Atlantic Cyclones May Decrease As Globe Warms - Study
US: May 19, 2008
MIAMI - Fewer but more intense hurricanes may form in the Atlantic Ocean as
the globe warms toward the end of this century, according to a new study
that counters predictions of more frequent cyclones due to climate change.
The study, published on Sunday in Nature Geoscience, adds fuel to a fierce
scientific debate over whether human-produced greenhouse gases have
contributed to a recent rise in hurricane activity in the Atlantic basin and
whether tropical cyclones are becoming stronger.
A simulation of Atlantic hurricane activity for the final decades of the
century projected an 18 percent decrease in hurricanes and a 27 percent
decrease in tropical storms, researchers at the US government's Geophysical
Fluid Dynamics Laboratory in New Jersey found.
"It does not support the notion that increasing greenhouse gases are causing
a large increase in Atlantic hurricane or tropical storm frequency," said
Thomas Knutson, one of the study's authors.
Recent studies have found links between rising sea surface temperatures in
the tropical Atlantic Ocean, where cyclones form, and some measures of
hurricane activity. Tropical storms and hurricanes draw their energy from
warm sea water.
Around 1995, scientists believe, the Atlantic entered a period of higher
hurricane activity that could last 25 to 40 years. Some researchers
attribute the rise to natural cycles, while others believe global warming
may be brewing more, and more intense, storms.
The frenzied hurricane seasons of 2004, when four strong hurricanes hit
Florida, and 2005, when a record-breaking 28 storms formed, fired up the
climate change debate.
The next six-month Atlantic hurricane season begins on June 1 and
forecasters expect it to be a busy one, well above the long-term average
season that produces about 10 tropical storms and six hurricanes.
A well-known forecasting team at Colorado State University predicts 15
storms, of which eight would become hurricanes.
Forecasters have been wrong in the last few years.
Global energy, commodities and insurance markets have paid close attention
since 2004 and 2005, when the steady stream of Atlantic storms -- including
$80 billion in damage from Hurricane Katrina -- marched through US oil and
gas fields in the Gulf of Mexico.
WIND SHEAR RISING
The study by the Geophysical Fluid Dynamics Laboratory used a global warming
scenario for the rest of the century projected by the UN Intergovernmental
Panel on Climate Change.
That included a rise of 1.7 degrees Celsius in Atlantic sea temperatures,
higher vertical wind shear and lower low- to mid-tropospheric humidity
across the Caribbean.
Researchers said water temperatures in the tropical Atlantic have risen more
in recent decades than in other oceans but the models project more uniform
warming in coming years.
"Now those models could be wrong ... (but) without that rapid increase in
the Atlantic compared to the other tropical basins, our models do not expect
a rapid increase in Atlantic storminess," said researcher Isaac Held.
Vertical wind shear, a difference in wind speeds at varying altitudes that
tends to tear apart nascent hurricanes, could be another contributor.
Researchers said the environment may become more hostile to cyclones before
the end of the century.
"We've had lower levels of vertical wind shear (in recent years), but the
climate model projections are not indicating reduced wind shear in the
Atlantic but rather ... increased vertical wind shear," Knutson said.
The model projects more rainfall from hurricanes toward the end of the
century and a modest increase in intensity.
Knutson said a past study forecast a 4 percent rise in intensity for every 1
degree C rise in sea temperature, but this latest study found a smaller rise
of 1 to 2 percent.
The researchers said their findings for the Atlantic basin would not
necessarily apply to other ocean basins, in part because wind shear was not
expected to rise elsewhere.
Noting the debate among hurricane and climate researchers on the issue of
global warming and hurricanes, Knutson said: "We don't regard this as the
last word on this topic."
(Editing by Michael Christie and John O'Callaghan)
Story by Jim Loney
REUTERS NEWS SERVICE
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