It sounds intuitive: Of course global warming should lead to more—and more powerful—tornadoes.
We're adding energy to the atmosphere by trapping heat with greenhouse gases, and tornadoes are the very picture of terrifying atmospheric energy.
Linking any particular weather event to climate change is always tricky, because weather is inherently random. But weather patterns can speak to a warming planet. Scientists can detect that extreme rain events, for instance, are already happening more often than they used to, and that a warmer atmosphere with more water vapor in it is making such events more likely.
Tornadoes are different. Global warming may well end up making them more frequent or intense, as our intuition would tell us. But it might also actually suppress them—the science just isn't clear yet.
Neither is the historical record.
There is no real evidence that tornadoes are happening more often. A lot more are being recorded now than in 1950, but a closer look at the data shows the increase is only in the weakest category, EF0. There's been no increase in stronger twisters, and maybe even a slight decrease in EF4s and EF5s.
That suggests we're just spotting more of the weak and short-lived tornadoes than we did back when the country was emptier (the U.S. population in 1950 was less than half what it is now), we didn't have Doppler radar, and Oklahoma highways weren't jammed with storm-chasers.
There is also no evidence that tornadoes have gotten more damaging, according to a study by Roger Pielke, Jr., of the University of Colorado and his colleagues. Even so, when you allow for inflation and increases in population and wealth in the United States, 2011 becomes the third worst year for tornado damage, after 1953 and 1965.
When National Geographic magazine asked "What's Up With the Weather" in a cover story last September, we put a tornado photo on the cover and six pages of twister pictures inside—including a large shot of the swath of destruction that an EF4 tornado cut through Tuscaloosa, Alabama, in 2011, killing 64 people there and in Birmingham.
But as writer Peter Miller made clear in that story, intuition is not a reliable guide to tornadoes.
Two Opposing Forces
Since we're changing the climate, the historical record is no more certain a guide to the future than intuition is. So what does physics tell us about the future of tornadoes in a CO2-warmed world?
"It really comes down to two ingredients in the atmosphere, in the environment in which storms form," says Jeff Trapp, an atmospheric scientist at Purdue University.
Trapp has been on the road in Kansas and Oklahoma since last week, launching weather balloons into supercells—large, tornado-producing thunderstorms—as part of an effort to improve forecasting. He was 20 or 30 miles away from Moore when the tornado hit on Monday.
The first ingredient needed to make a tornado, he explains, is energy in the form of warm, moist, unstable air. In Oklahoma, that comes on southerly winds off the Gulf of Mexico.
The second ingredient is wind shear—a measure of how much the wind changes speed and direction between the ground and higher levels of the atmosphere. "Essentially that's determined by the strength of the jet stream," which blows in from the west, says Trapp. Wind shear causes the warm, rising air inside a supercell to start rotating, a necessary condition for organizing the storm and allowing it to spawn funnel clouds.
And that gets at the nub of the question surrounding a potential nexus between warming and tornadoes: Although climate change is increasing the energy in the atmosphere, it's also expected to reduce wind shear.
That's because the jet stream is powered ultimately by the temperature difference between Earth's hot tropics and its cold poles, and that difference is decreasing with climate change, as the poles warm faster than the rest of the planet. So the same phenomenon that is rapidly melting the Arctic ice cap and marooning polar bears could lead to a weaker jet stream and fewer tornadoes.
But will it?
Severe thunderstorms can happen even when wind shear is lower, says Trapp. "Really it's the product of the two ingredients that matters most," he says.
The big question, which he and a small number of other climate scientists have been trying to answer with climate simulations, is what will happen to the product of energy times wind shear—to the two ingredients combined—as CO2 continues to warm the world.
"What we find in the models," Trapp says, "is there's actually an increase in the product. The decrease in wind shear is more than compensated [for] by the increase in energy. This tells us that the number of days that support severe thunderstorms generically should increase."
Still, that's just one study. And it says that the future environment should favor the storms that create tornadoes—but not necessarily tornadoes themselves. It's possible that in the future, severe thunderstorms will tend to spend themselves in violent hail or in straight-line winds. Neither is a pleasant prospect, but neither packs the damage potential of tornadoes.
Trapp is now at work on a study that will combine a global climate model with a local, high-resolution model, which will show tornadoes as if on a virtual radar screen.
This new study may offer a glimpse of what the future has in store for Oklahoma and other parts of Tornado Alley. Meanwhile, he can say one thing with certainty: "The last several days in Oklahoma, both the wind shear and the energy have been incredibly large."