Brace for more Katrinas, say experts
Tue Aug 30,10:55 AM ET
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.
2005 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
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
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
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
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
US National Center for Atmospheric Research suggests that hurricanes
are dumping more rainfall as warmer seas suck more moisture into the
air, swelling the stormclouds.
The indirect evidence for this is that water vapour over oceans
worldwide has increased by about two percent 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 US journal Science in
June. He said computer models "suggest a shift" toward the extreme in
in hurricane intensities.
Copyright 2005 Agence France Presse.
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