BEIJING, Aug. 31 (Xinhuanet) -- Americans can expect more severe thunderstorms and tornadoes because of global warming, NASA scientists said Thursday.

A new space agency climate model is the first to show the difference in strength between storms that occur over land and those over the ocean, and how storms strengths will change in general. The models don't directly simulate thunderstorms and lightning, but look for conditions that are conducive to severe storms.

The model was tested against current climate conditions and produced well-known major global storm features, including the high levels of lightning seen over Africa and the Amazon basin and the near absence of lightning in ocean storms.

Researchers then ran a program where carbon dioxide levels were double what they are now and increased the Earth's surface temperature 5 degrees Fahrenheit. The model's projections showed that continents warm more than oceans (a result which is expected because water needs to absorb more heat than land to raise its temperature) and that lightning occurs at a higher altitude where storms are usually stronger.

These effects would combine to cause more continental storms to be of the strongest kind we see today, though there would be fewer storms overall.

These results are unfortunate for the storm-prone portions of the central and eastern United States, where strong winds are a major source of weather-related casualties. The western United States are expected to get drier, but the storms that do form are likely to have more lightning, which could then trigger more wildfires.

"Drier conditions near the ground combined with higher lightning flash rates per storm may end up intensifying wildfire damage," said study leader Tony Del Genio of NASA's Goddard Institute for Space Studies in New York.

(Agencies)