Scientists who have been studying the sudden outbreak of larger tornado clusters in the United States have been expecting to tie it to climate change, but a group of researchers have made a rather incredible discovery: global warming appears to have nothing to do with it.

The number of tornadoes that touch down in the U.S. hasn’t really changed much in the last 50 years, but the most extreme outbreaks, which is when there are clusters of several twisters in a single weather event, is growing.

A team of researchers from Columbia University thought that climate change must be playing a role in these extreme events, but they didn’t find the telltale signs they thought they would. That doesn’t mean climate change isn’t behind the outbreaks of tornado clusters nationwide, but it certainly doesn’t boost the case for that theory, according to a statement from the university.

“This study raises new questions about what climate change will do to severe thunderstorms and what is responsible for recent trends,” says Michael Tippett, associate professor of applied physics and applied mathematics at Columbia Engineering. “The fact that we don’t see the presently understood meteorological signature of global warming in changing outbreak statistics leaves two possibilities: either the recent increases are not due to a warming climate, or a warming climate has implications for tornado activity that we don’t understand. This is an unexpected finding.”