After tornadoes barreled through six U.S. states late last week, killing at least 88 people and flattening warehouses, nursing homes, and factories, many were quick to blame the warming climate for at least some of the damages.
Over the weekend, many in the news media speculated that global warming may have played a role and pressed policymakers for answers. Deanne Criswell, the administrator of the Federal Emergency Management Agency, or FEMA, called the devastating tornadoes “our new normal.” President Joe Biden, responding to a question from reporters at a press conference on Saturday, said that “everything is more intense when the climate is warming — everything” but emphasized on Monday that “we can’t say with absolute certainty” that climate change was to blame.
The questions are hardly a surprise: After all, it’s become increasingly common — and scientifically supported — to blame floods, heat waves, and even intense hurricanes on the increasing temperatures from fossil fuel use. Friday’s record-breaking heat caused many to speculate that climate change might have played a role in the disaster; there’s also evidence that tornadoes are shifting eastward, perhaps because of changing temperatures.
But tornadoes, scientists say, are extremely difficult to link to climate change. According to a 2016 report from the National Academies of Sciences, tornadoes are the most challenging weather event to attribute to global warming — after heat waves, wildfires, extra-tropical cyclones, and even heavy snowfall. “They are the single extreme that we have the least confidence around,” said Zeke Hausfather, a climate scientist and the director of climate and energy at the Breakthrough Institute.
That’s for several different reasons. First, most studies linking climate change to weather events (a field known as “extreme event attribution”) model the world’s climate in two different ways: with human-caused warming and without. But those climate models don’t have a fine-enough resolution to simulate tornadoes, making it hard for scientists to see how twisters might change in the coming years.
“Our models don’t produce tornadoes,” Hausfather said. Climate models, he explained, split the world into 100- or 25-kilometer wide boxes and simulate what happens in each box. But tornadoes form in areas that are less than one kilometer wide — much too small to be easily modeled.
That means scientists have to use other methods to analyze how twisters will be affected by the warming climate. Tornadoes require a few different ingredients to form: first, potential energy created by warm, moist air near the ground and cool, dry air overhead. In so-called “Tornado Alley” in the central U.S., for instance, tornadoes are formed by warm air flowing in from the Gulf of Mexico to the southeast, and cool air coming from over the Rocky Mountains. Second, they require “wind shear,” a change in wind speed or direction from the ground to the sky, to wind up the tornado’s spin.
Harold Brooks, a senior scientist at the National Severe Storms Laboratory, says that there is evidence that a warming climate could affect one of those ingredients, but not the other. There’s good evidence, according to Brooks, that the potential energy of storms could increase as the world heats up. But wind shear could decrease or increase in a warming planet, and scientists aren’t yet sure which effect will win out.
To make matters more complicated, tornadoes can also be affected by the precise way that thunderstorms form — whether they form as isolated storms, as in Friday’s disastrous events, or as a huge line. Isolated storms, Brooks says, are more likely to create tornadoes. But, he adds, there’s no understanding yet of how global warming could influence the exact mechanisms of thunderstorm formation.
Researchers argue that there could come a point when research clearly demonstrates a link between tornadoes and a warming world. “Absence of evidence is not evidence of absence,” Hausfather said. But at the moment, there’s no clear data to indicate that tornadoes are increasing in number or intensity. (There are more tornadoes now than in the 1950s, but scientists say that’s because of changing monitoring techniques.)
And there are risks that come with getting too far ahead of the science. In a paper released last week in the journal WIREs Climate Change, researchers from Sweden and the U.S. made the case that connecting climate change to extreme weather events can obscure how poor planning and failing infrastructure turns weather events into disasters. “Even where science can attribute such events to human emissions of greenhouse gasses with some rigor,” the authors wrote, “the damages that follow are centrally a function of vulnerabilities on the ground.”
Stephen Strader, a professor of geography at Villanova University, similarly argues that focusing too much on the role of global warming can let policymakers off the hook. “It creates an air of ‘what can we do, our hands are tied,’” he said. “Climate change is only one side of the disaster coin.”
In this case, Strader explained, the tornadoes were particularly dangerous for a few reasons: Many of them struck at night, when people were more likely to be asleep or unwilling to evacuate, and they struck the southeast of the United States, where there are many mobile homes and homes without basements or tornado shelters. (Last year the Southeast experienced 83 percent of tornado-related deaths.) Many of the deaths, meanwhile, occurred in a few buildings — an Amazon warehouse and a candle factory — that were razed to the ground.
Strader said he hopes that the recent spate of tornadoes will boost efforts to prepare — and that policymakers will shore up building safety and learn how to encourage people to take shelter quickly and safely. “There’s a narrow window of opportunity to prevent and prepare for the next one,” he said.