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Dozens of tornadoes — including one massive storm that tore through more than 200 miles — struck Kentucky, Tennessee, Arkansas, Illinois, Missouri and Mississippi on Friday and Saturday, killing at least 14 people in four states and dozens more in Kentucky alone.
People following the devastating news out of the region may be wondering: (How) was the storm related to climate change?
After all, most of the extreme weather events that have dominated headlines recently — from wildfires in the U.S. to historic flooding in Western Europe — have had a clear connection to high temperatures, record rainfall and other effects of a warming planet.
The same can't exactly be said for tornadoes, however.
Scientists know that warm weather is a key ingredient in tornadoes and that climate change is altering the environment in which these kinds of storms form. But they can't directly connect those dots, as the research into the link between climate and tornadoes still lags behind that of other extreme weather events such as hurricanes and wildfire.
That's at least in part due to a lack of data — even though the U.S. leads the world in tornadoes, averaging about 1,200 a year.
Less than 10% of severe thunderstorms produce tornadoes, which makes it tricky to draw firm conclusions about the processes leading up to them and how they might be influenced by climate change, Harold Brooks, a tornado scientist at the National Severe Storms Laboratory, told The Associated Press.
Other factors that make that climate change attribution difficult include the quality of the observational record and the ability of models to simulate certain weather events. The National Oceanic and Atmospheric Administration says that's the case with tornadoes.
"The observational record is not consistent and relatively short, the models remain inconclusive as to replicating tornado activity, and our understanding of how global warming and climate change will influence the different atmospheric processes that produce tornadoes (wind shear, for example) is more limited," reads a page on its website.
While scientists may not be able to conclusively connect tornado frequency or intensity to human-caused climate change, they say there are signs pointing in that direction.
Here's what they do know:
What tornadoes are and when they occur
NOAA defines tornadoes as narrow, violently rotating columns of air that extend from a thunderstorm to the ground (while the wind part is invisible, tornadoes can form condensation funnels of water droplets, dust and debris). They can be among the most violent of natural disasters, ripping homes apart, tearing through infrastructure and sending debris flying.
Tornadoes can occur in any part of the U.S. at any time of year.
They have historically been associated with the Great Plains, though experts say the idea of a so-called "Tornado Alley" can be misleading since the tornado threat is a bit of a moving target. It shifts from the Southeast in the cooler months of the year, toward the southern and central Plains in May and June, and the northern Plains and Midwest during early summer.
When people talk about "tornado season," they are usually referring to the time of year when the U.S. sees the most tornadoes — which peaks in May and June in the southern Plains and later in the northern Plains and upper Midwest. This weekend's tornadoes were well outside of typical tornado season, but experts say that in itself isn't rare.
What kind of conditions caused this weekend's storm
Meteorologists are pointing to two contributing factors: warm temperatures and strong winds.
Thunderstorms happen when denser, drier cold air is pushed over warmer, humid air, as the AP explains, and an updraft is created when the warm air rises. Changes in the wind's speed and/or direction (known as "wind shear") can cause the updraft to spin, laying the groundwork for a tornado.
There's not usually a lot of wind instability in the winter because the air is typically not that warm or humid — but that wasn't the case over the weekend.
States across the Midwest and South were experiencing springlike temperatures on Friday. Memphis, Tenn., saw a record high of nearly 80 degrees Fahrenheit, for example.
"The atmosphere didn't know it was December — temperatures in the 70s and 80s," tweeted Mississippi-based meteorologist Craig Ceecee.
That could be a product of many things, from the La Niña climate pattern bringing warmer-than-average conditions to the Southern U.S., to the above-average water temperatures of the Gulf of Mexico, to the warm winter weather that is increasingly common as the planet heats up.
In any case, those high temperatures lent themselves to the warm, moist air that helped form thunderstorms. And once the storm formed, experts believe a strong wind shear (which is typical in the winter) prolonged the duration of its tornadoes.
Victor Gensini, a meteorology professor at Northern Illinois University, told AP that while tornadoes typically lose energy within minutes, this weekend's tornadoes lasted for hours.
The U.S. will likely see more tornadoes beyond their typical time and place
Experts say climate change is impacting the conditions in which tornadoes form and could lead to changes in when and where the U.S. sees them.
John T. Allen, a professor of meteorology at Central Michigan University, wrote in a USA Today opinion column that while ties to climate change are still uncertain, there appears to have been an "eastward shift in tornado frequency" and increasing frequency of tornadoes in outbreaks over the past few decades.
"Climate projections for the late 21st century have suggested that the conditions favorable to the development of the severe storms that produce tornadoes will increase over North America, and the impact could be greatest in the winter and fall," he added.
Brooks, of NOAA's National Severe Storms Laboratory, said the U.S. is likely to see more tornadoes in the winter (and fewer in the summer) as national temperatures rise above the long-term average.
And Gensini told Axios that projections show an increase in major outbreaks in the mid-South and Southeast. He also compared tornado-climate change attribution to the steroids era of baseball, as Axios put it: "Pinning an individual home run on steroid use is difficult, he said, but in the aggregate the trends are evident."
A version of this story first appeared in the Morning Edition live blog.
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