Climate change blamed for heatwaves

Another week, another row about blaming climate change for extreme weather events. A top climate scientist now says that the 2010 Russian heatwave, and last year's Texas drought, were both the result of global warming ? and the current US drought probably is too.

James Hansen of the NASA Goddard Institute for Space Studies in New York and colleagues compared global temperatures between 1981 and 2010 to the cooler climes of 1950 to 1980. Extreme warming events, when temperatures were more than three standard deviations above the 1950 to 1980 average, were an order of magnitude more common. Such heatwaves covered 4 to 13 per cent of the planet between 2006 and 2011.

Hansen says this means recent extremes can be firmly blamed on climate change. In an editorial in the Washington Post, he writes: "The odds that natural variability created these extremes are minuscule, vanishingly small. To count on those odds would be like quitting your job and playing the lottery every morning to pay the bills."

That's going too far, says Peter Stott of the UK Met Office in Exeter. "While we can provide evidence that the risk of heatwaves has increased, we cannot say that the chances of such heatwaves were negligible before global warming set in."

Stott and others address that problem by running climate models with and without the effects of global warming. If a given extreme event is more likely in the warmed model, climate change probably contributed to it. Following a few isolated studies, some climatologists are now systematically examining recent extremes to see if they can be blamed on climate change.

The extreme events Hansen discusses have already been attributed to global warming by other researchers. Last month a major study found that last year's Texas drought was made 20 times more likely by climate change. Meanwhile, a 2011 study claimed that the Russian heatwave was unlikely to have happened without climate change (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1101766108).

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1205276109

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