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IT'S OFFICIAL: HURRICANES ARE GETTING STRONGER
Science Blog / National Center for Atmospheric Research
September 15, 2005

http://tinyurl.com/7zo3z

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 September 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.

"This long period of sustained intensity change provides an excellent basis
for further work to understand and predict the potential responses of
tropical cyclones to changing environmental conditions", said NCAR's
Holland.

"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% of all hurricanes in the 1970s, but over
the last decade they accounted for about 35% 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 between one-half and one 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."

"NCAR is now embarking on a focused series of computer experiments capable
of resolving thunderstorms and the details of tropical cyclones," said
Holland. "The results will help explain the observed intensity changes and
extend them to realistic climate change scenarios."

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%.

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."

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Published by David Sunfellow
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