions of greenhouse gases from human activities.}}</ref> The [[United Nations Framework Convention on Climate Change]] (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.<ref>{{cite web | title = United Nations Framework Convention on Climate Change, Article I | publisher = [[United Nations Framework Convention on Climate Change]] | url = http://unfccc.int/essential_background/convention/background/items/2536.php | accessdate = 2007-01-15}}</ref> The term "anthropogenic global warming" (AGW) is sometimes used when focusing on human-induced changes.

==Causes==
[[Image:Radiative-forcings.svg|thumb|280px|right| Components of the current [[radiative forcing]] as estimated by the [[IPCC Fourth Assessment Report]].]]
{{main|Attribution of recent climate change|Scientific opinion on climate change}}

The Earth's climate changes in response to external forcing, including variations in its orbit around the [[Sun]] ([[orbital forcing]]),<ref>{{cite journal |last=Berger |first=A. |coauthors=''et al.'' |date=[[2005-12-10]] |title=On the origin of the 100-kyr cycles in the astronomical forcing |journal=Paleoceanography |volume=20 |issue=4 |pages= |id=PA4019 |url=http://www.agu.org/pubs/crossref/2005/2005PA001173.shtml |accessdate= 2007-11-05}}</ref><ref>{{cite journal |last=Genthon |first=C. |coauthors=''et al.'' |date=[[1987-10-01]] |title=Vostok Ice Core - Climatic response to CO<sub>2</sup> and orbital forcing changes over the last climatic cycle |journal=[[Nature (journal)|Nature]] |volume=329 |issue=6138 |pages=414–418 |url=http://www.nature.com/nature/journal/v329/n6138/abs/329414a0.html |accessdate= 2007-11-05}}</ref><ref>
{{cite journal |last=Alley |first=Richard B. |coauthors=''et al.'' |year=2002 |month=January |title=A northern lead in the orbital band: north-south phasing of Ice-Age events |journal=Quaternary Science Reviews |volume=21 |issue=1-3 |pages=431–441 |url=http://www.ingentaconnect.com/content/els/02773791/2002/00000021/00000001/art00072 |accessdate= 2007-11-05}}
</ref> [[volcano|volcanic]] eruptions,<ref>Robock, Alan, and Clive Oppenheimer, Eds., 2003: Volcanism and the Earth’s Atmosphere, Geophysical Monograph 139, American Geophysical Union, Washington, DC, 360 pp.</ref> and atmospheric [[greenhouse gas]] concentrations. The detailed [[attribution of recent climate change|causes of the recent warming]] remain an active field of research, but the [[scientific opinion on climate change|scientific consensus]]<ref>
{{cite web |title=Joint science academies' statement: The science of climate change | url=http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13619 | format=[[Active Server Pages|ASP]] | quote=The work of the Intergovernmental Panel on Climate Change (IPCC) represents the consensus of the international scientific community on climate change science| publisher = [[Royal Society]] | date =[[2001-05-17]] |accessdate=2007-04-01}}
</ref><ref>{{cite journal | date=[[2007-10-18]] | title=Rising to the climate challenge | journal=[[Nature (journal)|Nature]] | volume=449 | issue=7164 | page=755 | url=http://www.nature.com/nature/journal/v449/n7164/full/449755a.html | accessdate=2007-11-06}}</ref>
is that the increase in atmospheric greenhouse gases due to human activity caused most of the warming observed since the start of the industrial era. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain most of the temperature increase. One such hypothesis proposes that warming may be the result of variations in solar activity.<ref name="Svensmark2007">{{cite journal | first=Henrik | last=Svensmark | authorlink=Henrik Svensmark | year = 2007 | month = February | title = Cosmoclimatology: a new theory emerges | journal = Astronomy & Geophysics | volume = 48 | issue = 1 | pages = 18-24 | doi = 10.1111/j.1468-4004.2007.48118.x | url=http://www.spacecenter.dk/research/sun-climate/Scientific%20work%20and%20publications/svensmark_2007cosmoClimatology.pdf}}</ref><ref>{{cite web | url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch02.pdf | last=Forster | first=Piers | coauthors=''et al.'' | format=[[Portable Document Format|PDF]] | title=Changes in Atmospheric Constituents and in Radiative Forcing | 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 | pages=188-193 | accessdate=2007-09-17 | date=[[2007-02-05]] | publisher=[[Intergovernmental Panel on Climate Change]]}}</ref><ref>
{{cite journal |last=Bard |first=Edouard |coauthors=Frank, Martin |date=[[2006-06-09]] |title=Climate change and solar variability: What's new under the sun? |journal=Earth and Planetary Science Letters |volume=248 |issue=1-2 |pages=1-14 |url=http://www.ifm-geomar.de/fileadmin/personal/fb1/p-oz/mfrank/Bard_and_Frank_2006.pdf |accessdate= 2007-09-17}}</ref>

None of the effects of forcing are instantaneous. The [[Volumetric heat capacity|thermal inertia]] of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. [[Climate commitment]] studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about {{nowrap|0.5&nbsp;°C}} {{nowrap|(0.9&nbsp;°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 (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=[[Portable Document Format|PDF]] | accessdate=2007-02-11}}
</ref>

=== Greenhouse gases in the atmosphere ===
{{main|Greenhouse gas|Greenhouse effect}}

The greenhouse effect was discovered by [[Joseph Fourier]] in 1824 and was first investigated quantitatively by [[Svante Arrhenius]] in 1896. It is the process by which [[Absorption (electromagnetic radiation)|absorption]] and emission of [[infrared]] radiation by [[Atmosphere|atmospheric gases]] warm a [[planet]]'s lower atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33&nbsp;°C (59&nbsp;°F), without which Earth would be uninhabitable.<ref name="IPCC_WG1_AR4_Ch1">{{cite web | title=IPCC WG1 AR4 Report — Chapter 1: Historical Overview of Climate Change Science | url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch01.pdf | format=[[Portable Document Format|PDF]] | accessdate=2007-10-07 | 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><ref>Note that the Greenhouse Effect produces a temperature '''increase''' of about
33 °C (59 °F) with respect to black body predictions and not a '''surface temperature''' of 33 °C (91 °F) which is {{convert|32|°F|°C|abbr=on}} higher. The average surface temperature is about 14 °C (57 °F).
Also note that both the
Celsius and Fahrenheit temperatures are expressed to 2 significant figures even though the conversion formula produces 3.
</ref> On Earth, the major greenhouse gases are [[water vapor]], which causes about 36–70% of the greenhouse effect ([[Cloud forcing|not including clouds]]); [[carbon dioxide]] (CO<sub>2</sub>), which causes 9–26%; [[methane]] (CH<sub>4</sub>), which causes 4–9%; and [[ozone]], which causes 3–7%.<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| coauthors= Kevin E. Trenberth| format=PDF | journal=Bulletin of the American Meteorological Society| pages=197-208| volume=78| issue=2| month=February| year=1997| accessdate=2006-05-01}}</ref><ref>{{cite web| url=http://www.realclimate.org/index.php?p=142| year=6 Apr 2005| title=Water vapour: feedback or forcing?| publisher=[[RealClimate]]| accessdate=2006-05-01}}</ref>
The issue is how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of some greenhouse gases.

