The number of tornadoes in each outbreak has increased up to 50 percent in the past 60 years, between 1954 through 2014. Additionally, the study published online on Feb. 29 in the journal Nature Communications also reveal that the number of tornadoes has remained the same annually, proving that tornadoes come in a cluster with each outbreak.

“Over a recent 10 year period, tornadoes in the United States resulted in an average of 110 deaths per year and annual losses ranging from $500 million (AU$ 674.2 million) to $9.6 billion (AU$ 12.9 billion),” said researcher Joel Cohen, Abby Rockefeller Mauzé Professor and head of the Populations at The Rockefeller University. “Outbreaks are the main offender, but relatively little was known about the current or projected statistics of their severity. We wanted to know if they’ve gotten worse over time, and how intense they may become.”

Tornado. Photo from Wikimedia/NWS Louisville, KY

Tornado. Photo from Wikimedia/ NWS Louisville, KY

Along with co-author Michael Tippett of Columbia University, they analysed reports from the National Oceanic and Atmospheric Administration. Aside from studying the average number of tornadoes in an each outbreak, they also studied the variance or differences of tornado numbers in each outbreak.

“In the time period investigated, we observed that both the mean and variance of the number of tornadoes per outbreak were increasing,” Cohen added. “An increase in both of these values over time is noteworthy — if the mean number of tornadoes per outbreak is larger each year, and the range of the values is more widespread each year, it suggests that outbreaks with an extreme number of tornadoes are more likely in the future.”

Additionally, the researchers noted that variance is increasing four times faster than the mean. “If I were to speculate, I would say that certain physical drivers are accelerating, leading to increased energy in the atmosphere, which affects the forces behind tornadoes. Our results do not directly link climate change to the increasing severity of outbreaks, but we’ve found an indicator of change that’s hard to explain otherwise,” explained Cohen.