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Warmer Atlantic basin to strengthen hurricanes

Climate models that incorporate global warming show a gradual increase in the intensity of hurricanes.

By GRAHAM BRINK
Published May 2, 2005

ST. PETERSBURG - The four hurricanes that walloped Florida last year stoked debate over whether the storms are gaining in intensity.

The dialogue invariably turned to global warming and its effects on the Atlantic basin, where many hurricanes form.

Could all the extra carbon dioxide and other greenhouse gases spewing from vehicles and power plants be heating up the Atlantic Ocean, turning it into fertile ground for stronger hurricanes?

Scientists cannot say with certainty that recent hurricanes were more intense because of warming waters in the Atlantic basin. But recent studies employing high-powered computer models show that the extra heat will increase the intensity of future hurricanes.

Any significant rise in intensity, however, is likely decades away.

"The key is to recognize what is happening and what will be happening in the future," said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colo. "That helps us prepare."

The theory that increased ocean and atmospheric temperatures could increase the intensity of hurricanes has been around for years.

It works like this: Greenhouse gases such as carbon dioxide and methane trap heat in the atmosphere and warm the ocean. Warmer water produces more water vapor, which is the fuel that drives hurricanes. More fuel means increased intensity.

In 1987, a study from the Massachusetts Institute of Technology concluded that a doubling of carbon dioxide in the atmosphere would yield "a 40-50 percent increase in the destructive potential of hurricanes."

Another breakthrough came last year in a study written by scientists Thomas Knutson and Robert Tuleya. Using supercomputers, they ran models of simulated hurricanes. Nearly all the models revealed that increased greenhouse gases would create stronger hurricanes on average.

The trapped gases would push future hurricanes an average of a half-step up the five-step intensity scale over the next 75 years, the study showed. Knutson cautioned that the exact amount of buildup in greenhouse gases is tough to predict.

"But I think you can use what we found to get a rough indicator of how much more intense the storms will be in the next few decades," said Knutson, who works at the government's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

Current research shows that the waters in the Atlantic basin are as warm as they've been since scientists began recording temperatures more than 100 years ago. The average for the last decade was one-tenth of a degree centigrade higher than the previous high, recorded in the late 1940s and early 1950s.

The increase doesn't sound like much. But researchers such as Trenberth have concluded that it is not a normal fluctuation in ocean temperatures, or the effect of El Nino, a current of warm ocean water. It's an increase in greenhouse gases such as carbon dioxide, they say.

When researchers plug global warming data into their models, they can re-create the climate patterns of the past 30 years, Trenberth said. Without that data, the models fail to re-create the patterns.

"There is a man-made contribution," Trenberth said. "It's not outstanding at the moment. But it's there."

Hurricanes already may be dropping more rain as a result of the warming.

Trenberth pointed out that "very heavy" precipitation, the kind dropped during hurricanes, was up 20 percent over the previous century. And by 2080, the average rainfall near the center of hurricanes will increase by 18 percent, according to Knutson's models.

Such a trend would not be easily reversed, scientists say.

The climate system can take a long time to fully respond to stimuli, including increases in greenhouse gases. But once it begins moving in a direction - warmer or colder, for instance - it's difficult to stop.

Ocean levels are a good example.

Even if the concentration of greenhouses gases had stabilized in 2000, global warming would cause ocean levels to rise at least an additional 6 to 12 inches by the end of this century, according to a recent study published in Science magazine.

Increased sea levels, more rain and stronger hurricanes can be a recipe for more flooding.

"That means more damage," Trenberth said.

In theory, warmer oceans could also help hurricanes last longer. Cooler water temperatures help sap a storm's power. If those waters are warmer than in the past, the hurricanes might travel farther or survive into higher latitudes.

"Those are good question that have not been answered yet," Knutson said. "Cooler waters are just one of many reasons that cause hurricanes to lose their power."

Knutson and Trenberth emphasized that this isn't a doomsday scenario. Researchers cannot say whether warming of the waters in the Atlantic basin will cause more, or fewer, hurricanes. And the increase in average intensity will be gradual, they said.

Professor James B. Elsner, a climatologist at Florida State University, added that there is still a lot to be learned about hurricanes. The current models can only go so far in making accurate predictions, he said.

"Warming is a cause for some concerns ... something to pay attention to," he said "There are many clues that we haven't uncovered yet."

Whether the models and predictions are accurate or not, Trenberth advised taking a proactive approach.

He recommended working toward reducing the production of greenhouse gases. Communities should also make sure building codes are followed to ensure structures can handle stronger storms, he said.

Knutson had a another common-sense suggestion.

"We're putting up more structures and more expensive structures right near the coast," he said. "Probably the most effective mitigation against damage is not to build so much stuff so close to the ocean."

--Graham Brink can be reached at 727 893-8406 or brink@sptimes.com

[Last modified May 2, 2005, 01:35:17]