Hurricanes
more powerful, study says
Researcher
at MIT sees larger storms with stronger winds
http://www.boston.com/news/nation/articles/2005/08/01/hurricanes_more_powerful_study_says/
The destructive power of hurricanes in the
North Atlantic and North Pacific has nearly doubled over the past 30 years, at
least partly because of human-induced global warming, according to a
controversial new study by a prominent climate researcher at the Massachusetts
Institute of Technology.
Though the number of tropical cyclones
worldwide has hovered at 90 a year for decades, MIT hurricane specialist Kerry
Emanuel contends that the storms are growing larger and reaching higher maximum
wind speeds than in the past. Focusing on the cyclones that have been most
closely measured -- hurricanes striking the Eastern United States and typhoons
in Southeast Asia -- Emanuel concluded that today's storms, on average, release
far more energy than their predecessors did in the mid-1970s.
''There seems to be a clear
correlation" between increasing strength and length of storms and a
temperature increase of 0.5 degrees Celsius on the surface of the sea during
the same period, said Emanuel, whose paper was published online yesterday by
the journal Nature.
One of the nation's leading hurricane
forecasters, William Gray of Colorado State University, said Emanuel is leaping
to conclusions based on imprecise information about the strength of hurricanes,
especially in decades past. He said Emanuel's formula for calculating the
energy released by hurricanes obscures the fact that no one directly measured
the winds in many of the storms, roughly estimating speeds from satellite
images instead.
''It's a terrible paper, one of the
worst I've ever looked at,"
said Gray, who does not believe that cyclone intensity worldwide is increasing.
He also questioned Emanuel's contention that human actions, such as the burning
of oil and other fuels, have caused the surface of the ocean to warm. Gray said
the ocean-temperature increase is natural.
Suzana Camargo, a cyclone specialist at
Columbia University in New York, said Emanuel's findings should be taken
seriously, arguing that his conclusion about the growing power of hurricanes is
similar to the increase in the energy of typhoons she measured when tropical
Pacific temperatures rise by several degrees Celsius as a result of cyclical El
Nino weather events. ''You don't have more typhoons; you have more intense
ones. You have supertyphoons," she said.
Christopher Landsea, research
meteorologist at the National Oceanic and Atmospheric Administration in Miami,
said Emanuel deserves credit for taking on a tough issue where the violence of
the storms often destroys the equipment intended to measure their ferocity.
''It's certainly the first paper that does systematically connect hurricane
intensity . . . to ocean temperature rises that may be due to global
warming," said Landsea, who, like Gray, is concerned about the lack of
reliable measures of hurricane winds.
Mounting damage from hurricanes has been a
major political issue in the United States, at least since Hurricane Andrew
caused tens of billions of dollars in damage to Florida in 1992. After several
relatively quiet decades, the number of hurricanes originating in the tropics
between Africa and the Caribbean rose sharply in the mid-1990s, including two
storms last year, Charley and Ivan, that rank among the worst in US history for
total damage.
Though most analysts agree the uptick in
the number of Eastern hurricanes is a result of natural factors, some
environmentalists have argued that global warming intensifies the storms.
Global temperatures have increased by roughly one degree Fahrenheit in the past
century, at least partly as a result of carbon dioxide releases from the
burning of fossil fuels. During the election last fall, a group called
Scientists and Engineers for Change posted a billboard in Florida that read:
''Global warming = Worse hurricanes. George Bush just doesn't get it."
Until now, scientific arguments that
global warming can intensify extreme weather have been based mainly on
projections derived from models. Emanuel published a paper in 1987 suggesting
that a warming of the ocean's surface waters would give hurricanes more fuel to
churn faster. That theory is now supported by the main scientific panel
tracking global warming, the Intergovernmental Panel on Climate Change.
However, detecting an actual increase in
overall hurricane activity has proven elusive. Worldwide, the annual number of
cyclones -- a term that includes typhoons and hurricanes -- has fluctuated
consistently between 80 and 100 for decades. And scientists had generally
agreed that the small rise in ocean temperatures so far is not enough to cause
dramatic changes in hurricane intensity, so few researchers attempted to
measure for a trend.
Emanuel said he, too, was surprised at his
results, which found the total power of North Atlantic hurricanes has more than
doubled since the mid-1970s, while western North Pacific cyclones have increased
in intensity by 75 percent. He said his team reviewed all the wind speed
measurements for hurricanes in both regions, making adjustments for the
imprecision of past records, then feeding the numbers into his formula.
Colorado's Gray said he was appalled that
Emanuel would take such shaky data on wind speeds, then feed them into a
formula that puts such heavy weight on those numbers. You can get any result
you want, he said.
Emanuel acknowledges that his technique
has a large margin of error and that he would not have published the results if
the increase in storm intensity hadn't so closely mirrored the rise in ocean
temperatures over the past 30 years.
Even if Emanuel is proved right, the
increasing damage from hurricanes may have more to do with where people choose
to live than the power of the storms. Most researchers have generally agreed
that the mounting damage is mostly a consequence of people building on the
coast, effectively putting more people and property in harm's way.
The contention that storms are getting
stronger ''could be true, but the significance of the trend he's identified is
dwarfed by the damages from coastal development," Roger A. Pielke,
director of the Center for Science and Technology Policy at the University of
Colorado, said in an interview.
Emanuel noted that he agreed with Pielke a
few months ago that global warming probably was not a major factor in hurricane
activity. ''I wouldn't be so confident about that now," he said.
Scott Allen can be
reached at allen@globe.com. 