Scientist links rising ferocity of hurricanes to global warming

MIAMI — The accumulated power of hurricanes has more than doubled in the past 30 years, with a particularly dramatic spike since 1995, and global warming likely is a major cause, according to a study to be published this week.

The report by Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology, is the first to draw a statistical relationship between global warming and hurricane ferocity. He reviewed about five decades of data on hurricanes in the Atlantic and typhoons in the Pacific.

"The large upswing in the last decade is unprecedented and probably reflects the effect of global warming," Emanuel wrote in a study that is to be published Thursday in the journal Nature. Copies of the article were made available yesterday.

His study did not shed any light on the effect, if any, of global warming on the number of storms.

Scientists at the National Oceanic and Atmospheric Administration's hurricane-research division in Virginia Key, Fla., previously have concluded that, due to long-term natural cycles, we are in the middle of a decades-long period of more frequent hurricane formation.

Emanuel's study comes at a time when residents along the Gulf Coast and throughout the Caribbean are recovering from what forecasters are calling the most active start to the hurricane season on record.

Since June 1, six storms grew strong enough to merit names. Three became hurricanes. Two reached a potent Category 4, with winds exceeding 130 mph, according to forecasters at the National Hurricane Center in Miami. Hurricane Dennis, which reached Category 4 on July 7, ranks as the earliest Caribbean storm on record to reach that strength.

So, if both camps of scientists are correct, people in hurricane-prone areas could be facing stronger storms and more of them.

"My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential and — taking into account an increasing coastal population — a substantial increase in hurricane-related losses in the 21st century," Emanuel wrote.

He said his analysis of wind-speed reports by the National Hurricane Center and other sources shows that the accumulated power of hurricanes in the Atlantic basin, which includes the Caribbean Sea and the Gulf of Mexico, has more than doubled since 1970.

A particularly steep increase began in 1995, according to the study.

"This large increase in power dissipation over the past 30 years or so may be because storms have become more intense, on the average, and/or have survived at high intensity for longer periods of time," he wrote.

He said the trend is closely linked to an increase of about one degree in the average ocean surface temperature, which might not seem significant but can be crucial.

"It sounds like a small amount, but we know that as waters get even a little bit warmer, the potential exists for hurricanes to get dramatically stronger," said Chris Landsea, a NOAA scientist on Virginia Key and one of the nation's leading hurricane researchers.

Still, he is not fully convinced by Emanuel's study.

Landsea said the 1995-04 spike in accumulated hurricane power correlated precisely with the beginning of the period of increased hurricane formation.

"It's very difficult to separate out what's caused by this natural cycle of activity versus man-made warming," Landsea said.

He also raised concerns about some statistical procedures employed by Emanuel, whom he described as "a very respected researcher."

"This is a serious study and it needs to be taken seriously," Landsea said. "But when you take a close look at it, there's a lot of caveats. So, at this point, I'm not convinced he's found the smoking gun between global warming and hurricanes."

Some researchers argue that, in practical terms, the allure to live near the sea will do far more to boost society's risk from such storms over the next several decades than any effect global warming could have on the storms themselves.

Until he concluded this study, Emanuel says, he was among that group. Now, he says, global warming's impact on the storms may play a bigger role than previously believed in putting societies at risk, particularly in less-developed countries.

"I'd been thinking of a very modest response" of tropical cyclones to climate change, "and what we're seeing is not so modest," he said.

Material from The Christian Science

Monitor is included in this report