Paris (AFP) Jan 30, 2008
Warmer seas accounted for 40 percent of a dramatic surge in hurricanes from the mid-1990s, according to a study released on Thursday by the British journal Nature.
The paper -- the first to calculate the precise contribution of sea temperatures in driving hurricane frequency -- could be a major contribution to scientists struggling to understand impacts from global warming, its authors say.
Hurricanes -- the term for fierce cyclones that brew in the Atlantic and threaten Central America, the Caribbean and southern United States -- are known to have several causes.
One of them is the raw fuel of heat and moisture, provided by seas warmed to at least 27 degrees Celsius (80.6 degrees Fahrenheit).
Another, called vertical wind shear, is the angle of prevailing winds. These dictate whether the infant storm will develop into the wheeling shape of a hurricane or instead be torn to be pieces.
British researchers Mark Saunders and Adam Lea of the Benfield UCL Hazard Research Centre at University College London, looked at yearly US meteorological data for hurricanes between 1965 and 2005 and compared these to a 50-year average.
Over the half century, there were around six hurricanes per year on average, roughly half of which were intense hurricanes.
But for the 10 years from 1996 to 2005, the tally rose to about eight hurricanes per year, about four of which were intense ones.
Hurricanes that made landfall in the United States also became more frequent -- one extra storm every three years or so, statistically speaking.
After stripping out the role of wind in hurricane generation, the researchers calculate that an increase of 0.5 degrees Celsius (0.9 Fahrenheit) was responsible for about 40 percent of the rise in hurricane activity.
Saunders told AFP the findings could be a big help for computer modellers striving to understand whether global warming will stoke hurricanes, a scenario that has been sketched by many experts but remains hedged with many unknowns.
But he cautioned against the temptation to extrapolate that hurricane activity will triple or quadruple if the IPCC's predictions of a global temperature rise to 2100 come true.
"Extrapolation assumes that the vertical wind shear will remain constant, whereas some models suggest that it will go up. This would mean that vertical wind shear would have a suppressing effect, counterbalancing the enhancing effect from higher temperatures," he explained.
Nor is it a given, said Saunders, that in the future a sharp rise in sea temperatures will generate so much hurricane fuel, as happened from the mid-90s.
"It could be that currently we are close to the (temperature) threshold for hurricane generation," said Saunders. "It could be that as the sea waters warm more, maybe the sensitivity to sea warming might decrease. Scientists just don't know the answer to this one way, or another."
Oceanographers sometimes have "very fierce and quite acrimonious debates" about the impact of global warming on hurricanes, said Saunders.
Some say the surge in hurricanes at the end of the last century can clearly be pinned on warmer seas.
Others say it is wrong or premature to blame human interference with the climate system. They say there are natural cycles of hurricanes and reliable data -- usually accepted as the advent of weather satellites in the 1960s -- is far too recent to take this into account.
Last year, the UN's Intergovernmental Panel on Climate Change (IPCCC) said it was "likely" that tropical cyclones will become more intense this century. The storms could pack higher peak winds and heavier rainfall as tropical seas warmed.
The 1996-2005 decade climaxed with Hurricane Katrina, the most devastating storm ever to whack the United States.
But 2006 was quieter, and 2007 was unusually calm. Last year, sea temperatures in the tropical Atlantic were even slightly below the norm.
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