Editing General circulation model (section)

==Projections==
{{Main|Global warming}}

[[File:Animation of projected annual mean surface air temperature from 1970-2100, based on SRES emissions scenario A1B (NOAA GFDL CM2.1).webm|thumb|480px|right|alt=In the 21st century, changes in global mean temperature are projected to vary across the world|Projected annual mean surface air temperature from 1970 to 2100, based on [[Special Report on Emissions Scenarios|SRES]] emissions scenario A1B, using the NOAA GFDL CM2.1 climate model (credit: [[NOAA]] [[Geophysical Fluid Dynamics Laboratory]])<ref name="gfdl cm2.1 global warming projections">
{{citation
 | url=http://www.gfdl.noaa.gov/patterns-of-greenhouse-warming-ar4
 | title=NOAA GFDL Climate Research Highlights Image Gallery: Patterns of Greenhouse Warming
 | publisher=NOAA GFDL
 | date=9 October 2012
 | author=NOAA Geophysical Fluid Dynamics Laboratory (GFDL)
}}
</ref>]]
Coupled AOGCMs use [[transient climate simulation]]s to project/predict climate changes under various scenarios. These can be idealised scenarios (most commonly, CO<sub>2</sub> emissions increasing at 1%/yr) or based on recent history (usually the "IS92a" or more recently the [[Special Report on Emissions Scenarios|SRES]] scenarios). Which scenarios are most realistic remains uncertain.

The 2001 [[IPCC Third Assessment Report]] [https://web.archive.org/web/20160303212120/http://www.grida.no/climate/ipcc_tar/wg1/fig9-3.htm Figure 9.3] shows the global mean response of 19 different coupled models to an idealised experiment in which emissions increased at 1% per year.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/348.htm#fig93 |title=Climate Change 2001: The Scientific Basis |publisher=Grida.no |access-date=2012-02-18 |url-status=dead |archive-url=https://web.archive.org/web/20120218141613/http://www.grida.no/climate/ipcc_tar/wg1/348.htm#fig93 |archive-date=18 February 2012}}</ref> [https://web.archive.org/web/20160303170118/http://www.grida.no/climate/ipcc_tar/wg1/fig9-5.htm Figure 9.5] shows the response of a smaller number of models to more recent trends. For the 7 climate models shown there, the temperature change to 2100 varies from 2 to 4.5&nbsp;°C with a median of about 3&nbsp;°C.

Future scenarios do not include unknown events{{snd}} for example, volcanic eruptions or changes in solar forcing. These effects are believed to be small in comparison to [[greenhouse gas]] (GHG) forcing in the long term, but large volcanic eruptions, for example, can exert a substantial temporary cooling effect.

Human GHG emissions are a model input, although it is possible to include an economic/technological submodel to provide these as well. Atmospheric GHG levels are usually supplied as an input, though it is possible to include a carbon cycle model that reflects vegetation and oceanic processes to calculate such levels.

===Emissions scenarios===
{{See also|economics of global warming#Scenarios|l1=Economics of global warming: Scenarios}}
[[File:Projected change in annual mean surface air temperature from the late 20th century to the middle 21st century, based on SRES emissions scenario A1B.png|thumb|left|alt=In the 21st century, changes in global mean temperature are projected to vary across the world|Projected change in annual mean surface air temperature from the late 20th century to the middle 21st century, based on SRES emissions scenario A1B (credit: NOAA [[Geophysical Fluid Dynamics Laboratory]])<ref name="gfdl cm2.1 global warming projections" />]]
For the six SRES marker scenarios, IPCC (2007:7–8) gave a "best estimate" of global mean temperature increase (2090–2099 relative to the period 1980–1999) of 1.8&nbsp;°C to 4.0&nbsp;°C.<ref name="ar4 spm projections" /> Over the same time period, the "likely" range (greater than 66% probability, based on expert judgement) for these scenarios was for a global mean temperature increase of 1.1 to 6.4&nbsp;°C.<ref name="ar4 spm projections">{{cite book
 | title = IPCC Fourth Assessment Report: Climate Change 2007: Synthesis Report: Synthesis Report Summary for Policymakers
 | chapter = Chapter 3: Projected climate change and its impacts
 | url = http://www.ipcc.ch/publications_and_data/ar4/syr/en/spm.html
 | chapter-url = http://www.ipcc.ch/publications_and_data/ar4/syr/en/spms3.html
 | access-date = 3 December 2013
 | archive-date = 9 March 2013
 | archive-url = https://web.archive.org/web/20130309142919/http://www.ipcc.ch/publications_and_data/ar4/syr/en/spm.html
 | url-status = dead
 }}, in {{harvnb|IPCC AR4 SYR|2007}}</ref>

In 2008 a study made climate projections using several emission scenarios.<ref>
{{cite web
|year=2008
|title=Met Office: The scientific evidence for early action on climate change
|publisher=Met Office website
|author=Pope, V.
|url=http://www.metoffice.gov.uk/climatechange/policymakers/action/evidence.html| archive-url=https://web.archive.org/web/20101229170710/http://www.metoffice.gov.uk/climatechange/policymakers/action/evidence.html
 | archive-date=29 December 2010
}}
</ref> In a scenario where global emissions start to decrease by 2010 and then decline at a sustained rate of 3% per year, the likely global average temperature increase was predicted to be 1.7&nbsp;°C above pre-industrial levels by 2050, rising to around 2&nbsp;°C by 2100. In a projection designed to simulate a future where no efforts are made to reduce global emissions, the likely rise in global average temperature was predicted to be 5.5&nbsp;°C by 2100. A rise as high as 7&nbsp;°C was thought possible, although less likely.

