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From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish. It's been an unusually hot couple of years in many parts of the U.S., but just how unusual is this? That's a question climate scientists are getting asked a lot. They wonder whether a warming climate is to blame or whether it's just part of normal variation in the weather.

NPR's Christopher Joyce reports on a growing view that these latest heat waves are a result of climate change.

CHRISTOPHER JOYCE, BYLINE: Climate scientist James Hansen of NASA's Goddard Institute for Space Studies has been looking at the past century's temperatures all over the world. He's measured hot spells with what you might call a unit of weirdness, a standard deviation. It's a measure of abnormality. One standard deviation from what's normal might be throwing snake eyes three times in a row. The more snake eyes you roll in a row, the more standard deviations away from normal you are.

Hansen says current temperatures in the world are out of whack, even to people who don't know statistics.

JAMES HANSEN: If you look at the frequency with which things are happening, then you get to a point where the dice are so loaded that the public can see it and that, I think, is where we are now.

JOYCE: Hansen says heat waves like the one that hit the U.S. last year are a whole three standard deviations from normal summer weather - that's a lot of snake eyes in a row. But extremes do happen now and then. Without the climate change, think of the Dust Bowl drought of the 1930s.

But Hansen says the difference is these kinds of extreme events are happening much more often.

HANSEN: Now, between 10 to 12 percent of the planet in the last 10 years has been covered by these three standard deviation anomalies.

JOYCE: Hansen's analysis appears in the proceedings of the National Academy of Sciences. There's another odd thing. If the hot weather were part of normal variation, you'd expect a lot of very cold spells, too.

Jonathan Overpeck is a climate scientist at the University of Arizona.

JONATHAN OVERPECK: We really see a strong trend to many more hot records than cold records. In March alone, there were nearly 15,000 hot records broken in the United States and that really, in a sense, blows, you know, records away. I mean, it's just incredible.

JOYCE: Climate scientists point out that global warming doesn't mean increasingly severe heat waves and droughts will hit everywhere at once.

KEVIN TRENBERTH: The really hot spots certainly move around.

JOYCE: That's Kevin Trenberth from the National Center for Atmospheric Research in Colorado in a recent NPR interview.

TRENBERTH: You know, last year, it was in the South, in Texas, Arizona, New Mexico and Oklahoma. The previous year, it was in Russia and the outstanding event in 2009 was down in Australia.

JOYCE: One caveat in all this is the period scientists pick as normal to compare the present to. When, if ever, was the climate normal? Hansen shows the period 1951 to 1980, before the sharp upturn in carbon dioxide in the atmosphere. He acknowledges that picking any other time period for the norm would alter his results, but he says whatever you compare the present to, it still comes out abnormal.

Christopher Joyce, NPR News.

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