September 19, 2005
Monday

The number of hurricanes with sustained winds in excess of 131 mph - the catastrophic Category 4 and 5 strengths that Katrina reached at her peak in the Gulf of Mexico last month - has nearly doubled around the globe in the last 35 years, according to a new study.

The study is certain to add to the debate over whether global warming is fueling the stronger hurricanes.

The researchers from the Georgia Institute of Technology and the National Center for Atmospheric Research looked at the strength of tropical storms in all the world's oceans from 1970 through 2004 and found that the number of Category 4 and 5 storms increased from about 11 a year in the 1970s to 18 annually since 1990, according to their report published Friday in the journal Science.

There are about 90 hurricanes - called typhoons in the western Pacific and cyclones around the Indian Ocean - across the globe every year.

There has been no overall increase in the number of tropical storms over the past three decades, but "Category 4 and 5 hurricanes are making up a larger share of the total number of hurricanes,'' said Judith Curry, head of atmospheric sciences at Georgia Tech and a co-author of the study. She said the strongest storms "made up about 20 percent of all hurricanes in the 1970s, but over the last decade they account for about 35 percent of these storms."

Peter Webster, a Georgia Tech professor who led the study, argues that warming sea surface temperatures - an average of 1 degree Fahrenheit worldwide as a result of global greenhouse warming - is providing the water evaporation that fuels hurricanes and can make them more powerful.

Separately, the National Oceanic and Atmospheric Administration declared Thursday that Katrina, while not the strongest hurricane to ever make landfall in the United States, was the most destructive when it hit the central Gulf Coast Aug. 29.

Although Katrina's intensity at landfall was about 140 mph, weaker than the 160-mph winds assigned Hurricane Camille when it hit the same area in 1969, officials said Katrina was much more destructive because the storm's hurricane force winds extended over a wider area - about 120 miles from its center.

Katrina was fueled by water 2 to 3 degrees warmer than normal in the Gulf, and hurricane experts universally agree that warmer-than-normal sea temperatures have prevailed in the hurricane-breeding waters of the North Atlantic and the Caribbean since the mid-1990s.

But there's debate about whether global warming has really had so much effect on hurricanes. Kerry Emanuel, a meteorologist at the Massachusetts Institute of Technology, who published a paper reaching similar conclusions on global hurricane intensity last month in the journal Nature, noted that "the upswing has been attributed by many researchers to global warming."

Others, like James O'Brien at Florida State University, say there's no evidence that areas of warm water in the tropical oceans are increasing in size outside of natural cycles.

Chris Landsea, a leading hurricane researcher now overseeing scientific operations at the National Hurricane Center in Miami, said that while the new study shows that the percentage of stronger storms is up globally, neither the duration of the storms nor their average maximum wind speeds have increased. "That's not physically consistent with more intense storms, there's something fishy in that result,'' he said.

Landsea doesn't think the researchers did bad science, but may have relied on poor data - estimates of maximum wind speeds of hurricanes obtained by contrasting the temperature of a storm's eye and that of adjacent cloud tops as displayed in satellite images.

This technique wasn't developed by NOAA scientist Vernon Dvorak until 1973 and wasn't put into use worldwide until the 1980s, "so there was no way for even crude estimates of strength for storms outside the Atlantic and some in the western Pacific until the middle of the study period,'' Landsea said.

Only NOAA and the U.S. Air Force regularly fly reconnaissance aircraft into Atlantic hurricanes to measure wind speed, barometric pressure and other aspects. This information has been used to improve the satellite estimates over the last two decades.

Landsea said the fact that the study showed only a 5 percent increase in number of intense storms in the Atlantic during the past 15 years - during a period when storm activity picked up as part of a natural multi-decadal cycle - suggests that the storm data from other oceans may not be so reliable.

Greg Holland, a researcher at NCAR in Boulder, Colo., and a co-author of the study, agreed that there have been improvements in intensity measurements since the 1970s, and that the 30-year period might not capture some still unknown cycles of hurricane activity around the world. "But there is no doubt that there is a substantial increase in intensity here,'' he said.

On the Net:

http://www.sciencemag.org

www.noaa.gov

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