PARIS For all its numbing ferocity, Hurricane Katrina will not be a unique event, say scientists, who say that global warming appears to be pumping up the power of big Atlantic storms.
This year is on track to be the worst-ever year for hurricanes, according to experts measuring ocean temperatures and trade winds the two big factors that breed these storms in the Caribbean and tropical North Atlantic.
Earlier this month, Tropical Storm Risk, a London-based consortium of experts, predicted that the region would see 22 tropical storms during the six-month June-November season, the most ever recorded and more than twice the average annual tally since records began in 1851.
Seven of these storms would strike the United States, of which three would be hurricanes, it said.
Already, 2004 and 2003 were exceptional years: they marked the highest two-year totals ever recorded for overall hurricane activity in the North Atlantic.
This increase has also coincided with a big rise in Earth's surface temperature in recent years, driven by greenhouse gases that cause the Sun's heat to be stored in the sea, land and air rather than radiate back out to space.
But experts are cautious, also noting that hurricane numbers seem to undergo swings, over decades.
About 90 tropical storms a term that includes hurricanes and their Asian counterparts, typhoons occur each year.
The global total seems to be stable, although regional tallies vary a lot, and in particular seem to be influenced by the El Nino weather pattern in the Western Pacific.
"Atlantic cyclones have been increasing in numbers since 1995, but one can't say with certainty that there is a link to global warming," says Patrick Galois with the French weather service Meteo-France.
"There have been other high-frequency periods for storms, such as in the 1950s and '60s, and it could be that what we are seeing now is simply part of a cycle, with highs and lows."
On the other hand, more and more scientists estimate that global warming, while not necessarily making hurricanes more frequent or likelier to make landfall, is making them more vicious.
Hurricanes derive from clusters of thunderstorms over tropical waters that are warmer than 27.2 C (81 C).
A key factor in ferocity is the temperature differential between the sea surface and the air above the storm. The warmer the sea, the bigger the differential and the bigger the potential to "pump up" the storm.
Just a tiny increase in surface temperature can have an extraordinary effect, says researcher Kerry Emanuel of the Massachusetts Institute of Technology (MIT).
In a study published in Nature in July, Emanuel found that the destructive power of North Atlantic storms had doubled over the past 30 years, during which the sea-surface temperature rose by only 0.5 C (0.9 F).
Emanuel's yardstick is storm duration and windpower: hurricanes lasted longer and packed higher windspeeds than before.
Another factor in destructiveness is flooding. Kevin Trenberth of the U.S. National Center for Atmospheric Research suggests that hurricanes are dumping more rainfall as warmer seas [release] more moisture into the air, swelling the storm clouds.
The indirect evidence for this is that water vapor over oceans worldwide has increased by about 2% since 1988. But data is sketchy for precipitation dropped by recent hurricanes.
"The intensity of and rainfalls from hurricanes are probably increasing, even if this increase cannot yet be proven with a formal statistical test," Trenberth wrote in the U.S. journal Science in June. He said computer models "suggest a shift" toward the extreme in ihurricane intensities.