Some things you really can pin on climate change ? the heatwave that struck Texas last year, for instance. A long-term rise in temperatures due to greenhouse gas emissions made the hot weather 20 times more likely, modelling suggests.
The same study found that four other extreme weather events last year can be linked to climate change.
Until recently, climate scientists have been reluctant to blame individual weather events on climate change. There have been exceptions: studies have found that the European heatwave in 2003 was twice as likely because of climate change, and that the UK floods in 2000 were also made more likely. But now, for the first time, climate scientists are systematically examining recent extreme weather events to determine whether climate change played a role.
Peter Stott of the UK Met Office in Exeter headed a project looking at six unusual weather events in 2011. "This is the first time this has been done so soon after the events in question," he says.
Five of the six events could be blamed on climate change to some extent. "We can never say 100 per cent absolutely that this is due to climate change," Stott says. "But we are very confident."
Texas fried
In 2011, Texas suffered the hottest and driest growing season since records began in 1895, which resulted in a severe drought. Farmers were forced to sell their cattle because their pastures were so dry they could not feed them. Climate change is expected to bring more drought to the US Southwest, but few states are planning for this eventuality.
A study led by David Rupp of Oregon State University in Corvallis examined the causes of the Texas drought. One major factor was the La Ni?a in the Pacific, which changed seasurface temperatures and thus rainfall patterns, boosting the chances of a drought in the US Southwest. But that wasn't the only cause.
According to Rupp's models, such a severe heatwave is now 20 times more likely in a La Ni?a year than it was in the 1960s, when global temperatures were significantly cooler. "Ongoing climate change has exacerbated that existing vulnerability," says Stott.
Europe and UK
Three other events, two in Europe and one in Africa, were also affected by climate change.
In the UK, November 2011 was the second warmest since records began in 1659. "That warm November was about 60 times more likely than in the 1960s," Stott says.
Meanwhile, continental Europe had an unusually warm spring and autumn in 2011. This was partly the result of westerly winds bringing warm air from the tropics, but these circulation patterns cannot explain all the heat. "It wouldn't have been that unusual without the long-term warming," says Stott.
Famine struck East Africa last summer, after both the region's rainy seasons failed and caused a severe drought. Several studies linked this to changes in sea surface temperatures in the western Pacific and Indian Oceans, but it was not clear if this was part of a long-term trend. Stott now says there is "initial evidence" that climate change has, indeed, affected ocean temperatures, and partly caused the drought.
Reduced risk
Some events have been made less likely by climate change. December 2010 was the second coldest on record. Greenhouse gases have reduced the odds of such events in the UK. "It's about half as likely," Stott says.
And some events did not appear to be related to climate change at all. Thailand suffered severe floods from July 2011 as a result of a strong monsoon. The capital, Bangkok, was inundated in October. "We didn't find a clear human influence on that weather event," Stott says. Rather, there were changes to the management of rivers, which made the floods more severe than they otherwise would have been.
All the studies rely on the same basic method. The team used models to simulate climate with and without greenhouse-gas emissions, and compared the chances that a given extreme weather event would occur. If it was significantly more likely in the models that included emissions, climate change was judged to have been a factor.
Journal reference: Bulletin of the American Meteorological Society, in press
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