The exact link between tornadoes and climate change is hard to draw. Here's why

Rural communities in western Mississippi are surveying and cleaning up the damage after an unusual and powerful tornado tore through the area Friday night.

The tornado killed at least 25 people in Mississippi and one in Alabama, destroyed buildings and left hundreds without shelter.

The tornado landed in Rolling Fork around 8 p.m. local time before blowing through nearby towns including Silver City, Black Hawk and Winona, staying on the ground for an hour and 10 minutes.

The National Weather Service says it spanned roughly 170 miles and had a path of 59.4 miles, an unusually long distance compared to what it calls a typical tornado path of 1-2 miles.

The NWS has given the tornado a preliminary rating of 4 out of 5 on the Enhanced Fujita (EF) Scale, which estimates wind speeds (in this case, 166-200 mph) based on the damage caused.

"This is one of the more rare tornadoes that we've seen in recorded Mississippi history given its longevity and strength over a period of time," National Weather Service meteorologist Lance Perrilloux told NPR.

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 heat waves to atmospheric rivers to historic floods — have had a clear connection to high temperatures, record rainfall and other effects of a warming planet.

Years of research have shown how climate change intensifies rain storms, heat waves and hurricanes, as NPR has reported.

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 in 2021.

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.

Severe thunderstorms and tornadoes tend to start ramping up in the month of March (usually doubling from February), with the threat most concentrated in Southern states. That's the result of the clash between winter and spring weather patterns, with a still-strong jet stream and warmer air moving northward.

Recent Marches have been especially active, the Weather Channel notes: There were 236 recorded tornadoes in March 2022, the most in that month since 1950.

What kind of conditions shaped Friday's destructive storm

This particular tornado — one of at least 10 reported in Mississippi, Alabama and Tennessee from this storm — had its origins in a type of rotating thunderstorm known as a supercell.

The storm system started earlier in the week in California, where it spawned the strongest tornado to hit Los Angeles County since 1983. It then continued its journey east, triggering deadly floods in Arizona and the central swath of the country and gaining strength along the way.

Many states saw record-high temperatures during this period, and the Washington Post explains that the warm and humid air — exacerbated by unusually high sea surface temperatures over the Gulf of Mexico — helped energize the storm.

Warm winds from the south fueled the storm at ground level, it adds, while westerly winds of the jet stream generated extreme amounts of "wind shear" — the change in wind speed and/or direction with height that can lead to the development of tornadoes.

The National Oceanic and Atmospheric Administration (NOAA) says as few as 20% of all supercell thunderstorms produce tornadoes, but that those are the most common — and often the most dangerous — kind of twister.

NOAA compares supercells to cancer cells in a living organism, because "the rotation of their updraft enables them to overcome the self-limiting mechanisms that bring demise to regular storms," lasting for "an appreciable length of time" and causing damage all the while.

Storm chasers and meteorologists have described Friday's event as a "wedge tornado," a slang term meaning its funnel is at least as wide on the ground as it is tall.

Its maximum path width was 3/4 mile, the NWS says, which might help explain the extent of the damage.

Experts say other factors could include the disproportionate number of mobile homes (which are more vulnerable to tornado damage) in the Southeast U.S., the nighttime arrival of the tornado and a relatively short warning period: The NWS issued its first tornado warning just some 20 minutes before the storm reached Rolling Fork.

About 75% of Rolling Fork — a predominantly Black town of about 2,000 people — is "pretty much flattened," NPR's Debbie Elliott reported on Monday.

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 2021.