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 over the last 35 years can account for the rising strength of tempests in six oceans 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 climate variability expert Judith A. Curry, a senior author of the study who heads Georgia Tech's School of Earth and Atmospheric Sciences. "It is an important piece of the global warming debate."
Their 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 temperatures of tropical oceans, which warmed globally by 1 degree Fahrenheit during the same period. Warm water vapor rising from the sea helps energize massive storms.
Though 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, published Thursday in the online edition of the journal Science, comes after several unusually disruptive storm seasons worldwide.
Hurricanes during the 2005 North Atlantic storm season set records not only for their severity but also for the number that made landfall, such as Hurricane Katrina, which killed more than 1,300 people in Louisiana and Mississippi.
Also last year, five fully developed cyclones raged through the Cook Islands in a five-week period.
In March 2004, communities in southern Brazil suffered severe damage in the region's first recorded severe cyclone, as hurricanes are known in the South Atlantic. Meanwhile in the Pacific, 10 major tropical cyclones made landfall in Japan during 2004.
MIT meteorologist Kerry Emanuel said that the mercurial variables of local weather might be important for storms during a single hurricane season but that only sea surface temperatures showed any 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 depended on so many technical factors, such as the salinity of regional seawater and the pervasive influence of El Nino currents, that it was 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.
Several experts said the historical data used by the Georgia Tech researchers to estimate the frequency of severe storms over the years was inaccurate and incomplete.
"The estimates are flakey," said Hugh Willoughby, head of the International Hurricane Research 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 concluded that no single severe tropical storm during the last two years could 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.
Still, the evidence is continuing to build that the factors leading to warmer seas are growing worse.
This week, the National Oceanic and Atmospheric Administration announced that the concentration of carbon dioxide in the Earth's atmosphere had grown in 2005 to 381 parts per million, about 100 ppm higher than pre-industrial levels.
Carbon dioxide is one of several greenhouse gases that can alter the planet's climate by trapping heat from the sun.
The rate of increase has doubled in recent years, NOAA's carbon dioxide analysts said.
Levels of other greenhouse gases, such as nitrous oxide and methane, have also been rising steadily for decades.