Scientists Debate Global Warming Role in Hurricane Activity

By B. Blair Dedrick

Washington - When Hurricane Stan developed in the southern Gulf of Mexico last week, 2005 became the second busiest hurricane season in 154 years. And that was before Tropical Storm Tammy, the 20th named storm of the year, which has caused flooding along the entire East Coast.

With the high numbers and high intensities of recent hurricanes, like Katrina and Rita which both reached Category 5, the debate is whether this year is a natural cycle of high frequency, high intensity hurricanes or if global warming is increasing hurricane activity. James J. O'Brien, director of the Center for Ocean-Atmospheric Prediction Studies at Florida State University, said in a presentation Wednesday that he believes the last few years is just a normal period of increased hurricane activity not caused by global warming.

Using National Hurricane Center data from 1866 to today, O'Brien said hurricanes that made U.S. landfall had not increased in either frequency or intensity. "Does anyone see a trend?" O'Brien asked his audience while showing a slide of his data. 'No one sees a trend. You are all witnesses."

The George C. Marshall Institute, which assesses scientific issues to help guide public policy, sponsored the presentation. Kerry A. Emanuel, an atmospheric science professor at the Massachusetts Institute of Technology, agrees with O'Brien's analysis, up to a point. "If you look at landfall data, I don't see a trend either," he said in a telephone interview. "But, that doesn't say anything about global warming."

Emanuel, the author of an August article in Nature magazine on the subject, said there are too few landfall hurricanes to see any trends. Globally, however, Emanuel said data indicate an increase in hurricane frequency and duration that he calls 'pretty clear." "It sounds like a contradiction, but I don't think it is," he said, explaining that areas can be so dominated by local effects that global trends are not obvious.

Kevin E. Trenberth, the head of the climate analysis section of the National Center for Atmospheric Research in Boulder, Colo., said in a phone interview that O'Brien's data is too narrow. He called hurricanes that make landfall a subset of Atlantic hurricanes, which are a subset of global hurricanes. "Globally there is an increase in hurricane intensity and, especially, in hurricanes of a Category 4 and 5," Trenberth said. In June, Trenberth published an article in Science magazine, suggesting that global warming was expected to increase hurricane intensity and rainfall, but the effect on hurricane frequency and paths was unclear.

Researcher Judith A. Curry, co-author of a September article in Science magazine on hurricane trends, agreed that O'Brien's data is not global enough. She said in a phone interview that his statistics are like "using Californians over the age of 65 to try to determine the nation's voter preferences."

Curry's recent publication concluded that "global data indicate a 30-year trend toward more frequent and intense hurricanes," which, the report said, is not inconsistent" with global warming theories. The report adds, however, that only more data over more years will resolve the debate. O'Brien said the problem with Curry's report and other similar research is the short time period examined, while his research looks at the past 150 years.

Using data from the Hadley Center in Europe on North Atlantic Ocean temperatures, O'Brien said the only spots in the ocean that are warming up are near the equator not in the hurricane formation regions. "In fact, the ocean's been cooling down," he said. "Why? Because for 10 years, there have been more tropical storms, and storms take warmth out of the ocean."

O'Brien's data showed that the sea surface temperature for parts of the Gulf of Mexico cooled from almost 88 degrees pre-Katrina to about 82 degrees post-Katrina. He did not have similar data for Hurricane Rita. Curry said that the tropical sea surface temperature increased 1 degree over the past 35 years, a fact that is consistent with the effects of global warming.

While Hurricane Katrina did cool the ocean's surface, Curry said temperatures below the surface are also important. A hurricane needs water of at least 80 degrees to form.

"You normally wouldn't see a repeat so soon," she said. Just weeks after Katrina, Hurricane Rita went over a warm water layer west of Florida that boosted its intensity from Category 2 to Category 5 in just 24 hours.

Asked about the sea temperatures below the surface, O'Brien said only that the Gulf was a "very turbulent region" and that "was a rather advanced subject for another day."

Source: Scripps Howard Foundation