Human-driven climate change is to blame for a series of increasingly hot summers and the situation is already worse than was expected only two decades ago, said a top Nasa scientist.
James Hansen, who directs the Nasa Goddard Institute for Space Studies, wrote in the Washington Post at the weekend, that even his "grim" predictions of a warming future, made to the US Senate in 1988, were too weak.
"I have a confession to make: I was too optimistic," Hansen wrote.
"My projections about increasing global temperature have been proved true. But I failed to fully explore how quickly that average rise would drive an increase in extreme weather."
Hansen and his colleagues have published in the Proceedings of the National Academy of Sciences an analysis of the past six decades of global temperatures, revealing a "stunning increase in the frequency of extremely hot summers," he wrote. The peer-reviewed study shows that global temperature has been steadily rising due to a warming climate, about 0.8C in the past century, and that extreme events are more frequent.
The study echoes the findings of international research released last month that climbing greenhouse gas emissions increased the odds of severe droughts, floods and heat waves in 2011.
Hansen said the European heat wave of 2003, the Russian heat wave of 2010 and massive droughts in Texas and Oklahoma in the US last year can be attributed to climate change.
"Once the data are gathered in a few weeks' time it's likely that the same will be true for the extremely hot summer the US is suffering through right now," he said.
Another well-known US scientist and former sceptic of global warming, Richard Muller, last week made a very public turnaround, saying that a close look at the data had convinced him that his beliefs were unfounded.
"Call me a converted sceptic," wrote Muller, a professor at the University of California at Berkeley, in the New York Times.
"I'm now going a step further: "Humans are almost entirely the cause."
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