Thu Sep 15 11:00:01 2005 Pacific Time
Hurricanes Are Getting Stronger, Study Says
ATLANTA, Sept. 15 (AScribe Newswire) -- The number of Category 4
and 5 hurricanes worldwide has nearly doubled over the past 35 years,
even though the total number of hurricanes has dropped since the 1990s,
according to a study by researchers at the Georgia Institute of
Technology and the National Center for Atmospheric Research (NCAR). The
shift occurred as global sea surface temperatures have increased over
the same period. The research appears in the Sept. 16 issue of Science.
Peter Webster, professor at Georgia Tech's School of Earth and
Atmospheric Sciences, along with NCAR's Greg Holland and Georgia Tech's
Judith Curry and Hai-Ru Chang, studied the number, duration, and
intensity of hurricanes (also known as typhoons or tropical cyclones)
that have occurred worldwide from 1970 to 2004. The study was supported
by the National Science Foundation (NSF), NCAR's primary sponsor.
"What we found was rather astonishing," said Webster. "In the
1970s, there was an average of about 10 Category 4 and 5 hurricanes per
year globally. Since 1990, the number of Category 4 and 5 hurricanes
has almost doubled, averaging 18 per year globally."
Category 4 hurricanes have sustained winds from 131 to 155 miles
per hour; Category 5 systems, such as Hurricane Katrina at its peak
over the Gulf of Mexico, feature winds of 156 mph or more.
"Category 4 and 5 storms are also making up a larger share of
the total number of hurricanes," said Curry, chair of the School of
Earth and Atmospheric Sciences at Georgia Tech and coauthor of the
study. "Category 4 and 5 hurricanes made up about 20 percent of all
hurricanes in the 1970s, but over the last decade they accounted for
about 35 percent of these storms."
The largest increases in the number of intense hurricanes
occurred in the North Pacific, Southwest Pacific, and the North and
South Indian Oceans, with slightly smaller increases in the North
Atlantic Ocean.
All this is happening as sea surface temperatures have risen
across the globe anywhere from around one-half to 1 degree Fahrenheit,
depending on the region, for hurricane seasons since the 1970s.
"Our work is consistent with the concept that there is a
relationship between increasing sea surface temperature and hurricane
intensity," said Webster. "However, it's not a simple relationship. In
fact, it's difficult to explain why the total number of hurricanes and
their longevity has decreased during the last decade, when sea surface
temperatures have risen the most."
The only region that is experiencing more hurricanes and
tropical cyclones overall is the North Atlantic, where they have become
more numerous and longer-lasting, especially since 1995. The North
Atlantic has averaged eight to nine hurricanes per year in the last
decade, compared to six to seven per year before the increase. Category
4 and 5 hurricanes in the North Atlantic have increased at an even
faster clip: from 16 in the period of 1975-89 to 25 in the period of
1990-2004, a rise of 56 percent.
A study published in July in the journal Nature came to a
similar conclusion. Focusing on North Atlantic and North Pacific
hurricanes, Kerry Emanuel (Massachusetts Institute of Technology) found
an increase in their duration and power, although his study used a
different measurement to determine a storm's power.
But whether all of this is due to human-induced global warming
is still uncertain, said Webster. "We need a longer data record of
hurricane statistics, and we need to understand more about the role
hurricanes play in regulating the heat balance and circulation in the
atmosphere and oceans."
"Basic physical reasoning and climate model simulations and
projections motivated this study," said Jay Fein, director of NSF's
climate and large scale dynamics program, which funded the research.
"These results will stimulate further research into the complex natural
and anthropogenic processes influencing these tropical cyclone trends
and characteristics."
Webster is currently attempting to determine the basic role of
hurricanes in the climate of the planet. "The thing they do more than
anything is cool the oceans by evaporating the water and then
redistributing the oceans' tropical heat to higher latitudes," he said.
"But we don't know a lot about how evaporation from the ocean
surface works when the winds get up to around 100 miles per hour, as
they do in hurricanes," said Webster, who adds that this physical
understanding will be crucial to connecting trends in hurricane
intensity to overall climate change.
"If we can understand why the world sees about 85 named storms a
year and not, for example, 200 or 25, then we might be able to say that
what we're seeing is consistent with what we'd expect in a global
warming scenario. Without this understanding, a forecast of the number
and intensity of tropical storms in a future warmer world would be
merely statistical extrapolation."
NCAR is operated by the University Corporation for Atmospheric
Research under the sponsorship of NSF and other agencies. Opinions,
findings, conclusions, or recommendations expressed in this publication
do not necessarily reflect the views of any of UCAR's sponsors.
- - - -
CONTACTS:
Anatta, NCAR Media Relations, 303-497-8604,
David Terraso, Georgia Institute of Technology Media Relations,
404-385-2966,
Cheryl Dybas, NSF Public Affairs, 703-292-7734,
Media Contact: Anatta, 303-497-8604, David
Terraso, 404-385-2966, Cheryl Dybas,
703-292-7734,
.
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