Human activity since the industrial revolution has increased the concentration of various greenhouse gases, leading to increased [[radiative forcing]] from CO<sub>2</sub>, [[methane]], tropospheric [[ozone]], [[CFC]]s and [[nitrous oxide]]. Molecule for molecule, [[methane]] is a more effective greenhouse gas than [[carbon dioxide]], but its concentration is much smaller so that its total [[radiative forcing]] is only about a fourth of that from carbon dioxide. Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, [[nitrous oxide]] (N<sub>2</sub>O), is increasing in concentration owing to human activity such as agriculture. The [[Greenhouse gas#Increase of greenhouse gases|atmospheric concentrations]] of CO<sub>2</sub> and CH<sub>4</sub> have increased by 31% and 149% respectively since the beginning of the [[industrial revolution]] in the mid-1700s. 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>Neftel, A., E. Moor, H. Oeschger, and B. Stauffer. (1985). [http://www.nature.com/nature/journal/v315/n6014/abs/315045a0.html "Evidence from polar ice cores for the increase in atmospheric CO2 in the past two centuries"]. ''Nature'' 315:45-47.</ref> From less direct geological evidence it is believed that CO<sub>2</sub> values this high were last attained 20 million years ago.<ref>
{{cite journal| first=Paul N.| last=Pearson| coauthors=Palmer, Martin R.| journal=[[Nature (journal)|Nature]]| title= Atmospheric carbon dioxide concentrations over the past 60 million years| date=[[2000-08-17]]| volume=406| issue=6797| pages=695–699| url=http://www.nature.com/nature/journal/v406/n6797/abs/406695a0.html| doi=10.1038/35021000}}
</ref>
[[Fossil fuel]] burning has produced about three-quarters of the increase in CO<sub>2</sub> from human activity over the past 20 years. Most of the rest is due to land-use change, in particular [[deforestation]].<ref>
{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/006.htm |title=Summary for Policymakers |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=2007-01-18 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}
</ref>

[[Image:Mauna Loa Carbon Dioxide.png|thumb|280px|right|Recent increases in atmospheric carbon dioxide (CO<sub>2</sub>). The monthly CO<sub>2</sub> measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the [[Northern Hemisphere]]'s late spring, and declines during the Northern Hemisphere growing season as plants remove some CO<sub>2</sub> from the atmosphere.]]
The present atmospheric concentration of CO<sub>2</sub> is about 385 parts per million ([[parts-per notation|ppm]]) by volume.<ref>
{{cite web | title = Trends in Atmospheric Carbon Dioxide – Mauna Loa | last = Tans | first = Pieter | url = http://www.esrl.noaa.gov/gmd/ccgg/trends/ | publisher = [[National Oceanic and Atmospheric Administration]] | accessdate = 2008-02-15}}
</ref>
Future CO<sub>2</sub> levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC [[Special Report on Emissions Scenarios]] gives a wide range of future CO<sub>2</sub> 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 |last = Prentice |first = I. Colin |coauthors = ''et al.'' |title = 3.7.3.3 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=2007-04-28 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}
</ref>
Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if [[coal]], [[tar sands]] or [[methane clathrate]]s are extensively used.<ref>
{{cite web |url=http://www.grida.no/climate/ipcc/emission/104.htm |title=4.4.6. Resource Availability |work=IPCC Special Report on Emissions Scenarios |accessdate=2007-04-28 |publisher=[[Intergovernmental Panel on Climate Change]]}}
</ref>

===Feedbacks===
{{main|Effects of global warming#Positive feedback effects}}
The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. Warming by the addition of long-lived greenhouse gases such as CO<sub>2</sub> will cause more water to evaporate into the atmosphere. Since water vapor itself acts as a greenhouse gas, the atmosphere warms further; this warming causes more water vapor to evaporate (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 CO<sub>2</sub> 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 | page= 3354–3360 | url= http://www.gfdl.noaa.gov/reference/bibliography/2006/bjs0601.pdf | format= [[Portable Document 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}}
</ref>
This feedback effect can only be reversed slowly as CO<sub>2</sub> has a long average [[Greenhouse gas#Removal from the atmosphere and global warming potential|atmospheric lifetime]].

Feedback effects due to clouds are an area of ongoing research. 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. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the [[IPCC Fourth Assessment Report]].<ref name="soden1" />

A subtler feedback process relates to changes in the [[lapse rate]] as the atmosphere warms. The atmosphere's temperature decreases with height in the [[troposphere]]. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation emitted from the upper atmosphere is less than that emitted from the lower atmosphere. Most of the radiation emitted from the upper atmosphere escapes to space, while most of the radiation emitted from the lower atmosphere is re-absorbed by the surface or the atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height: if the rate of temperature decrease is greater the greenhouse effect will be stronger, and if the rate of temperature decrease is smaller then the greenhouse effect will be weaker. Both theory and climate models indicate that warming will reduce the decrease of temperature with height, producing a negative ''lapse rate feedback'' that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.<ref>[http://www.nap.edu/catalog.php?record_id=10850 Panel on Climate Change Feedbacks, Climate Research Committee, National Research Council, 2004: Understanding Climate Change Feedbacks.]</ref>

Another important feedback process is ice-albedo feedback.<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=[[Intergovernmental Panel on Climate Change]]}}
</ref>
When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

[[Positive feedback]] due to release of CO<sub>2</sub> and CH<sub>4</sub> from thawing [[permafrost]], such as the frozen [[peat]] [[bog]]s in [[Siberia]], is an additional mechanism that could contribute to warming.<ref>{{cite news | first=Ian | last=Sample | title=Warming Hits 'Tipping Point' | date=[[2005-08-11]] | url=http://www.guardian.co.uk/climatechange/story/0,12374,1546824,00.html | publisher=[[The Guardian]] | accessdate=2007-01-18}}</ref> Similarly a massive release of CH<sub>4</sub> from [[methane clathrate]]s in the ocean could cause rapid warming, according to the [[clathrate gun hypothesis]].

