Warming, storm intensity linked

Rising ocean temperatures have stoked the growing fury of hurricanes, according to a study made public Thursday that intensifies a debate over the link between global warming and the ferocity of storms.

Of all the factors that drive a major storm — such as humidity, wind shear or broad air-circulation patterns — only the steady increase in sea-surface temperatures during the past 35 years can account for the rising strength of tempests in six ocean basins around the world, including the North Atlantic, Pacific and Indian oceans, scientists at the Georgia Institute of Technology reported.

"This firms up the link between sea-surface temperatures and hurricane intensity," said senior study author Judith Curry, an expert in climate variability who is head of Georgia Tech's School of Earth and Atmospheric Sciences.

The research revealed that the increase in the most severe storms — category 4 and 5 hurricanes have doubled since 1990 — was directly linked to the rising temperature of tropical oceans, which warmed globally by 1 degree Fahrenheit during the same period. Warm water vapor rising from the sea helps fuel massive storms.

While many hurricane experts remained unconvinced of the connection between global warming and storm intensity, Curry said: "This trend can't be explained by natural cycles because the cycles are different for each basin. This is not natural variability."

The Georgia Tech study, posted online Thursday by the journal Science, comes after several unusually disruptive storm seasons worldwide.

Hurricanes during the 2005 North Atlantic storm season set records for severity and for the number that made landfall, such as Katrina, which killed more than 1,300 people in Louisiana and Mississippi.

Last March, communities in southern Brazil suffered severe damage in the region's first recorded severe cyclone, as hurricanes are known outside of the Atlantic and Caribbean Sea.

In the Pacific, 10 major tropical cyclones made landfall in Japan during 2004. Last year, five cyclones hit the Cook Islands in a five-week period.

Meteorologist Kerry Emanuel at the Massachusetts Institute of Technology said the mercurial variables of local weather may be important for storms during a single hurricane season, but only sea-surface temperatures showed long-term significance.

"When you are worried about longer time scales, the really big factor is sea-surface temperature," Emanuel said.

Many hurricane experts, however, said the severity of storms depends on so many factors, ranging from the salinity of regional seawater to the pervasive influence of El Niņo currents, that it is misleading to single out ocean temperatures as the dominant factor.

Long-term climate cycles also play a role, with the number of severe storms rising and falling naturally over the course of a century.

Moreover, the historical data used by the Georgia Tech researchers to estimate the frequency of severe storms over the years are inaccurate and incomplete, several experts said.

"The estimates are flaky," said Hugh Willoughby, head of the International Hurricane Center at Florida International University in Miami. "I think the [warming] signal is there, but the data problems are leading us to exaggerate it ... No matter how you do the statistics, we really don't know."

A panel of hurricane experts convened by the World Meteorological Organization in February to assess the effects of climate change formally concluded that no single severe tropical storm during the past two years can be "directly attributed to global warming."

A trend toward more severe storms may be emerging, the panelists said, but it was too soon to understand why.

"This is a hotly debated area for which we can provide no definitive conclusion," the panelists reported.