Hurricanes are growing fiercer, study finds

Findings sure to add fuel to global warming debate

September 16, 2005|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

When Hurricane Katrina roared out of the Gulf of Mexico with 145 mph winds, it was the latest in what scientists today cited as a growing number of fearsome Category 4 and 5 storms that form each year in the world's tropical oceans.

In a study published in the journal Science, researchers reported satellite data showing that the number of the most violent tropical storms increased almost 80 percent in 2004 over the number in 1975.

The paper - the first global assessment of hurricane activity - draws a link between the rising number of big storms and increases in tropical sea surface temperatures around the world.

Although the study itself stops short of blaming global warming for the trend, its authors were less reticent to draw a connection at a news conference this week.

"With some confidence we can say these two things [more violent storms and rising ocean temperatures] are connected, and there's probably a substantial contribution from greenhouse warming and not just natural variability," said study co-author Judith Curry of the Georgia Institute of Technology.

The findings are certain to add fuel to the debate over global warming and its influence on weather events.

The 2005 Atlantic hurricane season has been one of the most active in years, producing seven hurricanes so far. Dennis and Emily reached Category 4 on the Saffir-Simpson scale (131 mph), while Katrina hit Category 5, the highest, at 155 mph. But there are a variety of opinions as to why - and whether it's part of a larger trend.

Last spring, scientists at the National Hurricane Center independently predicted the increase in violent storms - but without invoking global warming.

They cited a recurring cycle of sea-surface temperatures, atmospheric pressure and wind conditions peculiar to the tropical Atlantic. The increased activity began in 1995 and could last for decades.

Today's study in Science, a publication of the American Association for the Advancement of Science, is much broader. It addresses a 35-year global rise in tropical ocean temperatures that the authors argue drives up the intensity of hurricanes in all ocean basins.

Curry, along with her Georgia Tech colleagues Peter J. Webster and Hai-Ru Chang, and Greg Holland of the National Center for Atmospheric Research, focused on the period from the 1970s through 2004. That so-called "satellite era" of weather records offered the best and most globally consistent set of observations.

Curiously, they found no increase in the total number and duration of tropical storms and hurricanes. In fact, in every ocean basin except the North Atlantic, their frequency and duration actually fell after 1995 - a period in which sea surface temperatures were rising the most.

"No global trend has yet emerged in the number of tropical storms and hurricanes," the paper concluded.

"We don't have a simple theory to explain that," Curry said.

But the data did reveal a sharp increase everywhere in the fiercest storms. Category 4 and 5 hurricanes were up about 80 percent between 1975 and 2004.

Those are the most powerful storms on the Saffir-Simpson hurricane scale - with winds over 131 mph.

During the same period, sea surface temperatures were rising in all tropical ocean basins, by about a half-degree Celsius. on average.

Because rising sea-surface temperatures are a global phenomenon, probably influenced by greenhouse warming, the scientists said, they offer the best explanation for global increases in the number of intense hurricanes.

Global warming theorists have predicted for decades that rising air and sea temperatures - driven by fossil fuel use and greenhouse gas emissions-would increase the frequency and intensity of storms around the world.

Brenda Ekwurzel, climatologist at the Union of Concerned Scientists, said the new results confirm earlier studies, including a controversial paper published in July by Kerry Emanuel of the Massachusetts Institute of Technology.

Emanuel concluded in the journal Nature that hurricanes in both the Atlantic and Pacific have increased in duration and intensity by about 50 percent since the 1970s. He linked the increase to rising air and ocean temperatures.

"It's like rolling the dice, only we're now rolling loaded dice," Ekwurzel added yesterday. "The good news is that since we are part of the problem, we can be part of the solution."

But not everyone buys the latest study's conclusions.

"The data is just not good enough to conclude that the [Category] 4 and 5s have gone up with this global warming," said William M. Gray, a professor of atmospheric science at Colorado State University and well-known hurricane forecaster.

Even so, he said: "People with a political agenda for human-induced global warming are going to come out of the woodwork and make as much of this as they can."

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