OxfordPress.com

Hurricane data suggest link to global warming

ATLANTA — The number of major hurricanes like Katrina has nearly doubled worldwide since 1990 — a finding that scientists say coincides with rising sea surface temperatures and seems certain to stoke the debate on global warming as well.

"What we found was rather astonishing," says Peter Webster of Georgia Tech's School of Earth and Atmospheric Sciences. "In the 1970s, there was an average of about 10 Category 4 and 5 hurricanes worldwide per year. Since 1990, the number has averaged 18 per year."

Although Webster and his fellow researchers stop short of attributing the increase directly to global warming, they say the worldwide increase in intense storms — like Hurricane Katrina — closely matches the predictions of computer climate models for a warmer world.

"It's impossible to say that a particular hurricane like Katrina, or any other storm, is due to climate change. But storms like Katrina have increased tremendously in all ocean basins of the world, so the trend doesn't appear to be a result of natural variability," says Webster.

The report by researchers from Georgia Tech and the National Center for Atmospheric Research was published Thursday in the journal Science. Because any given hurricane — or any hurricane season — is shaped by local and regional conditions, past efforts to show a connection between Earth's changing climate and the frequency or intensity of tropical storms have been the subject of vigorous debate.

Hurricane experts in and out of government, for instance, agree that this year's flurry of activity in the Atlantic and the Gulf of Mexico on top of last year's record four hurricanes that struck Florida are the result of a well-documented cycle that has come and gone every 20 years or so for more than a century, long before global warming was a concern.

As the Outer Banks of North Carolina mop up after Hurricane Ophelia, and New Orleans struggles with the aftermath of Katrina, the current Atlantic hurricane season continues at a furious pace.

So far this year, the Atlantic and Gulf have spawned 15 named storms, and forecasters say they expect the activity during the remaining 10 weeks of the season to continue at "near record levels."

A study by MIT climatologist Kerry Emmanuel earlier this year attributed the increase in powerful Atlantic storms to global warming, a contention that was quickly discounted by a number of prominent hurricane experts, including some at the National Hurricane Center.

Because increased hurricane activity in one ocean basin is often balanced by a lull in activity elsewhere around the globe, it has been difficult for scientists to link any global, long-term trend to climate change.

The latest study, however, is the first to look simultaneously at activity in all of the world's tropical oceans — the Atlantic, Pacific and Indian — over a period of 30 years.

"Looking at one hurricane or one season doesn't really tell you anything at all," says Greg Holland, of the National Center for Atmospheric Research. "But when you see a sustained increase like this, the chances of it being natural variation are pretty remote. The trend toward Katrina-type storms is increasing."

Worldwide, sea surface temperatures have risen about one degree in the last few decades. Regardless of the cause of the warming, researchers say even such a small increase dramatically affects the release of water vapor from the ocean surface, where tropical storms and hurricane gather their energy.

Webster and his colleagues say that while the warmer seas have produced a greater number of intense storms, in most of the ocean basins the total number of hurricanes of all categories has actually declined.

The one exception is the tropical Atlantic, where both the number of storms, their duration, and the number of intense hurricanes has increased.

In the 15 years prior to 1990, the Atlantic experienced 16 Category 4 and 5 hurricanes. In the 15 years since 1990, there were 25. And the total number of hurricanes has been above normal in nine of the last 11 seasons.

Mike Toner writes for The Atlanta Journal-Constitution. E-mail: mtoner@ajc.com