Study Sees a Higher Risk of Storms on the Horizon

September 20th, 2013 by Staff

(nytimes.com) The eastern and central United States likely will see a greater risk of severe weather by the middle of this century as rising temperatures trigger atmospheric changes that favor storms, a new study by climate scientists from Stanford and Purdue universities concludes.

By the century’s final 30 years, the study forecasts, the eastern United States could experience severe thunderstorms an average of nearly 7.5 spring days, an increase of almost 42 percent. A 15 percent increase is forecast during June, July and August.

The largest single increase, an average of more than 2.4 days, was likely from March through May across parts of Texas, Oklahoma and Louisiana.

The study’s data suggest — but do not flatly predict — that the number of days with conditions favorable to tornadoes will increase as well.

The peer-reviewed findings, by Noah S. Diffenbaugh and Martin Scherer of Stanford and Robert J. Trapp of Purdue, were published on Monday in the Proceedings of the National Academy of Sciences. The results were consistent across 10 different models that scientists worldwide use to estimate the impact of climate change, Dr. Diffenbaugh said in an interview.

The study measured the impact of projected increases in temperature on conditions that are known to produce severe thunderstorms. It makes theoretical projections of changes in those conditions daily — and even for parts of a day — throughout the rest of the century.

The calculations, never before made at such minute levels, required the use of computers at climate-change research centers around the world, Dr. Diffenbaugh said.

“We can look starting in the recent past and analyze how many severe thunderstorm environments there are in each model, in each year, all the way out to the end of 21st century,” he said.

All 10 models projected an overall increase in potential severe-weather days in the spring and autumn after about 2040. In some models, the risk of storms was found to decrease in part of the nation centered on Kansas and Nebraska, and only in summer.

The authors cautioned that some elements of thunderstorm formation, such as processes that stimulate atmospheric convection, were not included in their study. And they stressed that the conditions they did include are conducive to severe storms, but do not guarantee them.

Harold Brooks, a senior research scientist at the National Severe Storms Laboratory in Norman, Okla., called the findings “really intriguing,” noting that the study’s season-by-season analysis may add significantly to past studies that generally had made yearly predictions.

Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo., a second scientist whose own research has foreshadowed some of the findings, said he believes the study “actually underplays the worries.”

The likelihood of severe weather is “probably fairly robust and the prospects worrying,” he said in an e-mail.

The National Oceanic and Atmospheric Administration, which tallies severe-weather events that cause more than $1 billion in damage, recorded 13 such events in 2011 and 2012 involving thunderstorms and tornadoes. Those storms killed 628 people and caused nearly $45 billion in damage.

Various weather conditions help spawn thunderstorms, but among the most important are humidity, vertical wind shear and atmospheric instability, which can cause warm air to rush upward through colder air.

Because hot air can hold more moisture than cold air, rising temperatures ensure greater humidity in the future. The study predicted that the amount of energy available for a storm, called convective available potential energy, would also become more frequent as temperatures rise.

The number of days with wind shear likely would decrease, the study found — but almost all the decline would be on days when the air was calm, and thunderstorms were unlikely in any case. During periods of storm-spawning instability, the chances of wind shear were reported to rise.

That last point, which Dr. Diffenbaugh called a key finding, addresses questions about the projected decline in wind shear that have led some to question the link between higher temperatures and an increase in severe weather.

By some measures, severe weather has been on the rise in recent years, a development that some have attributed to climate change. Dr. Diffenbaugh said the study draws no such conclusion; the 10 models employed in the study did not reach general agreement on an increase in severe-weather days until about 2040, he said.

“We have a real challenge in trying to understand whether or not there have been changes in the likelihood of occurrence in the recent past,” he said. While temperatures have been assiduously recorded, he said, records of storms and tornadoes are considerably murkier


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