for National Geographic News
Warming ocean temperatures appear to be fueling stronger, more intense hurricanes around the world, a new study suggests.
The number of storms that reach Category Four and Fivethe most powerful, damaging hurricaneshas nearly doubled over the past 35 years, the study finds. However, the frequency and duration of hurricanes overall have stayed about the same.
Category Four hurricanes have sustained winds from 131 to 155 miles an hour (211 to 249 kilometers an hour). Category Five hurricaneslike Katrina at its peak in the Gulf of Mexicohave sustained winds of 156 miles an hour (251 kilometers an hour) or more.
The study finds that the increase in hurricane intensity coincides with a rise in sea surface temperatures around the world of about 1ºF (0.5ºC) between 1970 and 2004.
Writing in tomorrow's issue of the journal Science, the study's authors stop shy of pinning the increase in hurricane intensity on global warming. To do so would require a longer historical period of study and a better understanding of hurricane dynamics, they say.
But in an interview with National Geographic News, the study's lead author, Peter Webster, said, "I'm prepared to make an attribution to global warming."
If the increases in hurricane strength and sea surface temperatures were part of a natural cycle, as some scientists believe, then there would be decreases in other regions to compensate for them. But the increases found in the study are both worldwide.
"There's a plus and minus with oscillations," said Webster, an atmospheric scientist at the Georgia Institute of Technology in Atlanta. "What we see is a universal increase [in temperature] and a universal change in hurricane intensity."
Scientists know that warm ocean waters fuel hurricanes. Webster says it therefore follows that the more fuel there is, the bigger the storms will be. But why the frequency and duration of hurricanes aren't also rising is poorly understood.
"The relationship between sea surface temperature and intensity is not one that has surprised us," Webster said in a telephone briefing with reporters. "The other factors mentioned for hurricanes are more awkward."
Chris Landsea, a meteorologist with NOAA's National Hurricane Center in Miami, said that the increase in hurricane strength that Webster's team has observed is questionable.
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