Scientists now widely accept that greenhouse-gas emissions have led to a rising number of heatwaves. Recent examples include the Russian heatwave of 2010, the US heatwave of 2012 and a European heatwave this year. Abnormally high temperatures can be a devastating repercussion of climate change: the European heatwave of 2003, for instance, is estimated to have caused some 70,000 deaths.

Earlier this year, Dim Coumou at the Potsdam Institute for Climate Impact Research in Germany and others showed that, worldwide, there are on average five times more monthly local record-breaking temperature extremes than there would be in a world with no long-term global warming. The result implied that there is an 80% chance that any new monthly heat record is due to climate change.

Now Coumou, together with Alexander Robinson of the Complutense University of Madrid in Spain, has shown how the occurrence of abnormally high temperatures is likely to rise in the future. The pair defined abnormal temperatures by their statistical deviation from the mean, so that a three-sigma (three standard deviation) heat event is "extreme" and a five-sigma heat event is "unprecedented". They have performed simulations with the Coupled Model Intercomparison Project Phase 5 (CMIP5), an ensemble of climate models that is endorsed by the Intergovernmental Panel on Climate Change.

Coumou and Robinson have demonstrated that CMIP5 can retrospectively predict the observed rise in heat extremes. For the future, the climate models project that the frequency of boreal summer months with extreme and unprecedented heat will "strongly increase" under expected global warming. Although mitigation policies would strongly reduce the number of heat extremes in the second half of the 20th century, the researchers said, occurrences of extreme heat in the near-term will rise by at least a factor of four, regardless.

"It says that, in a high-emissions scenario, by the end of the century we will see serious heat-related impacts, and that these can be mitigated by reducing emissions," explained Coumou. "But another important political conclusion is that in the near-term we will see an increase in heat extremes anyway. This of course implies that several regions around the world will have to adapt to this."

Coumou and Robinson cannot say from their study exactly how their predicted rise in extreme heat will impact on societies across the globe. This is a difficult topic, partly because some areas, particularly those at mid-latitudes, are already more used to dealing with big variations in absolute temperature than, for example, the tropics. Coumou added, however, that one of his group's next goals is to analyse societal impacts.

The study is published in Environmental Research Letters (ERL).

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