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Extreme Tornado Outbreaks Are on the Rise, Study Says

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The average number tornado outbreaks that bring multiple twisters from a single weather event is on the rise in the U.S., according to new research, and the findings could change the way insurers and disaster preparedness officials respond to tornadoes.

The study, published in the journal Nature Communications, also shows an increased variability in the number of tornadoes from one outbreak to another. Higher variability means that large outbreaks that result in multiple tornadoes can be more common while the total number of tornadoes in a given year remains relatively constant. Tornado outbreaks result in dozens and sometimes hundreds of tornadoes each year and cause billions of dollars in damage. One such outbreak in 2011 resulted in 363 tornadoes in North America that killed more than 350 people.

“It means that when it rains, it really, really, really pours,” says study co-author Joel Cohen, a professor at Rockefeller and Columbia University, in a press release.

The reason behind the change in tornado patterns remains unclear. The short length, unpredictable arrival and relatively small size of tornadoes make them difficult to study. But researchers suggest that climate change may be a possible explanation for the change in patterns. The weather phenomenon occurs during periods of atmospheric instability and when there are large differences in wind speed in a given area known as “wind shear,” both of which could be affected by temperature increases.

“The science is still open,” says study co-author Michael Tippett, a climate and weather researcher at Columbia University, in a press release. “It could be global warming, but our usual tools, the observational record and computer models, are not up to the task of answering this question yet.”

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Write to Justin Worland at justin.worland@time.com