Tornado intensity climbs in the United States

SAN FRANCISCO —Tornadoes are getting stronger.

Over the last two decades, the intensity of twisters pummeling the United States has crept up, geographer James Elsner of Florida State University in Tallahassee reported at the American Geophysical Union’s fall meeting on December 10.

The National Weather Service ranks tornado intensity using the Enhanced Fujita Scale. The EF Scale lumps the storms into one of six categories based on damage to trees, light poles, buildings and other structures. Scientists assign a rating by looking at photographs of a tornado’s aftermath.

But fitting tornadoes into narrow categories gives scientists only a rough gauge of a tornado’s strength. “We need a continuous estimate of tornado intensity,” Elsner argued.

He and colleagues created such an estimate using a computer analysis that factors in the length and width of a tornado’s path. Then the team tapped into data from the U.S. Storm Prediction Center about tornadoes from 1994 to 2011.

According to the analysis, tornadoes have grown more intense. They’ve spent more time on the ground and left busted-up trails that get about 2 percent bigger each year, Elsner said.

He thinks the rise in intensity could be linked to climate change. A boost in temperature means more warm, moist air, which fuels whirling columns of wind.

How — and if — climate change affects tornadoes is still controversial, though, Elsner said. “When people see tornadoes and climate in the same sentence, their eyes roll.”

Even Elsner’s finding of an increase in tornado intensity is preliminary, cautions hurricane risk researcher Emmi Yonekura of Princeton University. Data from hurricanes and tornadoes can be inconsistent, she says, so analyzing trends is tricky.

Still, Elsner’s claim is “highly plausible,” says Peter Kalmus, a climate scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Five years from now, he says, “I’d be really surprised if everyone didn’t agree there’s a strong link to climate change.”