December 11, 2013 | 6
Sometimes scientists can’t help themselves from showing dramatic curves, even though they have so many caveats that no firm conclusions can be made from the data. James Elsner at Florida State University has a killer curve, and lots of caveats. The curve indicates that tornadoes in the U.S. may be getting stronger. The caveats indicate they may not be.
“If I were a betting man I’d say tornadoes are getting stronger,” he noted on Tuesday during a lecture at the annual American Geophysical Union fall meeting in San Francisco. But when asked directly at a press conference whether that is the case, he would not commit. “I’m not doing this [work] to establish the future intensity of tornadoes,” he explained, but to establish a method that someday could indeed determine if the storms are becoming more powerful.
Because the lecture was titled “Are tornadoes getting stronger?” the audience expected an answer. And their consternation rose when Elsner showed his final graph, adding up the kinetic energy of tornadoes each year from 1994 to 2012. The curve is flat from 1994 to about 2006 but then spikes upward through 2012. It was reminiscent of the now famous “hockey stick” graph produced by Michael Mann and colleagues a decade ago, indicating that Earth’s temperature had been flat for 1,000 years and began spiking upward in the mid-1800s. But Mann had 1,000 years of data; Elsner has 18. His data begin in 1994 because that’s when Doppler radar, the best at tracking tornadoes, began covering the entire U.S.
The point of the curve, however, is to show that measuring the length and width of a tornado’s damage path gives an accurate indication of its strength, which is driven by the storm’s peak wind speed. It is difficult if not impossible to measure that speed directly, as is done for hurricanes by ground instruments and planes that fly into the storms.
Despite the caveats, several interesting and solid conclusions do arise from Elsner’s painstaking work to map every single tornado that made landfall since 1994. Top winds speeds appear to be rising. And the stronger the storm, the longer it stays on the ground and the wider its path of destruction. Storms that ranked a 4 on the EF scale (1 to 5, with 5 the worst) cut paths with a mean length of 43 kilometers and a mean width of 809 meters. EF5 storms had a mean length of 67 kilometers and a mean width of 1,390 meters.
Also intriguing is that slightly more tornadoes are forming in the springtime, and in December, while the frequency in June and July is down a bit. Storm strength seems to be increasing most in the southern-most portions of the country.
Of course AGU attendees, and reporters, asked Elsner several times if climate change is or might be making tornadoes stronger. He would not bite. “I’m not claiming this is because of climate change,” Elsner told the audience. “But it is provocative, isn’t it?” He later noted that more moisture in the atmosphere, a general result of global warming, could provide more fuel for stronger tornadoes, but that no one has proven such a link. Changes in the El Nino – La Nina cycle of ocean and atmosphere conditions in the Pacific Ocean could be affecting the frequency of tornado formation, “but that research is just beginning,” he said.
Elsner was clear about not being able to say with certainty if tornadoes are getting stronger, and was careful to not speculate on a cause if they are. He also seemed to sense his audiences’ disappointment. The purpose of his work, he said, is to establish a scientifically rigorous method for determining tornado strength, which some day might answer the bigger questions. After years of skepticism, he noted, “the public has accepted that there is a link between hurricanes and climate change. We are just beginning” to determine whether there is any linkage for tornadoes.
Photo courtesy of Justin1569 at WikimediaCommons