Hansen and his colleagues argue in a paper published in the Proceedings of the National Academy of Sciences on Monday that the rapid global warming of recent years has loaded the "climate dice," thereby increasing the chances of extreme weather and tilting the overall trend towards hotter summers.
The researchers examined historical temperature data and compared summer temperature anomalies in recent years to temperature fluctuations in the summers between 1951 and 1980.
They found that very hot summers -- those with temperatures that deviated extremely far from the mean summer temperature from 1951 to 1980 -- have occurred much more frequently in recent years, and were practically absent during that base period.
"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," Hansen wrote in an editorial for the Washington Post on Sunday.
Hansen and his colleagues say the bigger heat waves of recent years have affected 10% of the Earth's surface. Comparatively, from 1951 to 1980, that figure was about 1%, according to the paper.
The authors say it's highly likely that high-profiled heat waves like the one that blanketed Texas and Oklahoma last summer, or a 2010 scorcher in Russia. That 2010 heat wave, which brought on drought and wildfires, was thought to have killed at least 7,000 people in Moscow alone.
"Such events, our data show, will become even more frequent and more severe," Hansen wrote in the Washington Post.
British climatologist Peter Stott of the Met Office told the climate news website Climate Central that Hansen's study falls in line with other research showing that hotter summers are more common, but took issue with attributing specific events to global warming caused by humans.
"I don't agree with how Hansen frames his conclusion in terms of the Texas 2011 and Moscow 2010 heat waves having been 'caused by global warming,'" Stott told Climate Central in an email. "Both of these heat waves were associated with unusual [atmospheric] circulation characteristics and a large part of the explanation for both lies in that."
SOURCE: Hansen et al. "Perception of climate change." PNAS published online 6 August 2012.