The heatwave that hit Russia last year, killing thousands and reducing the country's grain harvest, was primarily due to a weather event, not climate change caused by human activity, US scientists said.
There was plenty of circumstantial evidence pointing to human-caused greenhouse gas emissions and the resulting rise in carbon dioxide in the atmosphere, but investigation showed this was not a major factor, the scientists said in research published online in Geophysical Research Letters.
"It was an off-the-charts intensity event," Randall Dole of the US National Oceanic and Atmospheric Administration (NOAA) said on Wednesday. "It certainly was the most extreme event we had seen, dating back to at least 1880," when modern weather record-keeping began.
Temperatures soared above 37.7C in western Russia from July through to mid-August 2010. In Moscow, where long-term daily average temperatures for July range from 18.3 to 19.4C, daily average July 2010 temperatures climbed to 30.6C. Daily average temperatures include night time.
More severe heatwaves, intense droughts and wildfires were among the predictions made for a warming world in the 2007 report by the UN's Intergovernmental Panel on Climate Change.The first six months of 2010 were the hottest, globally, on record.
However, Mr Dole and his co-authors found that the kind of massive heatwave that hung over western Russia was due mainly to a natural phenomenon called atmospheric blocking.
This occurs when high atmospheric pressure builds up and refuses to budge, forcing any cool air and rains to move around it.
The blocking pattern in Russia was a weather pattern unlike any seen before around Moscow, the researchers said, and could not have been forecast far in advance.
This stubborn pattern in western Russia may have been related to the long period of rains that caused deadly flooding in Pakistan around the same time, said co-author Martin Hoerling, a research meteorologist at NOAA.
Whatever the cause of the heatwave, the record high temperatures combined with poor air quality from wildfires increased deaths by at least 56,000 in Moscow and other parts of western Russia, according to Munich Reinsurance, and led to massive crop failures in the region.
While climate change was not a major factor, the expected increase in atmospheric carbon dioxide and global average temperatures could make this kind of extraordinary event, which Mr Dole called a "climate surprise", more common.
Computer models show the risk of such heatwaves in western Russia could rise from less than 1 per cent in 2010 to 10 per cent or more by 2100 as the concentration of greenhouse gases in the atmosphere increases.
"It appears that parts of Russia are on the cusp of a period in which the risk of extreme heat events will increase rapidly," Mr Hoerling said.