Another no-reduction scenario resulted in a median warming over land (2090–99 relative to the period 1980–99) of 5.1&nbsp;°C. Under the same emissions scenario but with a different model, the predicted median warming was 4.1&nbsp;°C.<ref>{{cite journal
|year=2009
|title=Probabilistic Forecast for 21st century Climate Based on Uncertainties in Emissions (without Policy) and Climate Parameters
|author=Sokolov, A.P.|journal=Journal of Climate
|volume=22
|issue=19
|pages=5175–5204
|doi=10.1175/2009JCLI2863.1
|bibcode = 2009JCli...22.5175S |display-authors=etal|url=http://ocean.mit.edu/%7Estephd/Sokolovetal-jclim-2009.pdf
|hdl=1721.1/54833
|s2cid=17270176
|hdl-access=free
}}</ref>

===Model accuracy===
{{Update section|date=August 2015}}
[[Image:Hadcm3-era-sst-annual.png|thumb|SST errors in HadCM3]]
[[File:Climate model NA annual precipitation 2002.jpg|thumb|North American precipitation from various models]] [[File:Global Warming Predictions.png|thumb|Temperature predictions from some climate models assuming the SRES A2 emissions scenario]]

AOGCMs internalise as many processes as are sufficiently understood. However, they are still under development and significant uncertainties remain. They may be coupled to models of other processes in [[Earth system model]]s, such as the [[carbon cycle]], so as to better model feedback. Most recent simulations show "plausible" agreement with the measured temperature anomalies over the past 150 years, when driven by observed changes in greenhouse gases and aerosols. Agreement improves by including both natural and anthropogenic forcings.<ref name="f4">IPCC, [http://www.grida.no/climate/ipcc_tar/wg1/005.htm Summary for Policy Makers] {{webarchive|url=https://web.archive.org/web/20160307220102/http://www.grida.no/climate/ipcc_tar/wg1/005.htm |date=7 March 2016 }}, [http://www.grida.no/publications/other/ipcc%5Ftar/?src=/climate/ipcc_tar/wg1/figspm-4.htm Figure 4] {{webarchive|url=https://web.archive.org/web/20161021094407/http://www.grida.no/publications/other/ipcc_tar/?src=%2Fclimate%2Fipcc_tar%2Fwg1%2Ffigspm-4.htm |date=21 October 2016 }}, in {{citation 
 |year=2001 
 |author=IPCC TAR WG1 
 |author-link=IPCC 
 |title=Climate Change 2001: The Scientific Basis 
 |series=Contribution of Working Group I to the [[IPCC Third Assessment Report|Third Assessment Report]] of the Intergovernmental Panel on Climate Change 
 |editor=Houghton, J. T. 
 |editor2=Ding, Y. 
 |editor3=Griggs, D. J. 
 |editor4=Noguer, M. 
 |editor5=van der Linden, P. J. 
 |editor6=Dai, X. 
 |editor7=Maskell, K. 
 |editor8=Johnson, C. A. 
 |publisher=Cambridge University Press 
 |url=https://archive.org/details/climatechange2000000unse 
 |isbn=978-0-521-80767-8 
 |url-status=dead 
 |url-access=registration|archive-url=https://web.archive.org/web/20191215120519/https://archive.org/details/climatechange2000000unse 
 |archive-date=15 December 2019 
 }} (pb: {{ISBNT|0-521-01495-6}}).</ref><ref>{{cite web|title=Simulated global warming 1860–2000 |url=http://www.hadleycentre.gov.uk/research/hadleycentre/pubs/talks/sld017.html |url-status=dead |archive-url=https://web.archive.org/web/20060527001324/http://www.hadleycentre.gov.uk/research/hadleycentre/pubs/talks/sld017.html |archive-date=27 May 2006 }}</ref><ref>{{cite web|title = Decadal Forecast 2013|url = https://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/long-range/decadal-fc/2013 |website = Met Office|date = January 2014}}</ref>

Imperfect models may nevertheless produce useful results. GCMs are capable of reproducing the general features of the observed global temperature over the past century.<ref name="f4" />

A debate over how to reconcile climate model predictions that upper air (tropospheric) warming should be greater than observed surface warming, some of which appeared to show otherwise,<ref>The National Academies Press website press release, 12 Jan. 2000: 
 [https://web.archive.org/web/20060420125451/http://www4.nationalacademies.org/news.nsf/isbn/0309068916?OpenDocument Reconciling Observations of Global Temperature Change].</ref> was resolved in favour of the models, following data revisions.

