It's a question that has come up a lot this tornado season: Since global warming can make other types of extreme weather more likely, could it be behind these deadly twisters?
We asked some of the leading scientists who have been studying tornadoes and the physics of extreme weather and climate change to share their insights.
Many of them pointed out that it can be tough to detect tornado trends because comprehensive records only go back a few decades and there's a lot of variability in tornado activity year to year. But they said some shifts are starting to show: while tornado intensity doesn't appear to have changed, there are more days with multiple tornadoes now, and there may be a shift in which regions are especially prone to tornadoes.
There is growing evidence that "a warming atmosphere, with more moisture and turbulent energy, favors increasingly large outbreaks of tornadoes, like the outbreak we've witnessed in the last few days," said Penn State University climate researcher Michael Mann.
"There is also some evidence that we might be seeing an eastward shift in the regions of tornado genesis—again, consistent with what we are seeing," he added.
Tornadoes are complex, dynamic, short-lived and small, which makes them hard to study. But the deadly 2011 outbreak, which included the tornado that tore through Joplin, Missouri, spurred a new wave of studies that help explain how global warming affects tornado activity, said Harold Brooks, a senior scientist with the National Severe Storms Laboratory in Norman, Oklahoma.
More broadly, Brooks said, researchers are looking at severe storm development, because even without tornadoes, giant thunderstorms can produce damaging hail and destructive winds. There's a robust signal that global warming will make the atmosphere more likely to spawn such storms.
But for tornadoes, there are still more questions than answers.
Ingredients of a Tornado
Tornadoes need three things to form: warm, moist air near the ground, cold and dry air above, and horizontal winds generally getting stronger with height, and turning as you go up, Brooks said.
At ground level, the winds blow from the equator; at high elevation, from the west. The clash of those winds twists the rising bubbles of hot, moist air into destructive vortices.
The central U.S. is a hotbed for severe weather due to the presence of the Rocky Mountains and the Gulf of Mexico, said University of Missouri atmospheric scientist Anthony Lupo. "These geographical features set the stage for the collision of cold dry air from Canada meeting warm moist air from the Gulf of Mexico. This contrast in the density of air masses gives rise to storm systems that spawn severe weather."
As the planet warms, Brooks said, "we expect the warm moist air at low levels will increase. But the changes in winds with height (wind shear), is projected to decrease on average."
What matters most is the combination of ingredients, but it's hard to make accurate projections because our understanding right now of the how the averages will change is better than our understanding of how the combinations will change, he said.
What We Know
"What we do know for the U.S. is that we see no evidence for a change in intensity of any kind," Brooks said. Some statistics suggest changes in certain categories of tornadoes, but that's likely based on changes in tornado reporting since the 1970s, he added.
"What has changed is we have fewer days per year with at least one tornado, but many more days with many tornadoes, up to 20 or 30. Situations that can produce a lot of tornadoes are happening more often, big days have gotten bigger. That's something we have pretty good confidence has occurred," he said.
Tornado activity can vary so widely from year to year and the detailed record is so short that it's hard to say what's normal, so that makes it hard to identify a global warming fingerprint, said Columbia University's Michael Tippett, who studies variability in the climate system.
"Certainly what we're seeing this year is unusual. Over the last 11 days, there have been eight or more tornadoes each day. It's a record, a steady drumbeat of tornado activity day after day. It's unusual, but it's not crazy unusual," Tippett said. "And we don't have a way, at present, to say that this is due to climate change."
Tornadoes are too small to study with the models used for other climate attribution analyses, said Geert Jan van Oldenburgh, a climate scientist with World Weather Attribution, a research organization that studies links between extreme weather and global warming. But, he said, "you can look at the large-scale features that are associated with tornadoes and check how these change with global warming."
Prolonged tornado outbreaks also could potentially be linked with global warming through a jet stream pattern that is becoming more frequent and that keeps extreme weather patterns locked in place, Potsdam Institute for Climate Impact Research scientist Stefan Rahmstorf suggested on Twitter. (For more on the jet stream and climate change, read this.)
It is reasonable to expect that climate change has and will have some kind of effect on tornado activity, said Columbia University climate researcher Chiara Lepore. "Right now, we don't know how. I think the biggest challenges are the scale of tornado activity and the large natural variability of the process."
At best, there are hints as to what might happen with tornadoes in a warmer future. Lepore said recent research, including a 2013 study led by Stanford climate researcher Noah Diffenbaugh, projects that some of the environments conducive to severe weather will occur more often, but it's unclear if that means more tornadoes.
Diffenbaugh and his co-authors wrote that global climate models were zeroing in on "robust increases in the occurrence of severe thunderstorm environments over the eastern United States in response to further global warming," and suggested "a possible increase in the number of days supportive of tornadic storms."
Meeting the Paris climate agreement goal of capping global warming below 2 degrees Celsius would moderate the increase in severe storm scenarios, Diffenbaugh said.
Even subtle changes could have a big effect on people, Brooks said. Even if the overall number of tornadoes doesn't increase, the shift toward the southeast, toward areas that are more populated than the southern plains, would put more people in harm's way.
The trend toward more tornado clusters could also test the limits of emergency responders, and the economic impacts of tornado damage can spread beyond the area immediately affected, he said.
The damage caused by tornadoes and severe storms is already increasing, according to Munich Re, one of the world's top reinsurance companies. According to the company, loss trends show that the cost of the damage has been increasing steadily for 40 years, from an average of less than $2 billion in the 1980s to $22 billion in 2016.
Senior Munich Re research meteorologist Mark Bove warned in a 2017 insurance industry newsletter that "an increase of atmospheric heat and moisture due to our warming climate will likely increase the number of days per year that are favorable for thunderstorms and their associated hazards, including tornadoes."
Regardless of the effects of global warming, the central U.S. will continue to be a hotbed of severe storms that spawn tornadoes, Lupo said. And there is enough information about possible changes in tornado and severe storm activity to take meaningful action that protects lives and property.
"What will the future bring? It could be argued that a warmer world, regardless of the cause, will shift where severe weather occurs. More tornadoes and severe weather further north or east. But, it could also be argued that since a warmer world may experience a reduction in the equator to pole temperature contrast, the number of tornadoes and severe weather events would decrease," he said.
"We know where the most active severe weather areas are. We know that in the long term, this will likely not change since North America's geography won't change substantially," he said. "We can recommend better building codes, building severe storm shelters, and educating the populace as to how we warn people of severe weather. These things are being done, but we could do better."