Last year produced a record 15 Atlantic hurricanes, including four of the most severe Category 5 variety. One of those, of course, was Katrina, the most economically damaging storm in U.S. history. So it's no surprise that climate scientists are scrambling to learn whether these events merely constituted a freak coincidence or evidenced a frightening trend. Some studies have suggested that rising temperatures worldwide are to blame for the increased frequency of hurricanes (ScienceNOW, 27 June). The jury remains out on that issue, but new research suggests the first direct link between global warming and greater hurricane intensity.
James Elsner of Florida State University in Tallahassee studied data from the Intergovernmental Panel on Climate Change and the U.S. National Oceanographic and Atmospheric Administration stretching back over the past half century. In the 23 August Geophysical Research Letters, he reports that whenever average global air temperatures increased during the June-November hurricane season, water temperatures in the tropical Atlantic rose in lockstep. This nearly always produced a season with more powerful tropical storms.
Elsner's findings suggest that air temperatures tend to drive sea temperatures, but not the other way around--which implies a link between greenhouse gases and storm intensity. "The large increases in powerful hurricanes over the past several decades, together with the results presented here, certainly suggest cause for concern," Elsner says. "These results have serious implications for life and property throughout the Caribbean, Mexico, and portions of the United States."
Perhaps, says Gavin Schmidt, a climate specialist with NASA's Goddard Institute for Space Studies in New York City. Elsner's paper does add to evidence that recent increases in sea surface temperature result from the influence of greenhouse gases and other factors and don't simply reflect an upswing in a natural cycle, as some scientists have proposed, Schmidt says. He adds, however, that the findings do not necessarily pinpoint what increases in hurricane activity can be expected in the future, nor do they predict what level of hurricane damage increase we can expect.