[[clouds|Cloud]] effects are a significant area of uncertainty in climate models. Clouds have competing effects on climate. They cool the surface by reflecting sunlight into space; they warm it by increasing the amount of infrared radiation transmitted from the atmosphere to the surface.<ref>[https://web.archive.org/web/20000901022925/http://liftoff.msfc.nasa.gov/academy/space/greenhouse.html Nasa Liftoff to Space Exploration Website: Greenhouse Effect]. Archive.com. Recovered 1 October 2012.</ref> In the 2001 IPCC report possible changes in cloud cover were highlighted as a major uncertainty in predicting climate.<ref>{{cite web|url =http://webpages.icav.up.pt/PTDC/CVT/098487/2008/IPPC,%202001.pdf |title = Climate Change 2001: The Scientific Basis|publisher = IPCC|page = 90}}</ref><ref>{{cite journal |last1=Soden |first1= Brian J.|first2=Isaac M. |last2=Held |year=2006 |title=An Assessment of Climate Feedbacks in Coupled Ocean–Atmosphere Models |journal=J. Climate |volume = 19|issue= 14 |pages= 3354–3360 |doi= 10.1175/JCLI3799.1 |bibcode = 2006JCli...19.3354S |doi-access=free }}</ref>

Climate researchers around the world use climate models to understand the climate system. Thousands of papers have been published about model-based studies. Part of this research is to improve the models.

In 2000, a comparison between measurements and dozens of GCM simulations of [[ENSO]]-driven tropical precipitation, water vapor, temperature, and outgoing longwave radiation found similarity between measurements and simulation of most factors. However, the simulated change in precipitation was about one-fourth less than what was observed. Errors in simulated precipitation imply errors in other processes, such as errors in the evaporation rate that provides moisture to create precipitation. The other possibility is that the satellite-based measurements are in error. Either indicates progress is required in order to monitor and predict such changes.<ref>{{cite journal|title = The Sensitivity of the Tropical Hydrological Cycle to ENSO |first = Brian J.|last = Soden|date = February 2000|journal = Journal of Climate|volume = 13|issue = 3 |pages =  538–549|doi = 10.1175/1520-0442(2000)013<0538:TSOTTH>2.0.CO;2 |bibcode = 2000JCli...13..538S| s2cid=14615540 |doi-access = free}}</ref>

The precise magnitude of future changes in climate is still uncertain;<ref>Cubasch ''et al.'', [http://www.grida.no/climate/ipcc_tar/wg1/338.htm Chapter 9: Projections of Future Climate Change] {{webarchive|url=https://web.archive.org/web/20160416080318/http://www.grida.no/climate/ipcc_tar/wg1/338.htm |date=16 April 2016 }}, [http://www.grida.no/climate/ipcc_tar/wg1/339 Executive Summary] {{dead link|date=February 2018|bot=medic}}{{cbignore|bot=medic}}, in {{Citation 
 |year=2001 
 |author=IPCC TAR WG1 
 |author-link=IPCC 
 |title=Climate Change 2001: The Scientific Basis 
 |series=Contribution of Working Group I to the [[IPCC Third Assessment Report|Third Assessment Report]] of the Intergovernmental Panel on Climate Change 
 |editor=Houghton, J. T. 
 |editor2=Ding, Y. 
 |editor3=Griggs, D. J. 
 |editor4=Noguer, M. 
 |editor5=van der Linden, P. J. 
 |editor6=Dai, X. 
 |editor7=Maskell, K. 
 |editor8=Johnson, C. A. 
 |publisher=Cambridge University Press 
 |url=https://archive.org/details/climatechange2000000unse 
 |isbn=978-0-521-80767-8 
 |url-status=dead 
 |url-access=registration
 |archive-url=https://web.archive.org/web/20191215120519/https://archive.org/details/climatechange2000000unse 
 |archive-date=15 December 2019 
 }} (pb: {{ISBNT|0-521-01495-6}}).</ref> for the end of the 21st century (2071 to 2100), for SRES scenario A2, the change of global average SAT change from AOGCMs compared with 1961 to 1990 is +3.0&nbsp;°C (5.4&nbsp;°F) and the range is +1.3 to +4.5&nbsp;°C (+2.3 to 8.1&nbsp;°F).

The IPCC's [[IPCC AR5|Fifth Assessment Report]] asserted "very high confidence that models reproduce the general features of the global-scale annual mean surface temperature increase over the historical period". However, the report also observed that the rate of warming over the period 1998–2012 was lower than that predicted by 111 out of 114 [[Coupled Model Intercomparison Project]] climate models.<ref>{{cite web | url=http://www.climatechange2013.org/images/report/WG1AR5_Chapter09_FINAL.pdf | title=Evaluation of Climate Models | publisher=[[IPCC]] | date=2013| author=Flato, Gregory | pages=768–769}}</ref>