The Link Between Tornadoes and Climate Change Is Complicated

A line of severe storms cross the Mississippi River in Memphis, Tenn., passing by the Memphis Pyramid on Wednesday, May 25, 2011. Photo: Lance Murphey (AP)

In the wake of a devastating hurricane or wildfire, it’s fair to point out the ways that climate change is making those disasters more frequent and more intense. But when it comes to tornadoes—powerful vortices that can appear and destroy a community with little notice—the link to climate change is less clear.

There’s plenty of evidence that global warming fuels extreme weather, so it’s tempting to assume the same is true for tornadoes. But as Walker Ashley, a meteorologist and professor at Northern Illinois University, explained, experts remain unsure about how climate change influences individual tornadoes.

A tornado is a column of violently rotating air. The bottom of the column sometimes touches the ground, while the top is often connected to the base of a thunderstorm.

Tornadoes vary in size, distance traveled, and intensity. The deadly tornado that recently hit Mississippi traveled for 170 miles and was on the ground for about an hour. Strong tornadoes like that one can have wind speeds as high as 300 mph. Weaker tornadoes, which make up most of the tornadoes that form, have wind speeds 100 mph or less and may last only a few minutes.

Tornadoes require several ingredients to form. There needs to be warm, moist air close to the ground, with dryer, colder air higher up. A tornado also needs wind shear. This is the variation in wind speed and direction, and if this is strong enough, it enhances rotation in a thunderstorm and helps form the tornado. “Wind shear takes an ordinary storm and organizes it so that it can go on to produce perils like hail and tornadoes,” Ashley told Earther.

That type of storm is called a supercell. Think of the large, rotating thunderstorms that fill the sky with a huge anvil-shaped, ominous gray cloud. Supercell storms can last for hours and bring about a twister.

On a global scale, tornadoes are small weather events that begin and end within relatively short periods of time. It’s hard to connect those individual weather events to larger forces, like a changing climate. A clearer connection is found by studying the ingredients that come together to form tornadoes, according to Ashley.

“There’s a lot of steps in between those two,” he said. “The big fundamental question is, ‘is climate change contributing to the environments that support these, or sustain these, or make them more intense?’ The science really isn’t quite there yet.” This is because some of the factors that contribute to storms can be studied with global climate models, like temperatures and water vapor in the atmosphere. “Moisture is a slam dunk. It’s going to increase because we have a warming environment and warming oceans,” Ashley said.

Some research suggests that supercells in the U.S. have showed signs of change. Scientists at Northern Illinois University, including Ashley, compared years of those rotating thunderstorms. They also used smaller regional models and found that the number of supercells is likely to increase in frequency and intensity by the end of this century. The storms will be boosted by increased humidity and warmer temperatures.

John T. Allen, a meteorology professor at Central Michigan University, said that studying supercells is great, but it’s still not always clear if a tornado will form. As few as 20% of supercells create tornadoes, according to the National Oceanic and Atmospheric Administration. “[Sometimes] the Storm Prediction Center might even need to issue a tornado watch, and nothing happens,” Allen told Earther. So while the storms and storm environments are easier to study, researchers are still gathering data and learning about tornadoes. “The research is much younger than the field for hurricanes and for temperature,” he said.

“I think we’ll get there one day,” Ashley told Earther. “But even if the science advances, you’re going to be hard pressed to say, ‘5% was due to climate change’, or whatever the contribution might be.”