The verdict is in: climate change is guilty. Greenhouse gas emissions made five heatwaves in 2013 more likely.

That’s the conclusion of the third annual assessment of the role of global warming in extreme weather events from the previous year. For 2013, the research included five separate heatwaves in Australia, China, Japan, Korea and western Europe. The report found that climate change played a part in all of them.

Australia’s results were particularly damning. “The chances of observing such extreme temperatures in a world without climate change – it is almost impossible to imagine how that would have happened,” says Peter Stott of the UK Met Office, an editor of the report, a special supplement in the Bulletin of the American Meteorological Society called Explaining Extreme Events of 2013 from a Climate Perspective.

But the Californian drought was too close to call. While heatwaves depend only on how hot the temperature is, heavy rains, floods and dry spells are more complex events, relying on mix of factors including atmospheric patterns, sea surface temperatures and rainfall. That makes it harder to tease out the role of climate change from natural variations in the climate. Studies of these types of events were less conclusive, and three studies of different factors in California’s 2013-2014 drought found little or no role for global warming.


The extreme monsoon rains that hit India in June 2013, however, were found to have been made more likely by emissions.

The report has coincided with early data from the UK Met Office showing that this September was the driest for the UK on record – although we don’t yet know if this is due to global warming.