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How will global temperatures change?

Atmospheric scientists use the emission scenarios to predict how much greenhouse gas concentrations will increase in the atmosphere. They can then run Global Climate Models to predict how the temperatures will change in the future, alongside changes in other aspects of the weather like rainfall [i] Even the medium emission scenarios predict increases in average global temperatures by 3ºC, by the end of the centaury.

The climate science has improved with the release of the IPCC 5th Assessment Report (AR5). The results are similar though. This link takes you to the original global temperature change page, which is based on AR4 from 2007.

Changes to global average temperature

There is more to the climate model results than just estimates of a rise in global average temperature. The following figure is taken from the AR5 Physical Science Report - Summary for Policy Makers (SPM)

>  Increase of global mean surface temperatures for 2081–2100 relative to 1986–2005 is projected to likely be in the ranges derived from the concentration-driven CMIP5 model simulations, that is, 0.3°C to 1.7°C (RCP2.6), 1.1°C to 2.6°C (RCP4.5), 1.4°C to 3.1°C (RCP6.0), 2.6°C to 4.8°C (RCP8.5). The Arctic region will warm more rapidly than the global mean, and mean warming over land will be larger than over the ocean (very high confidence) (see Figures SPM.7 and SPM.8, and Table SPM.2).

Note that SRES B1 scenario has a mid estimate of 989 GtC between 1990 and 2100. This is equivalent to approx 3600 Gt CO2. Guidance on p1031 from above states "Carbon dioxide (CO2) represents about 80 to 90% of the total
anthropogenic forcing in all RCP scenarios through the 21st century". Estimate for all GHGs for SRES B1 is therefore approx 4170 Gt CO2 from 1990. Add approx 1500 Gt CO2 from 1870 to 1990 => 5670 Gt CO2. This is just beyond the 580-720 ppm CO2 from Figure SPM.10 of the "IPCC Fifth Assessment Report - Synthesis Report" presentation given on 2 Nov 2014. Equivalent to approvimately 3 degC temperature rise (relative to pre industrial levels).

The following graphics are taken from IPCC 4th Assessment Report (AR4 [i]): Working Group II Report "Impacts, Adaptation and Vulnerability" (Technical Summary).

Graphic showing global mean temperature rises associated with different SRES storylines (this has been cut from Figure TS.4, page 34 of the IPCC 'Impacts, Adaptation and Vulnerability' report mentioned in text).
Graphic showing global mean temperature rises associated with different greenhouse gas/carbon dioxide stabilisation targets (this has been cut from Figure TS.4, page 34 of the IPCC 'Impacts, Adaptation and Vulnerability' report mentioned in text).

Climate change predictions for a number of emission scenarios are also available in a table in the IPCC AR4 Synthesis Report. For example, by the 2090s the B1 scenario predicts that temperatures will rise by 1.8ºC (likely range between 1.1ºC and 2.9ºC). The A1FI scenario predicts that temperatures will rise by 4.0ºC (likely range between 2.4ºC and 6.4ºC). These changes are all relative to the 1980s, which in tern were 0.5ºC warmer than the period from 1850-1899.

What does the 2ºC limit look like?

That we "must limit global temperature rise to no more than 2ºC above pre-industrial levels" is a phrase you will hear a lot in climate change literature. The following graph is from the Copenhagen Diagnosis. Notice that each of the SRES scenarios reach temperatures that are much higher than anything we have experienced in the past 1,500 years. And notice as well that the 2ºC limit is lower than the predictions from the most benign B1 emission scenario. [i] It does not look very likely that we are going to meet that limit.

Graph showing global temperature from the year 500 to date, which is extended up to the year 2100 with high, medium and low emission scenarios. This is from page 52 of the report.

It is worth considering, when you are looking at the graph, that the rise in temperature does not stop at the year 2100. Only the B1 emission scenario shows any sign of starting to stabilise towards the end of the centaury. Our grandchildren (assuming you are my age) will find themselves in a much hotter world.

How does it vary across the world?

The following graphic is taken from AR4 Working Group I Report "The Physical Science Basis", Chapter 10. This demonstrates how mean annual air surface temperature is predicted to changes across the planet. Three emission scenarios are shown: B1, A1B and A2; and you can see the change increase through time, from 2011-2030 period to 2080-2099. Notice that the Artic is predicted to get a lot warmer than it is at the moment. Other parts of the world will be less effected. Notice as well the significantly higher temperatures predicted in the bigger emission scenarios.

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