In the wake of deadly storms that ravaged parts of the South and the Midwest this weekend, scientists had a warning: While the exact link between climate change and tornadoes remains uncertain, higher temperatures could add fuel to these violent disasters.
“A lot of people are waking up today and seeing this damage and saying, ‘Is this the new normal?’ ” said Victor Gensini, a meteorology professor at Northern Illinois University, adding that key questions still remain when it comes to tornadoes because so many factors come into play. “It’ll be some time before we can say for certain what kind of role climate change played in an event like yesterday.”
Still, he said the warm December air mass in much of the country and La Niña conditions created ideal conditions for a turbulent event. Thunderstorms — the raw material for tornadoes — happen when there is warm, moist air close to the ground and cooler, drier air above, creating a path for humidity to travel upward.
The tornadic thunderstorm carved a 250-mile path of destruction through northeast Arkansas, southeast Missouri, northwest Tennessee and western Kentucky, hurling debris into the sky for more than three straight hours. At times, the wreckage reached an altitude of 30,000 feet. As the twister blasted through Mayfield, Ky., it sheared entire homes off their foundations, indicating its top-tier intensity.
Whether or not scientists can pin down a link between this week’s horrific storms and climate change, Gensini said there’s no doubt that the tornadoes will go down as “one of the most devastating long-track tornadoes” in U.S. history — probably the worst since the Tri-State Tornado of 1925, which tore across three states over several hours and killed hundreds of people.
Meteorologists at the National Weather Service are surveying damage to determine whether this week’s storm was a single tornado or several. If it was a single tornado traveling on the ground without interruption for 250 miles, it will rank as the longest tornado track in U.S. history and the first to cross through four states.
As the death toll is expected to swell, it will become the deadliest December tornado outbreak on record and potentially among the most deadly in any month. Until Friday night, the deadliest December outbreak occurred on Dec. 5, 1953, which killed 38 people in Vicksburg, Miss.
Temperatures in the zone ravaged by tornadoes rose into the 70s to near 80 degrees Friday afternoon, providing the conditions for severe thunderstorms to develop that night. Dozens of record highs were set that day in the states hardest hit by the storms: In Memphis, temperatures soared to 79 degrees, breaking a 103-year-old record.
In a warming world, Gensini said: “It’s absolutely fair to say that the atmospheric environments will be more supportive for cool-season tornado events.”
But Gensini and other climate and weather experts noted that tornadoes are among the most difficult events to link definitively to global warming, partly because they are relatively small and short-lived events compared with the wildfires, heat waves and other climate disasters.
“Tornadoes are, unfortunately, one of the extreme events where we have the least confidence in our ability to attribute or understand the impact of climate change on specific events,” said Zeke Hausfather, a climate scientist who directs climate and energy programs at the Breakthrough Institute, a climate research center based in Oakland, Calif. “There is not much evidence to date that the number of strong tornadoes is different today than it was over much of the past century.”
The most recent National Climate Assessment, assembled by experts at numerous federal agencies, underscored this dynamic in 2018.
“Observed and projected future increases in certain types of extreme weather, such as heavy rainfall and extreme heat, can be directly linked to a warmer world,” the authors wrote. “Other types of extreme weather, such as tornadoes, hail, and thunderstorms, are also exhibiting changes that may be related to climate change, but scientific understanding is not yet detailed enough to confidently project the direction and magnitude of future change.”
The report went on to say tornado activity in the United States has become “more variable” in recent decades, with a decrease in the number of days per year that saw tornadoes but an increase in the number of tornadoes on these days.
Harold Brooks, a senior research scientist at the National Oceanic and Atmospheric Administration’s Severe Storms Laboratory, said extreme tornadoes are becoming “clumpier” over time, meaning they have become more concentrated in day-long bursts. He said it wasn’t clear whether there was a connection to climate change.
And warmer Decembers will probably lead to more December thunderstorms.
“Studies suggest that the frequency and intensity of severe thunderstorms in the U.S. could increase as climate changes, particularly over the U.S. Midwest and Southern Great Plains during spring,” Hausfather said.
The additional factors required to transform a thunderstorm into a tornado — among them, a change in wind speeds at different altitudes, with more intense winds up above — are so complex that scientists have a hard time drawing a straight line between warming temperatures and more frequent tornadoes. In fact, the number of intense tornadoes in the United States “hasn’t changed back into the fifties,” Brooks said, around 500 a year.
There may be some evidence that wintertime tornadoes are becoming more frequent, Brooks said. But the tornadoes are rare enough — and hard enough to measure and classify — that it is difficult to achieve statistical confidence.
What is unequivocal, Gensini said, is that the steady warming of the planet is making a broad array of extreme weather disasters more likely and more intense.
“When you look at these things in the aggregate, it becomes pretty clear that changes are happening,” he said. “There are definitely shifts in the probability of these things happening.”
That includes longer severe weather seasons and more instability in the atmosphere during “shoulder” seasons when such disasters historically would have been less common, he said. It also could include more variability than in the past — some years could be very intense and marked by multiple catastrophes, while others could prove mild when it comes to severe weather.
That is in line with the findings of a major report the U.N. Intergovernmental Panel on Climate Change released this summer, which found that as humans have continued to pump planet-warming gases into the atmosphere, weather-related disasters are growing more extreme and affecting every region of the world.
While the report didn’t draw a conclusion about the relationship between tornadoes and climate change, it noted that the frequency and intensity of extreme precipitation in the United States was increasing.
One thing that’s different now: More people and more “things” lie in the paths of future tornadoes and other extreme weather events.
“As populations continue to grow, as cities get larger, as we continue to have more things — this is what you get,” Gensini said. “I know for certain there will be more disaster as our human footprint continues to grow.”