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Cattle use a tree for shade as temperatures rose above 100 degrees in a pasture July 28, 2011, near Canadian, Texas.
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The last couple of years have certainly felt unusually hot in many parts of the U.S., but are they really all that unusual?
Many people wonder whether a warming climate is turning up the temperature or whether it's all just part of the normal variation in the weather. Among scientists, there's a growing view that these latest heat waves are indeed a result of climate change.
NASA climate scientist James Hansen has looked at the past century's temperatures all over the world. He has 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.
"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," he says. "And that, I think, is where we are now."
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 climate change — think of the Dust Bowl drought of the 1930s. But Hansen says the difference is that these kinds of extreme events are happening much more often.
"Now between 10 to 12 percent of the planet in the last 10 years has been covered by these three-standard-deviation anomalies," he says.
Hansen's analysis appears in the Proceedings of the National Academy of Sciences.
And there's another odd thing: If the hot weather were part of normal variation, you'd expect a lot of very cold spells, too.
"We really see a strong trend to many more hot records than cold records," says Jonathan Overpeck, a climate scientist at the University of Arizona. "In March alone, there were nearly 15,000 hot records broken in the United States. And that really, in a sense, really blows records away. I mean it's just incredible."
Climate scientists point out that global warming doesn't mean increasingly severe heat waves and droughts will hit everywhere at once.
"The really hot spots certainly move around," Kevin Trenberth, of the National Center for Atmospheric Research in Colorado, said in a recent NPR interview. "You know, last year it was in the South, in Texas, Arizona, New Mexico and Oklahoma. The previous year it was in Russia. The outstanding event in 2009 was down in Australia."
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 chose 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.