The ocean's ability to sequester carbon is expected to decline as it warms. This is because the resulting low nutrient levels of the [[mesopelagic zone]] (about 200 to 1000 m depth) limits the growth of [[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 (journal)|Science]] |volume=316 |issue=5824 |pages=567–570 |url=http://www.sciencemag.org/cgi/content/abstract/316/5824/567 |accessdate= 2007-11-16}}</ref>

===Solar variation===
[[Image:Solar-cycle-data.png|thumb|280px|right|Solar variation over the last thirty years.]]
{{main|Solar variation}}
A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at [[Duke University]], Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.<ref>
{{cite journal | first=Nicola | last=Scafetta | coauthors=West, Bruce J. | title=Phenomenological solar contribution to the 1900–2000 global surface warming | url = http://www.fel.duke.edu/~scafetta/pdf/2005GL025539.pdf | format = [[Portable Document Format|PDF]] | date=[[2006-03-09]] | journal=[[Geophysical Research Letters]] | volume=33 | issue=5 | id=L05708 | doi=10.1029/2005GL025539 | accessdate=2007-05-08}}
</ref> A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.<ref>{{Cite journal | first=Peter A. | last=Stott | coauthors=''et al.'' | title=Do Models Underestimate the Solar Contribution to Recent Climate Change? | date=[[2003-12-03]] | journal=[[Journal of Climate]] | volume=16 | issue=24 | pages=4079–4093 | doi=10.1175/1520-0442(2003)016%3C4079:DMUTSC%3E2.0.CO;2 | accessdate=2007-04-16 | url=http://climate.envsci.rutgers.edu/pdf/StottEtAl.pdf}}</ref>
They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in [[solar variation|solar output]], possibly amplified by cloud seeding via [[galactic cosmic ray]]s, may have contributed to recent warming.<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=[[Portable Document Format|PDF]] | doi=10.1023/A:1026723423896 | accessdate=2007-04-17}}
</ref> It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.<ref>{{cite journal |last=Svensmark |first=Henrik |authorlink=Henrik Svensmark |year=2000 | month=July |format=[[Portable Document Format|PDF]] |title=Cosmic Rays and Earth's Climate |journal=Space Science Reviews |volume=93 |issue=1-2 |pages=175-185 |url=http://winnetou.lcd.lu/physique/OSCIE2003/global_warming/Cosmic_rays_and_Earth_Climate_new_sven0606.pdf |accessdate= 2007-09-17}}</ref>

One predicted effect of an increase in solar activity would be a warming of most of the [[stratosphere]], whereas greenhouse gas theory predicts cooling there.<ref>{{cite web | url= http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter9.pdf | format=[[Portable Document 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 | accessdate=2008-02-01 | publisher=[[Intergovernmental Panel on Climate Change]] | last=Hegerl | first=Gabriele C. | coauthors=''et al.'' | pages=675}}
</ref> The observed trend since at least 1960 has been a cooling of the lower stratosphere.<ref>{{cite web|title=Climate Change 2001:Working Group I: The Scientific Basis (Fig. 2.12)|url=http://www.grida.no/climate/ipcc_tar/wg1/fig2-12.htm|date=2001|accessdate=2007-05-08}}
</ref> [[Ozone depletion|Reduction of stratospheric ozone]] also has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.<ref>[http://www.nas.nasa.gov/About/Education/Ozone/history.html Ozone History<!-- Bot generated title -->]</ref> Solar variation combined with changes in [[volcano|volcanic activity]] probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.<ref name="grida7" /> In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last thirty years. This effect is far too small to contribute significantly to global warming.<ref>
{{cite journal | first=Peter | last=Foukal | coauthors=''et al.'' | title=Variations in solar luminosity and their effect on the Earth's climate. | date=[[2006-09-14]] | journal=[[Nature]] | accessdate=2007-04-16 | url=http://www.nature.com/nature/journal/v443/n7108/abs/nature05072.html}}</ref><ref>{{cite pressrelease | title=Changes in Solar Brightness Too Weak to Explain Global Warming | url=http://www.ucar.edu/news/releases/2006/brightness.shtml#| publisher=[[National Center for Atmospheric Research]] | date=[[2006-09-14]] | accessdate=2007-07-13 }}</ref> A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.<ref>{{cite journal
| last = Lockwood
| first = Mike
| authorlink =
| 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 =
| issue =
| pages =
| date =
| 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
| id =
| accessdate = 2007-07-21 }}</ref> [[Henrik Svensmark]] and [[Eigil Friis-Christensen]], the main proponents of cloud seeding by galactic cosmic rays, disputed this criticism of their hypothesis.<ref>[http://www.spacecenter.dk/publications/scientific-report-series/Scient_No._3.pdf/view Reply to Lockwood and Fröhlich - The persistent role of the Sun in climate forcing — Spacecenter<!-- Bot generated title -->]</ref> A 2007 paper found that in the last 20 years there has been no significant link between changes in cosmic rays coming to Earth and cloudiness and temperature.<ref>{{cite news | publisher=[[BBC News Online]] | title='No Sun link' to climate change | url=http://news.bbc.co.uk/2/hi/science/nature/7327393.stm | author=Richard Black | date=[[3 April]] [[2008]]}}</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 | url=http://stacks.iop.org/1748-9326/3/024001}}<!-- remove url once doi starts working--></ref><ref>[http://arxiv.org/abs/0803.2298 preprint of this paper can be found here]</ref>

==Temperature changes==
[[Image:2000 Year Temperature Comparison.png|thumb|280px|right|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.]]
{{main|Temperature record}}

===Recent===
Global temperatures on both land and sea have increased by {{nowrap|0.75&nbsp;°C (1.35&nbsp;°F)}} relative to the period 1860–1900, according to the [[instrumental temperature record]]. This measured temperature increase is not significantly affected by the [[urban heat island]] effect.<ref>[http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter3.pdf Working group I, section 3.2.2.2 of the 2007 IPPC page 243]</ref> Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25&nbsp;°C per decade against 0.13&nbsp;°C per decade).<ref>
{{cite journal| last = Smith | first = Thomas M. | coauthors= Reynolds, Richard W. | title = A Global Merged Land–Air–Sea Surface Temperature Reconstruction Based on Historical Observations (1880–1997) | journal = [[Journal of Climate]] |volume = 18 |issue = 12 | issn = 0894-8755 | pages = 2021–2036 | url = http://www.ncdc.noaa.gov/oa/climate/research/Smith-Reynolds-dataset-2005.pdf | format = [[Portable Document Format|PDF]] | date = [[2005-05-15]] | accessdate = 2007-03-14}}
</ref>
Temperatures in the lower [[troposphere]] have increased between 0.12 and 0.22&nbsp;°C (0.22 and 0.4&nbsp;°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 possibly regional fluctuations such as the [[Medieval Warm Period]] or the [[Little Ice Age]].

