Historic number of tornadoes have left a path of death and destruction in 2023.
Trying to figure out what the warming climate means for tornado activity in the United States is a bit like trying to modify a recipe.
Add more of one ingredient and you get one result. Take away an ingredient or substitute one ingredient for another and you get an entirely different result.
Scientists studying the “recipe” for tornado activity in the United States, now and in the future, say it’s difficult to tease apart how all the pieces that have to interact for tornadoes to form – such as warmer temperatures and more intense rainfall – may affect storm activity in the future.
However, research announced this week by Northern Illinois University reports continued increases in carbon dioxide emissions could bring about more frequent and more intense supercell storms and tornado activity in the future, especially in the eastern U.S.
One thing’s for sure, the atmospheric ingredients are in overdrive so far this year.
How many tornadoes have there been in the U.S. this year?
Even before the March 31 outbreak, tornado activity – at 311 tornadoes through March 29 – was already the third busiest start to the year since records began, according to data from the National Oceanic and Atmospheric Administration.
Combined with the preliminary total of 104 tornadoes during the devastating March 31 outbreak, the 415 tornadoes for the first quarter would be the busiest start to the year on record. The average through the end of April between 1991-2020 is 337.9.
Why so much tornado activity?
A host of climate patterns and oceanic and atmospheric currents come together to create the conditions favorable for the supercell storms that spawn tornadoes.
“You really need to look at them all together holistically and understand that they all play together in an orchestra in a symphony, a very delicate symphony,” says Victor Gensini, an associate professor at Northern Illinois University.
Heat and humidity help create the instability that spins up supercell storms, intense, long-lived thunderstorms with a rotating updraft. Supercells are responsible for tornadoes and hail and cause billions of dollars in losses and hundreds of casualties every year.
One essential ingredient is moisture and the dial is cranked higher than normal in the Gulf of Mexico this year. Sea surface temperatures have been warmer than normal, thanks in part to a dearth of cold fronts and a persistent high pressure ridge in the region over the winter to help cool it down, said Gensini and others.
“We’re running anywhere from two to maybe four degrees Celsius warmer than average in the Gulf,” he said. When you have “bath water like this” and a southerly wind, it brings more moisture northward.
Combine that with the lingering effects of the La Nina and warm, dry winds from the west and it's the recipe for an active period.
“I don’t really see this going away either,” Gensini said. “I think we will end up with an above average April.”
What do researchers know about climate change and tornadoes?
For more than a year, Professor Walker Ashley and his colleagues in the Department of Earth, Atmosphere and Environment at Northern Illinois, including Gensini and Assistant Professor Alex Haberlie, have been running models that simulate storm activity.
Their results suggest the potential by century’s end for more supercell storms, hail, extreme rainfall and significant tornadoes. And that could have "disastrous consequences,” said Ashley, the lead author.
• They used two trajectories for potential greenhouse gas emissions, to see how that could influence the frequency or characteristics of tornado activity in the future.
• Under either trajectory, the number of annual supercell storms becomes more frequent and intense, with the mean supercell activity increasing in the U.S. by 7% to 15%.
• With increasing carbon dioxide emissions, the study projects an eastward shift in heightened supercell storm activity, particularly in the Ozarks and mid-South, with slight increases in the north and central regions of the Eastern U.S.
• The simulations documented “diminished” storm activity in much of the Great Plains, west of the I-35 corridor.
• Storm timing is expected to shift to earlier parts of the year, trailing off in the later months when temperatures climb in the summer.
Are we seeing climate change impacts already in U.S. tornadoes?
Possibly, but it's not as obvious as a heat wave or extreme rainfall.
Although a multitude of factors enhance conditions and available energy for storms, it's likely we are seeing a climate change signal in storm activity, Gensini said. Their projections, based on model simulations of the future, are consistent with changes already being seen in tornado frequency and location.
"The storms are more intense. They are longer-lived and they happen more frequently in the cool season,” he said. The distribution of tornadoes also is spreading out through the year and decreasing in the summer when temperatures get really hot and wind shear decreases.
Harold Brooks, a senior research scientist at the National Severe Storms Laboratory in Norman, Oklahoma, was a reviewer for the Northern Illinois study.
Its projected change in seasonal activity make sense, Brooks said. "If we make it warmer things should happen," he said. It's already been shown, for example, that fewer tornadoes occur when it's really hot during the summer.
Ashley noted their research is still in its early stages.
One of the more concerning findings, he said, is that the cumulative footprint of the strongest supercells is projected to increase at the same time that communities are becoming more vulnerable because of expanding populations and development, which creates bigger target areas for storms.