Are tropical cyclones getting stronger? Three years ago, the massively destructive Hurricane Katrina suggested as much, and two studies pegged global warming as a possible culprit (Science, 16 September 2005, p. 1807). Now another study bolsters the case, finding that the most intense storms have indeed been getting stronger over the past 30 years as the waters that fuel them get warmer.
The link between global warming and stronger cyclones makes sense in theory. The warmer the water beneath them, the higher their maximum wind speed. But the studies of 3 years ago drew on admittedly flawed records compiled using a variety of techniques, some more reliable than others.
To try to get beyond those limitations, tropical cyclone researchers James Elsner and Thomas Jagger of Florida State University in Tallahassee and James Kossin of the University of Wisconsin, Madison, drew on a record of satellite-determined storm winds compiled by Kossin and other colleagues (Science, 10 November 2006, p. 910). They then standardized the wind speeds from different satellites, which were based on infrared observations of cloud-top and storm-eye temperatures. From this uniform--if still imperfect--record, the researchers found that, globally, the top third of tropical cyclones rated by maximum wind speed became more powerful during the time period from 1981 to 2006, although the increase was smaller than found in the earlier studies. The intensification now appears to have reached 32 kilometers per hour among the strongest cyclones. The trend toward stronger storms was small in the North Pacific, where the most tropical cyclones occur, but considerable in the North Atlantic and the Indian oceans, the team reports in tomorrow's issue of Nature.
The work "is a big advance," says hurricane researcher Hugh Willoughby of Florida International University in Miami. It "really improves on all the work that has gone before," agrees hurricane researcher Gabriel Vecchi of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. However, he remains cautious about the role of global warming. "Is this trend going to continue or not? We're not in a position to say."