This report provides further information on the outcomes of pathways that could act to avoid dangerous climate change. We present an assessment of changes in temperature and precipitation based climate extremes using the HadGEM2 coupled ocean-atmosphere global climate model. We assess changing extremes at the end of the 21st Century under a “business-as-usual” scenario, and also under an “aggressive mitigation” scenario which sees carbon dioxide equivalent concentrations peak in 2045 before stabilising at around 450ppm during the 22nd Century. The aggressive mitigation scenario provides a preview of the results which might be obtained using the low-emissions scenario proposed for inclusion in the IPCC Fifth Assessment Report.
An increase in the frequency or intensity of temperature extremes is seen over most parts of the world under both scenarios, but mitigation results in a reduced frequency or intensity. When examined on a global scale, most indices of temperature extremes scale linearly with the increasing global mean temperature. This suggests that the use of pattern scaling, where a spatial response pattern from a GCM is scaled by a climate change projection from a simple climate model, does have some validity for temperature based extremes. It also suggests that halving global mean warming will approximately halve the change in temperature extremes on a global basis.
Precipitation shows more regional variability. Brazil stands out as one region where there is both a severe reduction in precipitation under business-as-usual and a strong reduction in precipitation extremes. Other regions (such as the USA and the UK) show more drying under business-as-usual, but a trend towards more intense precipitation extremes. Under the mitigation scenario, most regions of the world get wetter, or in some cases less dry, than they would under the business-as-usual scenario. However, the effect on precipitation extremes is opposite to the changes in mean precipitation in the majority of regions we have considered.
The relationship between global temperature and precipitation change is non-linear, with precipitation increasing at a faster rate under the mitigation scenario than under business-as-usual conditions. On a regional basis, mitigation results in a change of sign (decrease to increase) in precipitation over some regions, compared with business-as-usual. This regional variability suggests that pattern scaling should be used with caution for projecting future changes in precipitation patterns based on different emissions scenarios.