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.<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}}</ref> The [[Northern Hemisphere]] has more land than the [[Southern Hemisphere]], so it warms faster. The Northern Hemisphere also has extensive areas of seasonal snow and sea-ice cover subject to the ice-albedo feedback. More greenhouse gases are emitted in the Northern than Southern Hemisphere, but this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.

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 = [[Portable Document Format|PDF]]}}
</ref><ref>
{{cite web |url=http://grdc.bafg.de/servlet/is/4226/Pressemitteilung-WMO-23-Dez-05-743_E1.pdf |format=[[Portable Document Format|PDF]] |title=WMO STATEMENT ON THE STATUS OF THE GLOBAL CLIMATE IN 2005 |accessdate=2007-04-13 |date=[[2005-12-15]] |publisher=[[World Meteorological Organization]]}}
</ref> Temperatures in 1998 were unusually warm because the strongest [[El Niño]] in the past century occurred during that year.<ref>[http://data.giss.nasa.gov/gistemp/2005/ Data @ NASA GISS: GISS Surface Temperature Analysis: 2005 Summation<!-- Bot generated title -->]</ref>

Anthropogenic emissions of other [[pollutant]]s—notably sulfate [[aerosol]]s—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,<ref>
{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/462.htm |last = Mitchell |first = J. F. B. |coauthors = ''et al.'' |title = 12.4.3.3 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=2007-01-04 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}
</ref>
though the cooling may also be due in part to natural variability. [[James Hansen]] and colleagues have proposed that the effects of the products of fossil fuel combustion—CO<sub>2</sub> and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO<sub>2</sub> greenhouse gases.<ref>[http://www.pnas.org/cgi/content/full/97/18/9875 Global warming in the twenty-first century: An alternative scenario - Hansen et al. 97 (18): 9875 - Proceedings of the National Academy of Sciences<!-- Bot generated title -->]</ref>

Paleoclimatologist [[William Ruddiman]] has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.<ref>
{{cite journal |last=Ruddiman |first=William F. |authorlink=William Ruddiman |title=How Did Humans First Alter Global Climate? |volume=292 |issue=3 |journal=[[Scientific American]] |date=March 2005 |pages=46–53 |url=http://ccr.aos.wisc.edu/news/0305046.pdf |format=[[Portable Document Format|PDF]] |accessdate=2007-03-05}}
</ref>
Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.<ref>
{{cite journal |last=Schmidt |first=Gavin |authorlink=Gavin Schmidt |coauthors=''et al.'' |date=[[2004-12-10]] |title=A note on the relationship between ice core methane concentrations and insolation |journal=[[Geophysical Research Letters]] |volume=31 |issue=23 |id=L23206 |doi=10.1029/2004GL021083 |url=http://pubs.giss.nasa.gov/abstracts/2004/Schmidt_etal_2.html |accessdate=2007-03-05}}
</ref>

===Pre-human climate variations===
[[Image:Ice Age Temperature.png|thumb|280px|right|Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.]]
{{See|Paleoclimatology}}
{{See also|Snowball Earth}}
Earth has experienced warming and cooling many times in the past. The recent Antarctic [[European Project for Ice Coring in Antarctica|EPICA]] ice core spans 800,000 years, including eight glacial cycles timed by [[Milankovitch cycles|orbital variations]] with [[interglacial]] warm periods comparable to present temperatures.<ref>
{{cite journal | first=James | last=Hansen | coauthors=''et al.'' | url=http://www.pnas.org/cgi/reprint/103/39/14288.pdf | title=Global temperature change | journal=[[Proceedings of the National Academy of Sciences|PNAS]] | volume=103 | number=39 | pages=14288–14293 | date=[[2006-09-26]] | format=[[Portable Document Format|PDF]] | accessdate=2007-04-20 | doi:10.1073/pnas.060291103}}
</ref>

A rapid buildup of greenhouse gases amplified warming in the early [[Jurassic]] period (about 180 million years ago), with average temperatures rising by 5&nbsp;°C (9&nbsp;°F). Research by the [[Open University]] indicates that the warming caused the rate of rock [[weathering]] to increase by 400%. As such weathering locks away carbon in [[calcite]] and [[dolomite]], CO<sub>2</sub> levels dropped back to normal over roughly the next 150,000 years.<ref>
{{cite press release |title=The Open University Provides Answers on Global Warming |publisher=[[Open University]] |date=[[2004-01-30]] |url=http://www3.open.ac.uk/earth-sciences/downloads/Press%20Release.pdf |format=[[Portable Document Format|PDF]] |accessdate=2007-03-04}}
</ref><ref>
{{cite journal | last = Cohen | first = Anthony S. | coauthors = ''et al.'' | year = 2004 | month = February | title = Osmium isotope evidence for the regulation of atmospheric CO<sub>2</sub> by continental weathering | journal = [[Geology (journal)|Geology]] | volume = 32 | issue = 2 | pages = 157–160 | doi = 10.1130/G20158.1 | url = http://sheba.geo.vu.nl/~vonh/imagesanddata/data/Cohenetal2004.pdf | format = [[Portable Document Format|PDF]] | accessdate = 2007-03-04}}
</ref>

Sudden releases of methane from [[clathrate compound]]s (the [[clathrate gun hypothesis]]) have been hypothesized as both a cause for and an effect of other warming events in the distant past, including the [[Permian–Triassic extinction event]] (about 251 million years ago) and the [[Paleocene–Eocene Thermal Maximum]] (about 55 million years ago).

