Adding a grim exclamation point to a year of deadly climate extremes, the early December tornadoes that killed at least 90 people in the Southeast were some of the most intense storms on record so late in the year. 

The storms fired up in Arkansas the night of Dec. 10, during weather far too hot and humid for the season, and raced across Missouri, Illinois, Tennessee and Kentucky on Dec. 11. It will take weeks of analyzing data to make final classifications of the tornado outbreak’s intensity. But some of the mega-twisters that destroyed lives, livelihoods and communities may have raked the ground for 250 miles and thrown debris 30,000 feet high into the atmosphere. 

So far in 2021, nine severe storm episodes (not just tornadoes) have caused $15 billion in damage and accounted for half of the climate-related events on the federal billion-dollar disaster list. The increasing trend of damages from severe storms has also been tracked by the insurance industry, which shows losses steadily increasing for 40 years.

 In 2017, a research meteorologist with Munich Re, a global reinsurance company, wrote in a newsletter that “an increase of atmospheric heat and moisture due to our warming climate will likely increase the number of days per year that are favorable for thunderstorms and their associated hazards, including tornadoes.”

It’s not yet clear if and how global warming fuels individual tornadoes, because they are so small they can’t be reproduced by climate models. But after a Northern Hemisphere summer of floods, droughts, smoky wildfires and heat waves, climate scientists and meteorologists on social media and in broadcast interviews placed the December tornadoes squarely in the context of global warming.

Swiss climate scientist Sonia Seniveratne, an author of the latest Intergovernmental Panel on Climate Change report, noted in a tweet over the weekend that the document “affirms that days with a large number of tornadoes have become more frequent in the US.” A 2014 study showed that clear increase in tornado clustering, climate scientist Zack Labe added, in his own tweet. 

More clues to the connection between global warming and tornadoes can be found in research showing that a warmer atmosphere increases the frequency of some upper air wind patterns that favor extremes. After a May 2019 tornado outbreak in the Great Plains, climate scientist Stefan Rahmstorf showed on Twitter how those increasingly persistent loops in hemispheric winds can set up tornado conditions.

The 2014 National Climate Assessment also documented an increase of severe storm frequency, with “new and stronger evidence … that some of these increases are related to human activities,” but some puzzle pieces are still elusive, said Columbia University climate researcher Chiara Lepore, because there is so much natural variability in tornado activity from year to year that it’s hard to find a trend. 

Tornadoes, even the biggest ones, are smaller than pinpoints on the global climate scale. “Large scale rainfall events, droughts or hurricanes, are several orders of magnitude bigger and easier to attribute,” she said. “It is reasonable to expect that climate change has and will have some kind of effect on tornado activity. Right now we don’t know how.” 

Columbia University researcher Michael Tippett said a deadly 2011 tornado outbreak sparked his research interest, and explained how scientific understanding of tornadoes has increased since then. What seems increasingly certain, as reflected by the IPCC, is that climate change is driving a “creeping increase” in atmospheric conditions conducive to tornado formation, he said.

Scientists also know more about how a cyclical cooling phase of the Pacific Ocean called La Niña drives more frequent tornado outbreaks. Now it’s time to put that all together in the latest climate system models that can analyze all that information with more accurate results, he said.

The recent study he co-authored with Lepore suggests that increases in conditions favorable to breeding severe storms and tornadoes are between 5 and 20 percent for every 1 degree Celsius of warming, “depending on exactly where you are in the world,” he said. “What little confidence we have is toward the frequency side.”

Lepore was a co-author of a study published last month analyzing that trend with the latest climate models that combine more ingredients and can look at smaller scale patterns. She said the findings are in line with previous data that “project increases in frequency in many parts of our planet for conditions conducive to severe weather, especially for the northern hemisphere and northern latitude.” The study doesn’t focus only on tornadoes but includes severe thunderstorm conditions like hail storms and straight line wind storms, she added.

“This work definitely helps to shed some light on the relationship between a warming climate and conditions conducive to severe weather,” she said. “But there is still a gap in our understanding between these projected changes in frequency of severe storm conditions and the realization of these events into actual storms. This gap is still unresolved, I think. We need more research on it.”