A crew of residents gather to clean the debris from a convenience store Thursday, Dec. 1, 2016, in Rosalie, Ala., the day after a reported tornado struck. (AP Photo/Brynn Anderson)

(Bloomberg) – Tornadoes kill more people when they occur in “outbreaks,” storm systems that spin out more than a half dozen or so funnels in a limited time and area. Such twisters killed 49 people in 2015, and almost 80 percent of tornado fatalities from 1972 to 2010 occurred during outbreaks. 

That’s an increasing concern for citizens of “tornado alley” and the insurers and reinsurers who cover them, as outbreaks in the U.S. are becoming more extreme, according to a study published Thursday in the journal Science. The work, building on previous research, has left researchers with some unexpected questions. For one, the data are inconsistent with how global warming is expected to change our world.

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