==Climate models==
[[Image:Global Warming Predictions.png|thumb|280px|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 assumes no action is taken to reduce emissions.]]
[[Image:Global Warming Predictions Map.jpg|thumb|280px|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).]]
{{main|Global climate model}}

Scientists have studied global warming with [[computer models]] of the climate. These models are based on physical principles of fluid dynamics, [[radiative transfer]], and other processes, with simplifications being necessary because of limitations in computer power and the [[complex system|complexity]] of the climate system. All modern climate models include an atmospheric model that is coupled to an ocean model and models for ice cover on land and sea. 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 | format=[[Portable Document 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=[[2007-02-05]] | publisher=[[Intergovernmental Panel on Climate Change]]}}</ref>These models predict that the effect of adding greenhouse gases is to produce a warmer climate.<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 | date = 2000 |publisher= Health Press}}</ref> However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of [[climate sensitivity]].

Including uncertainties in future greenhouse gas concentrations and climate modeling, 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" /> Models have also been 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.

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>
{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/007.htm |title=Summary for Policymakers |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=2007-04-28 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}
</ref>
These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made [[greenhouse gas]] emissions.

Global climate model projections of future climate are forced by imposed greenhouse gas emission scenarios, most often from the IPCC [[Special Report on Emissions Scenarios]] (SRES). Less commonly, models may also include a simulation of the [[carbon cycle]]; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200&nbsp;ppm of CO<sub>2</sub>). Some observational studies also show a positive feedback.<ref>
{{cite journal |last=Torn |first=Margaret |coauthors=Harte, John |date=[[2006-05-26]] |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}}
</ref><ref>
{{cite journal |last=Harte |first=John |coauthors=''et al.'' |date=[[2006-10-30]] |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}}
</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 = [[2006-05-26]] |accessdate = 2007-05-04}}
</ref>

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.<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=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}
</ref>

==Attributed and expected effects==
[[Image:Glacier Mass Balance.png|left|thumb|280px|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]].]]
{{main|Effects of global warming}}
Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader [[effects of global warming|changes]], including [[Glacier mass balance|glacial retreat]], [[Arctic shrinkage]], and worldwide [[sea level rise]]. Changes in the amount and pattern of [[precipitation (meteorology)|precipitation]] may result in [[flood]]ing and [[drought]]. There may also be changes in the frequency and intensity of [[extreme weather]] events. Other effects may include changes in agricultural yields, addition of new trade routes,<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> reduced summer [[streamflow]]s, species [[extinction risk from climate change|extinctions]], and increases in the range of [[Vector (biology)|disease vectors]].

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 [[Effects of global warming#More extreme weather|extreme weather events]], are being attributed 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 = [[Intergovernmental Panel on Climate Change]] |date = [[2001-02-16]] |accessdate = 2007-03-14}}
</ref>
While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and 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>

Increasing deaths, displacements, and economic losses projected due to [[extreme weather]] attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some benefits, such as fewer deaths due to cold exposure.<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 = [[Portable Document Format|PDF]] |publisher = [[Intergovernmental Panel on Climate Change]] |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, 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|110 to 770 millimeters}} {{nowrap|(0.36 to 2.5&nbsp;ft)}} between 1990 and 2100,<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/409.htm |last = Church |first = John A. |coauthors = ''et al.'' |title = Executive Summary of Chapter 11 |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=2005-12-19 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> [[Climate change and agriculture|repercussions to agriculture]], [[Shutdown of thermohaline circulation|possible slowing of the thermohaline circulation]], reductions in the [[ozone layer]], increased intensity of [[Effects of global warming#More extreme weather|hurricanes and extreme weather events]], [[Ocean acidification|lowering]] of ocean [[pH]], and the spread of diseases such as [[malaria]] and [[dengue fever]]. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be [[extinction risk from climate change|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 (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= [[Portable Document 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= [[Proceedings of the National Academy of Sciences|PNAS]] |volume= 99 |issue= 9 |pages= 6070–6074 |doi= 10.1073/pnas.052131199 |url= http://www.nd.edu/~hellmann/pnas.pdf |format= [[Portable Document Format|PDF]] |accessdate= 2007-03-29}}</ref> and one study suggests that projected rates of extinction are uncertain.<ref>{{cite journal |last=Botkin |first=Daniel B. |authorlink= |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}}</ref>

===Economic===
[[Image:IPCC AR4 WGIII GHG concentration stabilization levels.png|thumb|right|280px|The 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 the work of [[IPCC Fourth Assessment Report|IPCC AR4]].]]
{{main|Economics of global warming|Low-carbon economy}}
Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from [[United States dollar|US$]]-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).<ref name="WGII SPM AR4" />
One widely publicized report on potential economic impact is the [[Stern Review]]; it suggests that extreme weather might reduce global [[gross domestic product]] by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.<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 report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of [[discounting]] and its choices of scenarios,<ref>Tol and Yohe (2006) "A Review of the Stern Review" ''World Economics'' '''7'''(4): 233-50. See also other critiques in ''World Economics'' '''7'''(4).</ref> while others have supported the general attempt to quantify economic risk, even if not the specific numbers.<ref>{{web cite|url=http://delong.typepad.com/sdj/2006/12/do_unto_others.html|title= Do unto others...|author=[[J. Bradford DeLong]]}}</ref><ref>{{web cite|url=http://johnquiggin.com/wp-content/uploads/2006/12/sternreviewed06121.pdf|title=Stern and the critics on discounting|author=[[John Quiggin]]}}</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= [[Portable Document 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 |date=2002}}
</ref> Developing countries dependent upon agriculture will be particularly harmed by global warming.<ref>{{cite web | url= http://www.rff.org/Documents/RFF-Resources-164_Thomas%20Schelling.pdf | title = Thomas Schelling: Developing Countries Will Suffer Most from Global Warming| accessdate=2008-03-01 | work = Resources 164 }} </ref>

===Security===
In November 2007, the [[Center for Strategic and International Studies]] and the [[Center for a New American Security]] published a report highlighting the national security effects of climate change.<ref>{{cite web|author=Kurt M. Campbell, Jay Gulledge, J.R. McNeill, John Podesta, Peter Ogden, Leon Fuerth, R. James Woolsey, Alexander T.J. Lennon, Julianne Smith, Richard Weitz, Derek Mix|title=The Age of Consequences: The Foreign Policy and National Security Implications of Global Climate Change|url=http://www.cnas.org/attachments/contentmanagers/1278/CSIS-CNAS_AgeofConsequences_October07.pdf|format=PDF|date=Oktober 2007|accessdate=2007-11-06}}</ref> These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

==Adaptation and mitigation==
{{main|Adaptation to global warming|Mitigation of global warming|Kyoto Protocol}}

