The study's authors said in a press release that they're not sure what exactly is driving the increases.
"The science is still open," said lead author Michael Tippett, a climate and weather researcher at Columbia University's School of Applied Science and Engineering and Columbia's Data Science Institute. "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."
Tippett said that many scientists expect the frequency of atmospheric conditions favorable to tornadoes to increase in a warmer climate - but even today, the right conditions don't guarantee a tornado will occur "When it comes to tornadoes, almost everything terrible that happens, happens in outbreaks," Tippett explained. "If outbreaks contain more tornadoes on average, then the likelihood they'll cause damage somewhere increases."
NEWS: El Nino Can Predict Tornado Season's Severity
The study was co-authored by Joel Cohen, director of the Laboratory of Populations, which is based jointly at Rockefeller University and Columbia's Earth Institute.