A new study concludes that rising sea temperatures have been accompanied by a significant global increase in the most destructive hurricanes, adding fuel to an international debate over whether global warming contributed to the devastation wrought by Hurricane Katrina.
The study, published today in the journal Science, is the second in six weeks to draw this conclusion, but other climatologists dispute the findings and argue that a recent spate of severe storms reflects nothing more than normal weather variability.
Katrina's destructiveness has given a sharp new edge to the ongoing debate over whether the United States should do more to curb greenhouse gas emissions linked to global warming. Domestic and European critics have pointed to Katrina as a reason to take action, while skeptics say climate activists are capitalizing on a national disaster to further their own agenda.
According to data gathered by researchers at the School of Earth and Atmospheric Sciences at Georgia Tech and the National Center for Atmospheric Research, the number of major Category 4 and 5 hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes, including weaker ones, has dropped since the 1990s. Katrina was a Category 4 storm when it made landfall.
Using satellite data, the four researchers found that the average number of Category 4 and 5 hurricanes -- those with winds of 131 mph or higher -- rose from 10 a year in the 1970s to 18 a year since 1990. Average tropical sea surface temperatures have increased as much as 1 degree Fahrenheit during the same period, after remaining stable between 1900 and the mid-1960s.
Georgia Tech atmospheric scientist Judith A. Curry -- co-author of the study with colleagues Peter J. Webster and Hai-Ru Chang, and NCAR's Greg J. Holland -- said in an interview that their survey, coupled with computer models and scientists' understanding of how hurricanes work, has given the researchers a better sense of how rising sea temperatures are linked to more-intense storms.
"There is increasing confidence, as the result of our study, that there's some level of greenhouse warming in what we're seeing," Curry said. "Is it the whole story? We don't know."
Higher ocean temperatures result in more water vapor in the air, which, combined with certain wind patterns, helps power stronger hurricanes, Webster said. Small increases in sea temperature, he added, can "exponentially provide more and more fuel for the hurricanes."
Other studies and computer models also have pointed to an increase in storm intensity: Massachusetts Institute of Technology atmospheric scientist Kerry A. Emanuel wrote last month in the journal Nature that the duration and maximum wind speeds of storms in the North Atlantic and North Pacific have increased about 50 percent since the mid-1970s. The storms' growing violence stemmed in part from higher ocean temperatures, he concluded.
Some researchers, however, question the connection with more severe hurricanes and cyclones. Gerry Bell, the lead seasonal hurricane forecaster at the National Oceanic and Atmospheric Administration, said the rise in strong hurricanes reflects a natural weather pattern spanning several decades. Hurricanes in the Atlantic Ocean were more powerful in the 1950s and '60s, weakened in the 1970s, '80s and early '90s, and have strengthened again since 1995.
"It's not linked to global warming or anything like that," Bell said. "This is normal climate variability. It's just that this trend lasts for decades."
Florida State University meteorology and oceanography professor James O'Brien, who writes for the online free-market journal Tech Central Station, said his survey of government data on Atlantic storms between 1850 and 2005 shows that "there's no indication of an increase in intensity."
But both Emanuel and Gavin Schmidt, a climatologist at NASA's Goddard Institute for Space Studies, said today's Science paper is important because it examines worldwide hurricane patterns.
"If you look at it on the global basis, it makes that signal of global warming easier to see," Schmidt said. "You have to be extremely conservative -- with a small 'c' -- to think [rising sea temperatures and stronger hurricanes] are not related."
And some hurricane experts who previously have questioned the influence of global warming now say the evidence is mounting that it has contributed to recent intense tropical storms.
Florida International University researcher Hugh Willoughby, who headed NOAA's hurricane research division between 1995 and 2003, said the recent two hurricane studies are "very persuasive" and helped move him "toward the climate corner" of the debate.
"It's really hard to find any holes in this, and I'm the kind of person who's inclined to look for holes," he said of the new study in Science. The arguments against the connection between climate change and more intense storms, he added, are "looking weaker and weaker as time goes by."
Katrina reanimated a transatlantic argument over global warming policy as critics of the Bush administration have seized on it to promote mandatory limits on greenhouse gas emissions.
"The American president shuts his eyes to the economic and human damage that the failure to protect the climate inflicts on his country and the world through natural catastrophes like Katrina," Germany's environmental minister, Jurgen Trittin, wrote in an opinion piece printed Aug. 30 in the Frankfurter Rundschau newspaper.
But Bill Holbrook, spokesman for Environment and Public Works Committee Chairman James M. Inhofe (R-Okla.), said the senator has no intention of pushing for new emissions curbs.
"It is reprehensible for a politician to promote an agenda by twisting a tragedy Americans feel so deeply about, particularly when there is no merit to his ideas," Holbrook said of Trittin. "Policy decisions should be based on sound science, and the notion that Katrina's intensity is somehow attributable to global warming has been widely dismissed by scientific experts."
Arguing that the science of global warming remains uncertain, President Bush in 2001 disavowed the Kyoto treaty that sets mandatory targets for reducing greenhouse gas emissions, and he has pursued policies calling for more research and voluntary efforts to limit emissions.