The [[Scientific opinion on climate change|broad agreement]] among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage [[Individual and political action on climate change|individual action]] against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO<sub>2</sub> 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 = [[Portable Document 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/ | format = HTML | accessdate = 2007-12-22}}</ref>

There has also been [[business action on climate change]], including efforts at increased energy efficiency and limited moves towards use of [[alternative fuels]]. One important innovation has been the development of greenhouse gas [[emissions trading]] through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming 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 globally and over 55% of global greenhouse gas emissions.<ref>{{cite web | url=http://unfccc.int/files/essential_background/kyoto_protocol/application/pdf/kpstats.pdf | format=[[Portable Document 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> Only the [[United States]] and [[Kazakhstan]] have not ratified the treaty, with the United States historically being the world's [[greenhouse gas emissions by the United States|largest emitter]] of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.<ref>[http://www.boston.com/news/science/articles/2007/05/14/climate_talks_face_international_hurdles/ Climate talks face international hurdles], by Arthur Max, Associated press, 5/14/07.</ref>

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like [[China]] and [[India]] from the treaty, [[George W. Bush]] contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.<ref>{{cite web |url=http://www.whitehouse.gov/news/releases/2001/03/20010314.html|title=Text of a Letter from the President to Senators Hagel, Helms, Craig, and Roberts |accessdate=2007-11-21 |author=[[George W. Bush]] |date=March 13, 2001|publisher=Office of the Press Secretary}}</ref> Bush has promoted improved energy technology as a means to combat climate change,<ref>[http://www.whitehouse.gov/stateoftheunion/2008/index.html State of the Union Address], retrieved 2008-01-28. "The United States is committed to strengthening our energy security and confronting global climate change. And the best way to meet these goals is for America to continue leading the way toward the development of cleaner and more energy-efficient technology."</ref> and various state and city governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis; an example of this being the [[Regional Greenhouse Gas Initiative]].<ref>{{cite web| url=http://www.rggi.org/ |title=Regional Greenhouse Gas Initiative| accessdate=2006-11-07}}</ref>

China and India, though exempt from its provisions as [[developing countries]], have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier [[Wen Jiabao]] has called on the nation to redouble its efforts to tackle [[pollution]] and global warming.<ref>{{cite web|url=http://www.planetark.com/dailynewsstory.cfm/newsid/43027/story.htm|title=Wen Urges Greater China Effort to Fight Pollution, July 10, 2007, Reuters, via planetark.com|accessdate=2007-07-27}}</ref>

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 [[IPCC Fourth Assessment Report]], they conclude 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% increase and 3% decrease in global [[gross domestic product]].<ref>{{cite web | url= http://arch.rivm.nl/env/int/ipcc/pages_media/FAR4docs/final_pdfs_ar4/SPM.pdf | format=[[Portable Document Format|PDF]] | title=Summary for Policymakers | work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | accessdate=2007-12-09 | date=[[2007-05-04]] | publisher=[[Intergovernmental Panel on Climate Change]]}}</ref>

==Social and political debate==
{{main|Global warming controversy|Politics of global warming|Economics of global warming}}
{{see also|Climate change denial}}
[[Image:GHG per capita 2000.svg|thumb|280px|Per capita greenhouse gas emissions in 2000, including [[Land use, land-use change and forestry|land-use change]].]]
[[Image:GHG by country 2000.svg|thumb|280px|Per country greenhouse gas emissions in 2000, including land-use change.]]
Over the past several years, increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate.<ref>{{Citation | 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 suggested effects of global warming, while their actual 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>
At the same time, [[developing country]] exemptions from provisions of the [[Kyoto Protocol]] have been criticized by the [[United States]] and [[Australia]], and used as part of a rationale for continued non-ratification by the U.S.<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>
In the [[Western world]], the idea of human influence on climate has gained wider acceptance in [[Europe]] than in the United States.<ref>
{{cite news | title=More in Europe worry about climate than in U.S., poll shows | first=Thomas | last=Crampton | date=[[2007-01-04]] | publisher=[[International Herald Tribune]] | url=http://www.iht.com/articles/2007/01/04/news/poll.php | accessdate = 2007-04-14}}</ref><ref>{{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>

The issue of climate change has sparked debate weighing the benefits of limiting [[industrial process|industrial]] [[Air pollution|emissions]] of [[greenhouse gas]]es against the [[Economics of global warming|costs]] that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting [[Alternative energy|alternative energy sources]] in order to reduce carbon emissions.<ref>{{cite news |url= http://news.bbc.co.uk/2/hi/europe/6432829.stm |title= EU agrees on carbon dioxide cuts |date= [[2007-03-09]] |publisher= [[BBC]] |accessdate=2007-05-04}}</ref> Organizations and companies such as the [[Competitive Enterprise Institute]] and [[ExxonMobil]] have emphasized more conservative climate change scenarios while highlighting the potential 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= [[American Broadcasting Company|ABC]] |date= [[2007-01-03]] |accessdate= 2007-04-27}}
</ref> Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential [[Effects of global warming|risks of climate change]] and promote the implementation of stricter controls. 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=April 28, 2004| publisher=Ceres|accessdate=2007-07-27}}</ref>

Another point of contention is the degree to which [[Newly industrialized country|newly developed economies]] such as [[India]] and [[China]] should be expected to constrain their emissions. China's [[List of countries by carbon dioxide emissions|gross national CO<sub>2</sub> emissions]] are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.<ref> {{cite news| url=http://www.usatoday.com/tech/science/2007-06-20-124188869_x.htm| title= Group: China tops world in CO2 emissions| publisher=[[Associated Press]] | date=[[2007-06-20]] | accessdate=2007-10-16}}; {{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| publisher=[[Reuters]] | date=[[2007-06-20]] | accessdate=2007-10-16}} </ref> China has contended that it has less of an obligation to reduce emissions since its [[List of countries by carbon dioxide emissions per capita|per capita emissions]] are roughly one-fifth that of the United States.<ref>[http://www.newsvine.com/_news/2007/12/07/1147788-china-says-west-should-deal-with-warming China: US should take lead on climate], by Michael Casey, Associated Press, via newsvine.com 12/7/07. </ref> India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.<ref>[http://www.oregonlive.com/science/oregonian/index.ssf?/base/science/1184718333231440.xml&coll=7 India's glaciers give grim message on warming], by Somni Sengupta, 7/17/07, New York Times via oregonlive.com.</ref> However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.<ref> [http://news.bbc.co.uk/2/hi/science/nature/6610653.stm Chinese object to climate draft], BBC, 5/1/07; [http://www.washingtonpost.com/wp-dyn/content/article/2007/06/05/AR2007060502546_2.html?hpid=topnews In Battle for U.S. Carbon Caps, Eyes and Efforts Focus on China],by Steven Mufson, Washington Post, 6/6/07. </ref>

