Global Warming to Fuel More Severe Tornadoes and Thunderstorms

We can all stop arguing about whether the climate is changing. Evidence is overwhelming, from shrinking glaciers to melting polar ice caps and seas rising at twice the rate of the pre-industrial era. Animals are changing migration and mating patterns; in the North, 125 lakes disappeared; river ice is melting sooner in spring. This year is expected to be the hottest, stormiest and driest on record. The big remaining question is how much of the trend is natural (scientists admitted they know little about the Sun's role!) and how much is exacerbated by greenhouse gas emissions. Meanwhile, a host of studies made dire predictions about the inevitability of rising temperatures and swamped coastlines over the next century. Nasty side effects were predicted: more intense rainstorms; worse droughts; stronger hurricanes; increased allergies; ice-free arctic summers; and economic costs. A couple novel solutions were proposed: altering airline flights and lofting a ring of miniature satellites to shade the equator. Tempers rose in 2005, too, with the year closing on a low note from the perspective of more than 150 nations who pledged to do something about the problem, without the support of the United States or China.

Global warming will make severe thunderstorms and tornadoes a more common feature of U.S. weather, NASA scientists said today.

Climate models have previously shown that Earth will see more heavy rainstorms as the atmosphere warms, but a new climate model developed by NASA researchers 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 ripe for severe storms to form.

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 the model for a future climate scenario where carbon dioxide levels were double their current level and the Earth's surface was 5 degrees Fahrenheit warmer than it is now. 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 more vigorous.

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 conclusions are particularly bad news 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 won't catch a break either—while it is expected to get drier, 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.

The results of the study are detailed in the Aug. 17 issue of the journal Geophysical Research Letters.

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