Climate 'causing more and worse heat waves'

EMBARGOED till 0001 GMT on Tuesday 15 January
Researchers say there are now on average five times as many record-breaking hot months worldwide as we could expect without long-term global warming, with human activities responsible for four-fifths of them.

LONDON, 15 January – Climate change has already increased the incidence of dangerous heat waves five-fold, and 80% of them have been caused by man-made emissions. The last decade has been particularly affected.

These conclusions, by researchers at the Potsdam Institute for Climate Impact Research and the Complutense University of Madrid, look at the increasing incidence of record-breaking hot months since 1880. The last 40 years has seen a steady rise, but in the last decade the upward curve has been far greater. Even these disturbing figures do not include the recent record temperatures and disastrous fires in Australia.

The new study relies on 131 years of monthly temperature data for more than 12,000 grid points around the world, provided by NASA.

Heat waves matter because they kill large numbers of people through heat stress, cause forest fires, reduce crop yields and damage ecosystems, which are not adapted to high temperatures.

Although the worldwide decadal average was five times more heat waves than expected without long-term warming, there were large regional differences. Hardest hit, with 10 times more damaging heat waves, were eastern Europe, Africa, southern Asia and Amazonia.

The Potsdam paper, published in the journal Climatic Change, concludes: “Our statistical analysis does not consider the causes behind climatic change, but given the overwhelming evidence that global warming in the second half of the 20th century is anthropogenic one may conclude that 80% of the monthly heat records would not have occurred without human influence on climate.”

This trend is expected to continue, with the number of records globally by 2040 being more than 12 times as high as in a climate with no long-term warming.

There has been a steep global warming trend over the last 40 years. Superimposed on this long-term rise, the team’s data show the effect of natural variability, with especially high numbers of heat records during years with El Niño events. This natural variability, however, does not explain the overall development of the record events they found, the researchers say.

Lead author Dim Coumou says: “The last decade brought unprecedented heat waves; for instance in the US in 2012, in Russia in 2010, in Australia in 2009, and in Europe in 2003. Heat extremes are causing many deaths.”

The study projection that the number of new monthly records will be 12 times as high in 30 years is misleading, the researchers say, because although it sounds very bad, the reality – on present trends – will be even worse.

The new records set in the 2040s will be hot not just by today’s standards. “To count as new records, they actually have to beat heat records set in the 2020s and 2030s, which will already be hotter than anything we have experienced to date,” explains Coumou.

“And this is just the global average – in some continental regions, the increase in new records will be even greater.”

“Statistics alone cannot tell us what the cause of any single heat wave is, but they show a large and systematic increase in the number of heat records due to global warming,” says Stefan Rahmstorf, a co-author of the study and co-chair of PIK’s research domain Earth System Analysis.

“Today this increase is already so large that by far most monthly heat records are due to climate change. The science is clear that only a small fraction would have occurred naturally.” – Climate News Network