==Related climatic issues==
{{main|Ocean acidification|global dimming|ozone depletion}}
A variety of issues are often raised in relation to global warming. One is [[ocean acidification]]. Increased atmospheric CO<sub>2</sub> increases the amount of CO<sub>2</sub> dissolved in the oceans.<ref>
{{cite web |url=http://science.hq.nasa.gov/oceans/system/carbon.html |title=The Ocean and the Carbon Cycle |accessdate=2007-03-04 |date=[[2005-06-21]] |work=[[NASA]]}}
</ref>
CO<sub>2</sub> dissolved in the ocean reacts with water to form [[carbonic acid]], resulting in 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=[[Portable Document Format|PDF]] |doi = 10.1029/2004JD005220 |accessdate=2007-04-28}}
</ref>
and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO<sub>2</sub>.<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>
Since organisms and ecosystems are adapted to a narrow range of pH, this raises [[extinction]] concerns, directly driven by increased atmospheric CO<sub>2</sub>, that could disrupt [[food chain#Food web|food webs]] and impact human societies that depend on marine ecosystem services.<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= [[Active Server Pages|ASP]] |accessdate= 2007-05-04}}
</ref>

[[Global dimming]], the gradual reduction in the amount of global direct [[irradiance]] at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.<ref name="grida7" />

[[Ozone depletion]], the steady decline in the total amount of [[ozone]] in Earth's [[stratosphere]], is frequently cited in relation to global warming. Although there are [[Ozone depletion#Ozone depletion and global warming|areas of linkage]], the relationship between the two is not strong.

==See also==
* [[Glossary of climate change]]

==Notes and references==
{{reflist|2}}

==Further reading==
{{Wikinewscat|Climate change}}
{{Portal|Environment}}
{{EnergyPortal}}
<div class="references-small">
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| title = Financial Risks of Climate Change
| author = Association of British Insurers
| year= 2005-06
| url=http://www.abi.org.uk/Display/File/Child/552/Financial_Risks_of_Climate_Change.pdf
| format = [[Portable Document Format|PDF]]
}}

* {{cite journal

| last = Barnett | first = Tim P.
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| date = [[2005-11-17]]
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| volume = 438 | issue = 7066 | pages = 303–309
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* {{cite journal

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| coauthors = Robert T. O'Malley, David A. Siegel, Charles R. McClain, Jorge L. Sarmiento, Gene C. Feldman, Allen G. Milligan, Paul G. Falkowski, Ricardo M. Letelier, Emanuel S. Boss
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| format = [[Portable Document Format|PDF]]
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}}

* {{cite journal

| first = Onelack | last= Choi
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* {{cite journal

| last=Hansen | first=James
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* {{cite journal

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* {{cite news

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* {{cite book

| last = Kenneth | first = James P.
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| url = https://www.agu.org/cgi-bin/agubooks?book=ASSP0542960
}}

* {{cite news

| last = Keppler | first = Frank
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| title = Global Warming - The Blame Is not with the Plants
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}}

* {{cite journal

| author = Kurzweil, Raymond
| authorlink = Raymond Kurzweil
| year = 2006–07
| title = Nanotech Could Give Global Warming a Big Chill
| journal = Forbes / Wolfe Nanotech Report
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| format = [[Portable Document Format|PDF]]
}}

* {{cite journal

| title = The effect of increasing solar activity on the Sun's total and open magnetic flux during multiple cycles: Implications for solar forcing of climate
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| coauthors = Y.M. Wang, N.R. Sheeley
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}}

* {{cite book

| last = Lerner | first = K. Lee
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* {{cite journal

| last = McLaughlin | first = Joseph B.
| coauthors = Angelo DePaola, Cheryl A. Bopp, Karen A. Martinek, Nancy P. Napolilli, Christine G. Allison, Shelley L. Murray, Eric C. Thompson, Michele M. Bird, John P. Middaugh
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| journal = [[New England Journal of Medicine]]
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| url = http://content.nejm.org/cgi/content/abstract/353/14/1463
}}''(online version requires registration)''

* {{cite journal

| last = Muscheler, Raimund
| coauthors = Fortunat Joos, Simon A. Müller, Ian Snowball
| date = [[2005-07-28]]
| title = Climate: How unusual is today's solar activity?
| journal = [[Nature (journal)|Nature]]
| volume = 436 | issue = 7012 | pages = 1084–1087
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* {{cite journal

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| date = [[2005-04-29]]
| title = Extracting a Climate Signal from 169 Glacier Records
| journal = [[Science (journal)|Science]]
| volume = 308 | issue = 5722 | pages = 675–677
| url=http://www.cosis.net/abstracts/EGU05/04572/EGU05-J-04572.pdf
| format = [[Portable Document Format|PDF]]
| doi = 10.1126/science.1107046
}}

* {{cite journal

| last = Oreskes | first = Naomi
| authorlink=Naomi Oreskes
| date = [[2004-12-03]]
| title = Beyond the Ivory Tower: The Scientific Consensus on Climate Change
| journal = [[Science (journal)|Science]]
| volume = 306 | issue = 5702 | pages = 1686
| url = http://www.sciencemag.org/cgi/reprint/306/5702/1686.pdf
| format = [[Portable Document Format|PDF]]
| doi = 10.1126/science.1103618
}}

* {{cite journal

| last = Purse | first = Bethan V.
| coauthors = Philip S. Mellor, David J. Rogers, Alan R. Samuel, Peter P. C. Mertens, Matthew Baylis
| title = Climate change and the recent emergence of bluetongue in Europe
| journal = [[Nature Reviews Microbiology]]
| volume = 3 | issue = 2 | pages = 171–181
| date = February 2005
| doi = 10.1038/nrmicro1090
| url=http://www.nature.com/nrmicro/journal/v3/n2/abs/nrmicro1090_fs.html
}}

* {{cite news

| last = Revkin | first = Andrew C
| date = [[2005-11-05]]
| title = Rise in Gases Unmatched by a History in Ancient Ice
| publisher = [[The New York Times]]
| url = http://www.nytimes.com/2005/11/25/science/earth/25core.html?ei=5090&en=d5078e33050b2b0c&ex=1290574800&adxnnl=1&partner=rssuserland&emc=rss
}}

