Increasing Tornado Outbreaks—Is Climate Change To Blame?

Extreme meteorological environments associated with severe thunderstorms showed consistent upward trends, but the trends did not resemble those currently expected to result from global warming. They looked at two factors: convective available potential energy (CAPE) and a measure of vertical wind shear, storm relative helicity. Modeling studies have projected that CAPE will increase in a warmer climate leading to more frequent environments favorable to severe thunderstorms in the U.S. However, they found that the meteorological trends were not due to increasing CAPE but instead due to trends in storm relative helicity, which has not been projected to increase under climate change.

“Tornadoes blow people away, and their houses and cars and a lot else,” says Joel Cohen, coauthor of the paper and director of the Laboratory of Populations, which is based jointly at Rockefeller University and Columbia’s Earth Institute. “We’ve used new statistical tools that haven’t been used before to put tornadoes under the microscope. The findings are surprising. We found that, over the last half century or so, the more extreme the tornado outbreaks, the faster the numbers of such extreme outbreaks have been increasing. What’s pushing this rise in extreme outbreaks is far from obvious in the present state of climate science. Viewing the thousands of tornadoes that have been reliably recorded in the U.S. over the past half century or so as a population has permitted us to ask new questions and discover new, important changes in outbreaks of these tornadoes.”

Adds Harold Brooks, senior scientist at NOAA’s National Severe Storms Laboratory, who was not involved with this project, “The study is important because it addresses one of the hypotheses that has been raised to explain the observed change in number of tornadoes in outbreaks. Changes in CAPE can’t explain the change. It seems that changes in shear are more important, but we don’t yet understand why those have happened and if they’re related to global warming.”

Better understanding of how climate affects tornado activity can help to predict tornado activity in the short-term, a month, or even a year in advance, and would be a major aid to insurance and reinsurance companies in assessing the risks posed by outbreaks. “An assessment of changing tornado outbreak size is highly relevant to the insurance industry,” notes Kelly Hererid, AVP, Senior Research Scientist, Chubb Tempest Re R&D. “Common insurance risk management tools like reinsurance and catastrophe bonds are often structured around storm outbreaks rather than individual tornadoes, so an increasing concentration of tornadoes into larger outbreaks provides a mechanism to change loss potential without necessarily altering the underlying tornado count. This approach provides an expanded view of disaster potential beyond simple changes in event frequency.”

Tippett notes that more studies are needed to attribute the observed changes to either global warming or another component of climate variability. The research group plans next to study other aspects of severe thunderstorms such as hail, which causes less intense damage but is important for business (especially insurance and reinsurance) because it affects larger areas and is responsible for substantial losses every year.

The study was partially funded by Columbia University Research Initiatives for Science and Engineering (RISE) award; the Office of Naval Research; NOAA’s Climate Program Office’s Modeling, Analysis, Predictions and Projections; Willis Research Network; and the National Science Foundation.