The relentless heat that has blistered the United States and other parts of the world in recent years is so rare it must be caused by man-made global warming, a top Nasa scientist has warned.

The research, by a man often called the "godfather of global warming" says the likelihood of such temperatures occurring from the 1950s to the 1980s was rarer than one in 300. Now the odds are closer to one in 10, according to the study by Professor James Hansen.

He says statistically what is happening is not random or normal, but climate change. "This is not some scientific theory. We are now experiencing scientific fact," Mr Hansen said.

Mr Hansen is a scientist at Nasa's Goddard Institute for Space Studies in New York and a professor at Columbia University. But he is also a strident activist seeking Government action to curb greenhouse gases.

His study, published online in the Proceedings of the National Academy of Science, is unlikely to sway climate change sceptics, but several scientists have praised the new work.

In a blunt departure from most climate research, Mr Hansen's study blames three heatwaves purely on global warming. He singles out last year's devastating Texas-Oklahoma drought, the 2010 heatwaves in Russia and the Middle East, which killed thousands, and the 2003 European heatwave blamed for tens of thousands of deaths.