[Paleopsych] CHE: Hurricanes Have Grown More Intense Since 1970, Researchers Say, and Global Warming Is a Prime Suspect
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Sun Sep 25 01:54:49 UTC 2005
Hurricanes Have Grown More Intense Since 1970, Researchers Say, and Global
Warming Is a Prime Suspect
News bulletin from the Chronicle of Higher Education, 5.9.19
By RICHARD MONASTERSKY
The number of intense hurricanes developing around the globe has
climbed markedly in the past 35 years, according to researchers who
mined storm data from the tropical ocean basins and are publishing
their findings in a paper in today's issue of Science.
While the total number of tropical cyclones has waxed and waned over
the decades with no overall change, the proportion that reach the
status of Category 4 and 5 hurricanes -- the strongest storms -- has
climbed from 20 percent to 35 percent since 1970, according to Peter
J. Webster, a professor of earth and atmospheric sciences at the
Georgia Institute of Technology, and his colleagues.
The researchers say they cannot tell whether the increase is part of a
natural cycle or an indication that global warming has altered the
behavior of storms. But they note that the temperature of the sea
surface has climbed in all tropical ocean regions over the same period
and that such changes, in theory, could spawn stronger storms.
Judith A. Curry, chairwoman of Georgia Tech's School of Earth and
Atmospheric Sciences, says that when researchers see sea-surface
temperatures and hurricane intensity "relentlessly rising," it gives
them confidence that "these two things are connected and that there is
probably a substantial contribution from greenhouse warming and it's
not just natural variability."
"This could be a very long-period cycle, and it could go down in the
next 30 years," says another author of the paper, Greg J. Holland,
director of the Mesoscale and Microscale Meteorology Division at the
National Center for Atmospheric Research. "Unfortunately I don't think
The new findings are consistent with a study, published last month in
Nature by Kerry A. Emanuel, an atmospheric scientist at the
Massachusetts Institute of Technology (The Chronicle, September 8).
Mr. Emanuel reported that the total power released by tropical storms
in the North Atlantic and North Pacific Oceans had increased
substantially in the past 30 years, a trend that dovetails with the
changes expected to occur because of global warming. Forecasts using
computer climate models have suggested that warming of the tropical
oceans could spur the growth of more strong hurricanes, such as
Katrina, which was a Category 4 storm when it made landfall.
Mr. Webster and his colleagues say the random nature of hurricanes
makes it impossible to discern whether the long-term change they have
identified has influenced any one storm. But the trend they detected
suggests there are more Katrina-type storms now than in the past. "Who
knows?" he asks. "Had this trend not been there, then Katrina may have
been a Category 2 or a Category 3 and done a lot less damage."
Some scientists have criticized Mr. Emanuel's report on hurricane
power, and the new paper is also likely to generate a scientific
storm. Christopher W. Landsea, science-and-operations officer at the
National Hurricane Center, in Miami, says the new study relies on
incomplete storm data that were collected using different techniques
over the years. He questions the veracity of the reported increase in
intense storms. "My conclusion is that it's an artifact of the
database," he says.
Mr. Webster and his colleagues analyzed the number, intensity, and
duration of tropical storms. They chose to limit their study to the
period of satellite storm observations, which began in 1970. For each
storm, meteorologists used the satellite images to estimate wind
speeds by examining the cloud patterns in the storm and
characteristics of the cyclone's eye.
Mr. Landsea says that the techniques and the satellites have changed
with time, making it difficult to identify any trend over the decades
in storm wind speeds. "They didn't take into account the changes in
methodology," he says of Mr. Webster and his colleagues.
Unlike other ocean basins, however, the Atlantic has good wind
measurements taken by aircraft that flew through the center of the
hurricanes. Mr. Landsea notes that the Atlantic data show the smallest
change over time. The most intense storms went from being 20 percent
of the total in the late 1970s and 1980s to being 25 percent of the
total from 1990 through 2004. That change, he says, is consistent with
a cyclical pattern in hurricane activity seen in the Atlantic since
the late 1920s.
But Mr. Holland responds that his team took into account the changes
in the data. He also notes that storms have grown more intense in all
ocean basins, including the well-studied Atlantic and the eastern
North Pacific, where meteorologists have some aircraft measurements to
check against the satellite data. "We've done the best we can," he
says. "If it is a factor of the database, it is a very, very remote
chance that is the case."
2. mailto:rich.monastersky at chronicle.com
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