NASA: More twisters ahead

Warming may put storms on steroids

WASHINGTON -- As the world warms, the United States will face more severe thunderstorms with deadly lightning, damaging hail and the potential for tornadoes, a trailblazing study by NASA scientists suggests.

Other research has warned of broad weather changes on a large scale, such as more extreme hurricanes and droughts. But the new study predicts that even smaller events like thunderstorms will become more dangerous because of global warming.

The basic ingredients for whopper U.S. inland storms are likely to be more plentiful in a warmer, moister world, said lead author Tony Del Genio, a NASA research scientist.

And when that happens, watch out.

"The strongest thunderstorms, the strongest severe storms and tornadoes are likely to happen more often and be stronger," Del Genio said in an interview Thursday from his office at the Goddard Institute for Space Studies in New York. The paper he co-authored was published online this month in the journal Geophysical Research Letters.

Other scientists caution that the study is not the last word on the subject. But some coming studies point in the same direction.

With a computer model, Del Genio explores an area that most climate scientists have avoided. Simple thunderstorms are too small for their massive models of the world's climate. So Del Genio looked at the forces that combine to make thunderstorms.

A unique combination of geography and weather patterns already makes the United States the world's hottest spot for tornadoes and severe storms in spring and summer. The large land mass that warms on hot days, the contours of the atmosphere's jet stream, the wind coming off the Rocky Mountains and warm moist air coming up from the Gulf of Mexico all combine to produce severe weather.

Del Genio's computer model shows global warming will mean more strong updrafts, when the wind moves up and down instead of sideways.

"The consequences of stronger updrafts are more lightning and bigger hail," he said.

On a normal sunny day, updrafts move at less than 1 mile per hour. In a big but not severe rainstorm, it's about 2 mph. In a severe storm they can increase to 20 to 30 mph. The faster the updraft, the worse the storm.

Enough trouble for everyone

The Southeast and Midwest lie in the path of most of the worst storms.

However, the new study also forecasts danger for the Western United States. It predicts lightning will increase about 6 percent as the amount of carbon dioxide -- the chief global warming gas -- doubles from what it is today.

Previous studies have shown that the West will get drier, making it a tinderbox for more wildfires. This study shows that there will be more matches in the form of lightning strikes to start those fires, Del Genio said.

Other pending and recent research, especially from the National Oceanic Atmospheric Administration, points in the same general direction, said several scientists who weren't involved in Del Genio's study. But they said research in this area is so new that the NASA study is not the final word.

"It's certainly a plausible result," said Leo Donner, a climate modeling scientist at NOAA's Geophysical Fluid Dynamics Lab in Princeton, N.J. Earlier this year, Donner came out with a study predicting more heavy rain as temperatures rise.

Harold Brooks, a top scientist at NOAA's severe storms laboratory in Norman, Okla., has led soon-to-be-published studies with results similar to the new NASA study, especially when it comes to hail. Some of the severe hail could be baseball-sized and come down at 100 mph, "falling like a major league fastball," he said.

He argued that it's not possible to predict whether more tornadoes will result from climate change, however.