* {{cite book

| last = Ruddiman | first = William F.
| authorlink=William Ruddiman
| date = [[2005-12-15]]
| title = Earth's Climate Past and Future
| location = New York
| publisher = [[Princeton University Press]]
| isbn = 0-7167-3741-8
| url = http://www.whfreeman.com/ruddiman/
}}

* {{cite book

| last = Ruddiman | first = William F.
| authorlink=William Ruddiman
| date = [[2005-08-01]]
| title = Plows, Plagues, and Petroleum: How Humans Took Control of Climate
| location = New Jersey
| publisher = [[Princeton University Press]]
| isbn = 0-691-12164-8
}}

* {{cite journal

| last = Solanki | first = Sami K.
| authorlink=Sami Solanki
| coauthors = I.G. Usoskin, B. Kromer, M. Schussler, J. Beer
| date = [[2004-10-23]]
| title = Unusual activity of the Sun during recent decades compared to the previous 11,000 years.
| journal = [[Nature (journal)|Nature]]
| volume = 431 | pages = 1084–1087
| url = http://cc.oulu.fi/%7Eusoskin/personal/nature02995.pdf
| format = [[Portable Document Format|PDF]]
| doi = 10.1038/nature02995
}}

* {{cite journal

| last = Solanki | first = Sami K.
| authorlink=Sami Solanki
| coauthors = I. G. Usoskin, B. Kromer, M. Schüssler, J. Beer
| date = [[2005-07-28]]
| title = Climate: How unusual is today's solar activity? (Reply)
| journal = [[Nature (journal)|Nature]]
| volume = 436
| pages = E4-E5
| url = http://cc.oulu.fi/%7Eusoskin/personal/sola_nature05.pdf
| format = [[Portable Document Format|PDF]]
| doi = 10.1038/nature04046
}}

* {{cite journal

| last = Sowers | first = Todd
| date = [[2006-02-10]]
| journal = Science
| volume = 311 | issue = 5762 | pages = 838–840
| title = Late Quaternary Atmospheric CH<sub>4</sub> Isotope Record Suggests Marine Clathrates Are Stable
| doi = 10.1126/science.1121235
}}

* {{cite journal

| last = Svensmark | first = Henrik
| authorlink=Henrik Svensmark
| coauthors = Jens Olaf P. Pedersen, Nigel D. Marsh, Martin B. Enghoff, Ulrik I. Uuggerhøj
| year = [[2007-02-08]]
| title = Experimental evidence for the role of ions in particle nucleation under atmospheric conditions
| journal = [[Proceedings of the Royal Society]] A
| volume = 463 | issue = 2078 | pages = 385–396
| publisher = FirstCite Early Online Publishing
| doi = 10.1098/rspa.2006.1773
}}''(online version requires registration)''

* {{cite journal

| last = Walter | first = K. M.
| coauthors = S. A. Zimov, Jeff P. Chanton, D. Verbyla, [[F. Stuart Chapin III|F. S. Chapin]]
| date = [[2006-09-07]]
| title = Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming
| journal = [[Nature (journal)|Nature]]
| volume = 443 | issue = 7107 | pages = 71–75
| doi = 10.1038/nature05040
}}

* {{cite journal

| last = Wang | first = Y.-M.
| coauthors = J.L. Lean, N.R. Sheeley
| date = [[2005-05-20]]
| title = Modeling the sun's magnetic field and irradiance since 1713
| journal = [[Astrophysical Journal]]
| volume = 625 | pages = 522–538
| url = http://climatesci.colorado.edu/publications/pdf/Wang_2005.pdf
| format = [[Portable Document Format|PDF]]
| doi = 10.1086/429689
}}</div>

== External links ==

; Scientific
* [http://www.ipcc.ch Intergovernmental Panel on Climate Change] and [http://www.planetark.com/dailynewsstory.cfm/newsid/44456/story.htm UN Climate Panel Report's Key Findings].
* [http://www.nature.com/climate/index.html Nature Reports Climate Change]
* [http://www.metoffice.gov.uk/research/hadleycentre/index.html The UK Met Office Hadley Centre site]
* [http://www.ncdc.noaa.gov/oa/climate/globalwarming.html#INTRO NOAA's Global Warming FAQ]
* [http://www.aip.org/history/climate Discovery of Global Warming] – An extensive introduction to the topic and the history of its discovery, written by [[Spencer R. Weart]]
* [http://news.bbc.co.uk/1/hi/world/6460635.stm Caution urged on climate 'risks']
* [http://www.virtualcentre.org/en/library/key_pub/longshad/A0701E00.htm Impact of Livestock on Global Warming (UN Report)]

; Educational
* {{HSW|global-warming}}
* [http://green.nationalgeographic.com/environment/global-warming/gw-overview.html What Is Global Warming?] – Shockwave presentation from [[National Geographic]]
* [http://edgcm.columbia.edu/ The EdGCM (Educational Global Climate Modelling) Project] – A free research-quality simulation for students, educators, and scientists alike, with a user-friendly interface that runs on desktop computers
* [http://pm-esip.msfc.nasa.gov/amsutemps/ Daily global temperatures and trends from satellites] – Interactive graphics from [[NASA]]
* [http://www.pewclimate.org/ The Pew Center on global climate change]
* [http://osulibrary.oregonstate.edu/specialcollections/events/2007paulingconference/video-s3-4-washington.html Video] of a talk by [[Warren Washington]] titled "The Evolution of Global Warming Science: From Ideas to Scientific Facts"

; Other
* [http://www.istl.org/01-fall/internet.html Science and Technology Sources on the Internet] – Extensive commented list of Internet resources
* [http://www.ucsusa.org/global_warming/ Union of Concerned Scientists Global Warming page]
* [http://www.abc.net.au/catalyst/stories/s1647466.htm Watch and read 'Tipping Point'], Australian science documentary about effects of global warming on rare, common, and endangered wildlife
* [http://www.un.org/climatechange Gateway to the UN System's Work on Climate Change]

{{Global warming}}

[[Category:Global warming|*]]
[[Category:Carbon Finance]]
[[Category:Climate change feedbacks and causes]]
[[Category:Economic problems]]
[[Category:History of climate]]

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Revision as of 01:47, 13 April 2008

Fuck man am I established?

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