The tornadoes that recently struck the U.S. are some of the most destructive and deadly in history. The death toll in Kentucky, the hardest hit state, reached 80 on Monday, with dozens still unaccounted for.
The scale of destruction and timing of the tornadoes so late in the year—most tornadoes occur in the spring and summer —is fueling discussion about how climate change may have influenced this deadly outbreak.
“In my 40 years as a meteorologist, this was one of the most shocking weather events I’ve ever witnessed,” says Jeff Masters, a meteorologist at Yale Climate Connections. “Watching these storms on Friday night, my thought was, ‘Is no season safe?’ Extreme tornadoes in December. That was mind blowing to me.”
Unlike heat waves and floods, the link between a warming world and tornadoes is complicated and inconclusive. Scientists have several theories about how tornado behavior may change. December tornadoes and a shift toward the southeastern United States are possible. Whether climate change will make tornadoes more intense or frequent remains to be seen.
While there’s been a recorded increase in the overall number of observed tornadoes since 1950, according to National Oceanic and Atmospheric Administration data, experts say that’s largely a result of better technology such as Doppler radar. There’s been no observed increase in the frequency of major tornadoes over time.
For example, 59 of the most severe F5 tornadoes in the U.S. have occurred since 1950. But if the storm that devastated Kentucky turns out to have been an F5, it will have been the first since 2013–bringing to end the longest recorded period so far without one of those disastrous twisters.
Nevertheless, given the pervasive influence of global warming on the atmosphere, it makes sense that it should be having an impact on tornadoes too, says Victor Gensini, an extreme weather expert at Northern Illinois University.
“Instead of asking: ‘Did climate change cause this tornado?’ It’s better to operate under the assumption that climate change did play a role,” he says. “Start from the premise that every extreme event is being affected by climate change.”
Tornado weather and how it’s changing
To understand how scientists theorize climate change is impacting tornadoes, it helps to understand how warm, moist air flowing beneath cool, dry air creates unstable atmospheric conditions.
As warm air rises into cooler air, wind shear—a sudden change in the wind’s speed, direction, or both—can set this upward-moving air spinning like a top, creating a tornado.
As the climate warms, it’s putting more heat into the atmosphere and creating more energy that can feed tornadoes. Large December tornadoes are rare because December tends to be cool—but the U.S. is experiencing unusual warmth this year, including over the Gulf of Mexico, where the moisture that fuels tornado-forming thunderstorms originates.
“[The Gulf] is running a fever right now,” says Gensini. “On Friday morning, we looked at weather maps, and it looked like a spring day.”
That same day, Memphis, Tennessee, saw record-high temperatures; the tornado traveled across parts of the state.
How climate change will alter the winds that bring tornadoes to life is still unclear.
Warmer conditions actually may be diminishing the wind shear that spins up tornadoes; the Arctic is warming more quickly than lower latitudes, reducing the temperature difference between them and making the winds produced by the jet stream weaker overall. That reduces the strength of wind shear.
“It’s anybody’s guess how that’s going to play out,” says Masters. When conditions are ripe for a tornado, however, more heat will mean “you can have bigger outbreaks because there’s more energy” stored up, Master says. That means there could be more time between tornado outbreaks, but more tornadoes potentially spinning off when an outbreak does occur. One study did indeed find an increase over decades of the average number of tornadoes per outbreak.
Research on tornadoes is challenging because the events often happen at much smaller scales than other types of extreme weather, making it hard to have enough data to draw strong conclusions. While the tornado that struck Kentucky traveled more than 200 miles for an estimated three hours, small tornadoes have been historically more difficult to observe, leading to a poor database to compare today’s tornadoes to.
One study published in 2018 looked at tornado observations since 1979 and observed a shift in tornado locations, from slightly west of the Mississippi River to east of the river, in more populated states like Kentucky and Arkansas.
“We think it might be climate change. But it might also be natural variability,” says Gensini, an author on the study. “We stepped on the scale, and we know we gained 15 pounds, but we don’t know if it’s poor diet or lack of exercise.”
However even a slight geographic shift could have major consequences if storms become more common in more densely populated parts of the country. A tornado striking a cornfield is less dangerous than one tearing through a subdivision.
“The scary part of this is, in the Southeast [there are] an increasing number of manufactured homes,” says Stephen Strader, a geographer studying extreme weather risk at Villanova University. “The odds are stacked against us [there].”
Overall, Americans have made great progress in protecting ourselves against tornadoes: When population increase is taken into account, the tornado fatality rate has declined dramatically over the past century, in large part because of improved weather forecasting and warning systems.
But how we build our communities—and how soft a target we offer tornadoes—will have a major impact on how much death and destruction they cause in future.
“We could save countless lives if we improved how mobile homes are anchored to the ground,” Strader notes.
While the science on climate change and tornadoes isn’t clear, meteorologists are predicting that conditions conducive to tornado formation will persist this month.
“I’m worried for the rest of the year to be honest. I don’t think we’re out of the woods yet,” Gensini says.