An increasing trend of tornado outbreaks in the U.S. : is climate change to blame?

Researchers from Columbia University have identified a clear upward trend of tornado outbreaks in the U.S. When it comes to the contributor of this increasing trend, not so clear, though. Theoretically, an unstable atmosphere in a changing climate should give birth to more severe storms. However, this study, based on extreme value analyses, has not been able to "associate the observed changes in our environmental proxy, and by extension the changes in tornado outbreak statistics, with previously identified features of a warmer climate." The authors also cautioned, "this conclusion is, of course, subject to revision by the discovery of other implications of a warmer climate for severe thunderstorm environments."