Human-driven climate change is to blame for a series of increasingly hot summers and the situation is already worse than was expected just two decades ago, a top NASA scientist said on Saturday.

James Hansen, who directs the NASA Goddard Institute for Space Studies, wrote in the Washington Post that even his "grim" predictions of a warming future, delivered before 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.

Describing "deeply troubling ramifications for not only our future but also for our present," Hansen said the analysis is based not on models or predictions, "but actual observations of weather events and temperatures that have happened."

The peer-reviewed study shows that global temperature has been steadily rising due to a warming climate, about 1.5 degrees Fahrenheit (0.8 degrees Celsius) 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 boosted 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 last year can each be attributed to climate change.

"And 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 United States is suffering through right now," he said.

Another well-known US scientist and former skeptic 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 skeptic," wrote Muller, a professor at the University of California Berkeley, in an op-ed in the New York Times.

"I'm now going a step further: Humans are almost entirely the cause."

Hansen, too, while being a long-time proponent of humans as the main cause of global warming though pollution and fossil fuel consumption, expressed his increasing certainty that other causes could not be blamed.

"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," he wrote.