Scientists disagree on whether warming means more hurricanes -- It seems similarly logical that as the world warms, hurricanes will be more frequent or more powerful or both. After all, they draw their strength from warm ocean waters. But while many scientists hold this view, there is far less consensus, in part because of new findings on other factors that may work against stronger, more frequent storms.

Scientists disagree on whether warming means more hurricanes
Other factors may work against bigger, more frequent storms

By CORNELIA DEAN The New York Times

WHEN PEOPLE WORRY about the effects of global warming, they worry more
about hurricanes than anything else. In surveys, almost three-quarters
of Americans say there will be more and stronger hurricanes in a
warming world. By contrast, less than one-quarter worry about
increased coastal flooding.

But as far as the scientific consensus is concerned, people have
things just about backward.

There is no doubt that as the world warms, seas will rise, increasing
the flood risk, simply because warmer water occupies more space. (And
if the Greenland or Antarctic ice sheets melt, the rise will be far
greater.)

It seems similarly logical that as the world warms, hurricanes will be
more frequent or more powerful or both. After all, they draw their
strength from warm ocean waters. But while many scientists hold this
view, there is far less consensus, in part because of new findings on
other factors that may work against stronger, more frequent storms.

"Global warming is as real as it gets," Richard A. Anthes, president
of the University Corporation for Atmospheric Research, said last
month at a weather conference in the Bahamas, where most of the
conversation focused on hurricanes. But as for its link to hurricanes,
Anthes said, "I don't think it's been proved conclusively."

In a consensus statement issued last year, the World Meteorological
Organization said it was likely that there would be some increase in
hurricane wind speeds in a warmer world. But the organization, which
is the UN weather agency, noted that decades-long periods of high and
low hurricane activity, unconnected to any climate change, had been
recorded before. (Climate experts say a period of high activity began
in 1995.)

Also, measurement techniques have greatly improved in recent decades,
making it difficult to compare data and detect trends.

So as the annual hurricane season begins on June 1, scientists are
pressing on a number of fronts to learn how hurricanes form and move,
what factors limit or expand their lethal potential and how to tell
with greater precision when and where they will strike.

Perhaps the best-known proponent of the idea that warming and
hurricanes may be connected is Kerry A. Emanuel, an atmospheric
scientist at the Massachusetts Institute of Technology. His conclusion
that the total power released in Atlantic and western Pacific
hurricanes had increased perhaps by half in recent decades, reported
in 2005 in the journal Nature, is one of the most discussed ideas in
the debate.

He is not alone. Last year, researchers led by Carlos D. Hoyos of the
Georgia Institute of Technology analyzed the frequency of Category 4
and 5 storms, the most powerful, and concluded that their increased
frequency since 1970 was "directly linked to the trend in sea-surface
temperature," which is increasing. They reported their findings in the
journal Science.

Other experts challenge the idea that a warmer world means more and
stronger storms. For example, researchers at the National
Oceanographic and Atmospheric Administration and the University of
Miami have been studying how vertical wind shear - the differences in
wind direction or speed at different altitudes - can inhibit hurricane
formation.

In work reported last month in Geophysical Research Letters, the
researchers said that in a warming world, wind shear in the Atlantic
would increase, possibly enough to cancel out the hurricane-forcing
effects of warmer water.

Last week, researchers at the Woods Hole Oceanographic Institution in
Massachusetts reported in the journal Nature that periods of frequent
storminess had occurred in the past, even though things were cooler
than they are now. They also concluded that wind currents were a
crucial factor.

But even these researchers call the question open. "This doesn't
settle the issue," said Gabriel Vecchi, the lead author of the wind
shear study and a research scientist at the NOAA Geophysical Fluid
Dynamics Laboratory, in Princeton, N.J.

In February, researchers led by James Kossin, an atmospheric scientist
at the University of Wisconsin, recalibrated recent and early
satellite data on hurricanes using information from the National
Climatic Data Centre, a NOAA archive in Asheville, N.C. They concluded
that hurricane frequency had increased, but only in the Atlantic,
possibly because temperatures there are just about warm enough for
storms; so even modest warming makes hurricanes more likely.

But when Christopher W. Landsea analyzed historical records of
hurricane activity, he concluded that satellite observations and other
new techniques had increased scientists' ability to detect major
storms, skewing the frequency data. Landsea, a meteorologist at the
National Hurricane Centre, reported this conclusion this month in EOS,
an electronic publication of the American Geophysical Union.

This kind of he-said-he-said debate often leads people to dismiss a
subject as one about which nothing will ever be known with confidence.
In fact, the give and take is an example of the way scientists tug and
haul at their own and others' findings until a consensus takes shape.

In the current debate over global warming and hurricanes, the problem
is relatively new and the data are hard to obtain and analyze.

For example, atmospheric researchers are wrestling with an enormous
amount of new data as they study factors that contribute to the
formation and maintenance of the hurricane's characteristic eye and
the bands of wind and rain that howl around it.

They hope to use the data to predict better how strong hurricanes will
be when they strike land. But the effort is complicated by the way
storms gain or lose strength as they move over warm or cool water, and
by the way their structures evolve.

One question meteorologists and climate experts can answer quickly is
an obvious one: What happened to the hurricane season of 2006? Viewed
from the perspective of the Atlantic and Gulf coasts, it was a bust
(or a boon). Not a single hurricane struck the U.S.

But last year a persistent Bermuda high, sitting unusually far out in
the Atlantic, and air currents from an unexpected and quick-forming El
Nino system, which developed in the Pacific in August, diminished the
storms' potential to strike the U.S. As a result, it felt like a year
with no storms, even though there were only slightly fewer named
storms than average (nine instead of 11), about as many became
hurricanes (five instead of six) and, as in an ordinary year, two
hurricanes with winds of more than 111 miles per hour, the standard
for Category 3 on the Saffir-Simpson Hurricane Scale.

This year, we will probably not be so lucky, forecasters at the
National Hurricane Centre said at a news conference last week. They
said they expected 13 to 17 named storms this season, seven to 10 of
them hurricanes and three to five of them major storms. The first of
the named storms, Andrea, formed off the Southeast coast in mid-May,
more than three weeks before the season's official start.


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© 2007 The Halifax Herald Limited

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