Editing General circulation model (section)

===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>