According to some climate scientists, such earlier-than-normal outbreaks of tornadoes, which typically peak in the spring, will become the norm as the planet warms.

"As spring moves up a week or two, tornado season will start in February instead of waiting for April," said climatologist Kevin Trenberth of the National Center for Atmospheric Research.

Whether climate change will also affect the frequency or severity of tornadoes, however, remains very much an open question, and one that has received surprisingly little study.

"There are only a handful of papers, even to this day," said atmospheric scientist Robert Trapp of Purdue University, who led a pioneering 2007 study of tornadoes and climate change.

"Some of us think we should be paying more attention to it," said atmospheric physicist Anthony Del Genio of the Goddard Institute for Space Studies, part of NASA.

The scientific challenge is this: the two conditions necessary to spawn a twister are expected to be affected in opposite ways. A warmer climate will likely boost the intensity of thunderstorms but could dampen wind shear, the increase of wind speed at higher altitudes, researchers say.

Tomorrow's thunderstorms will pack a bigger wallop, but may strike less frequently than they have historically, explained Del Genio.

"As we go to a warmer atmosphere, storms - which transfer energy from one region to another - somehow figure out how to do that more efficiently," he said. As a result, thunderstorms transfer more energy per outbreak, and so have to make such transfers less often.

In a 2011 paper, Del Genio calculated that, "especially in the central and eastern United States, we can expect a few more days per month with conditions favorable to severe thunderstorm occurrence" by the latter part of this century if the global climate grows warmer.

Indeed, the world has been experiencing more violent storms since 1970, the Intergovernmental Panel on Climate Change reported in its most recent assessment.

EXTENDING TORNADOES' PATH

Purdue's Trapp and colleagues got a similar result in their 2007 study, which they confirmed in research published in 2009 and 2011. "The number of days when conditions exist to form tornadoes is expected to increase" as the world warms, he said.

In addition, they found, regions near the Gulf of Mexico and Atlantic coasts not normally associated with tornadoes will experience tornado-making weather more frequently. They projected a doubling in the number of days with such conditions in Atlanta and New York City, for instance.

More powerful thunderstorms would be expected to produce more tornadoes, but wind shear could prove a mitigating factor.

Because climate change is not uniform, Del Genio wrote in the 2011 paper, "in the lower troposphere, the temperature difference between low and high latitudes decreases as the planet warms, creating less wind shear."

Other scientists are not so sure, and they see a surge in tornadoes last year as ominous. April 2011 was the most active tornado month on record, with 753, according to the National Oceanic and Atmospheric Administration (NOAA), compared to the previous record of 267 in April 1974.

"I have no doubt that there will be many times when wind shear is plenty strong to create a tornado," said Trenberth.

That is what Trapp's team concluded in their 2007 study. "Over most of the United States," they wrote, the increase in the power of thunderstorms will "more than compensate for the relative decreases in shear."

As a result, "the environment would still be considered favorable for severe convection" of the kind that creates tornadoes.

From March to May the projected increase in severe storms is "largest over a 'tornado-alley'-like region extending northward from Texas," Trapp found. From June through August, the eastern half of the country is projected to experience such an increase.

If there are more days in the future when wind shear is too weak to produce a tornado from a thunderstorm, said Trenberth, then "the frequency of tornadoes may decrease but the average intensity might increase. You could have a doozy of an outbreak, and then they could go away for a while."

On average, about 800 tornados are reported annually in the United States. About 70 percent are "weak," finds NOAA, with winds less than 110 mph. Just under 29 percent are "strong," with winds between 110 and 205 mph. Only 2 percent of all tornadoes are what NOAA characterizes as "violent," with winds in excess of 205 mph, but they account for 70 percent of all twister deaths.

(Editing by Michele Gershberg and Sandra Maler)

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