A pioneering activist and proponent of human-driven global warming says he has underestimated the severity of hot weather and drought plaguing much of the United States and other parts of the world in the past decade.

James Hansen, who directs the NASA Goddard Institute for Space Studies, says he failed in his 1980s research to explore how quickly the average rise in temperatures would drive an increase in extreme weather.

Writing in the Washington Post, Hansen says his predictions, delivered in a landmark presentation to the U.S. Senate in 1988, were too weak.

The essay comes as Hansen and other researchers published an analysis of the past six decades of global temperatures, revealing what he says is “a stunning increase in the frequency of extremely hot summers.” The study appears in the Proceedings of the National Academy of Science on Monday.

He cites the deadly European heat wave of 2003, the fiery Russian summer of 2010 and the catastrophic drought in the United States last year, and attributes all three disasters to climate change spawned in large part by carbon pollution.

Hansen suggests there is still time to avoid a further climate deterioration, and advocates a gradually rising fee on carbon collected from fossil fuel companies.

The Associated Press quotes a science policy expert from the University of Colorado who suggests that Hansen's research and noted environmental activism is “a deeply flawed model of policy change.” Roger Pielke Jr. suggests the lasting consequence of the study will be to strengthen the resolve of global warming skeptics who have repeatedly attacked research linking global warming to human activity.