Report links global warming, storms
Keay Davidson, Chronicle Science Writer

Tuesday, September 12, 2006


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Scientists say they have found what could be the key to ending a
yearlong debate about what is making hurricanes more violent and common
-- evidence that human-caused global warming is heating the ocean and
providing more fuel for the world's deadliest storms.

For the past 13 months, researchers have debated whether humanity is to
blame for a surge in hurricanes since the mid-1990s or whether the
increased activity is merely a natural cycle that occurs every several
decades.

Employing 80 computer simulations, scientists from Lawrence Livermore
National Laboratory and other institutions concluded that there is only
one answer: that the burning of fossil fuels, which warms the climate,
is also heating the oceans.

Humans, Ben Santer, the report's lead author, told The Chronicle, are
making hurricanes globally more violent "and violent hurricanes more
common" -- at least, in the latter case, in the northern Atlantic
Ocean. The findings were published Monday in the latest issue of the
Proceedings of the National Academy of Sciences.

Hurricanes are born from tropical storms fueled by rising warm, moist
air in the tropics. The Earth's rotation puts a spin on the storms,
causing them to suck in more and more warm, moist air -- thus making
them bigger and more ferocious.

In that regard, the report says, since 1906, sea-surface temperatures
have warmed by between one-third and two-thirds of a degree Celsius --
or between 0.6 and 1.2 degrees Fahrenheit -- in the tropical parts of
the Atlantic and Pacific oceans, which are hurricane breeding grounds.

Critics of the theory that greenhouse gases are making hurricanes worse
remained unconvinced by the latest research.

Chris Landsea, a top hurricane expert, praised the Proceedings paper as
a worthwhile contribution to science, but said the authors failed to
persuasively counter earlier objections -- that warmer seas would have
negligible impact on hurricane activity.

Landsea, science and operations officer at the U.S. National Hurricane
Center in Miami, noted that modern satellite observations have made
hurricanes easier to detect and analyze, and that could foster the
impression of long-term trends in hurricane frequency or violence that
are, in fact, illusory. The surge in hurricane activity since the
mid-1990s is just the latest wave in repeating cycles of hurricane
activity, he said.

Philip Klotzbach, a hurricane forecaster at Colorado State University,
said that "sea-surface temperatures have certainly warmed over the past
century, and ... there is probably a human-induced (global warming)
component." But his own research indicates "there has been very little
change in global hurricane activity over the past 20 years, where the
data is most reliable."

Researchers report in the Proceedings paper an 84 percent chance that
at least two-thirds of the rise in ocean temperatures in these
so-called hurricane breeding grounds is caused by human activities --
and primarily by the production of greenhouse gases.

Tom Wigley, one of the world's top climate modelers and a co-author of
the paper, said in a teleconference last week that the scientists tried
to figure out what caused the oceans to warm by running many different
computer models based on possible single causes. Those causes ranged
from human production of greenhouse gases to natural variations in
solar intensity.

Wigley said that when the researchers reviewed the results, they found
that only one model was best able to explain changing ocean
temperatures, and it pointed to greenhouse gases in the atmosphere. The
most infamous greenhouse gas is carbon dioxide, a product of human
burning of fossil fuels in cars and factories.

Wigley estimated the odds as smaller than 1 percent that ocean warming
could be blamed on random fluctuations in hurricane activity, as some
scientists suggest.

The debate among scientists was triggered in August 2005, a few weeks
before Hurricane Katrina struck New Orleans, when hurricane expert
Kerry Emanuel of MIT wrote an article for the journal Nature proposing
that since the 1970s, ocean warming had made hurricanes about 50
percent more intense in the Atlantic and Pacific oceans.

Later, two scientific teams, both at Georgia Tech, estimated that
warmer sea-surface temperatures were boosting both hurricane intensity
and the number of the two worst types of hurricanes, known as Category
4 and Category 5 storms.

Nineteen scientists from 10 institutions were involved in the
Proceedings paper. In addition to Lawrence Livermore, other U.S.
institutions included Lawrence Berkeley National Laboratory, the
National Center for Atmospheric Research, NASA, UC Merced, Scripps
Institution of Oceanography in La Jolla (San Diego County), and the
National Oceanic and Atmospheric Administration.

Santer's co-authors included six Livermore colleagues -- Peter J.
Gleckler, Krishna AchutaRao, Jim Boyle, Mike Fiorino, Steve Klein and
Karl Taylor -- and 12 other researchers from elsewhere in the United
States and from Germany and England.

Assuming that warmer water equals more bad hurricanes, scary times
could be ahead for inhabitants of hurricane-prone regions.

That's because "the models that we've used to understand the causes of
(ocean warming) in these hurricane formation regions predict that the
oceans are going to get a lot warmer over the 21st century," Santer
said in a statement. "That causes some concern."



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How Hurricanes form in the Atlantic Ocean Hurricanes are born in far
western Africa, where modest windstorms known as tropical disturbances
pick up moisture from the warm sea and begin to whirl. As atmospheric
pressures drop, tropical depressions form with wind speeds up to 38
mph. As they speed westward they become tropical storms, lashing the
ocean with sheets of rain and winds blowing up to 70 mph or more,
finally building into hurricanes with winds exceeding 100 mph. --
Tropical disturbance -- Tropical depression -- Tropical storm --
Hurricane Source: NOAA, The New York Times Joe Shoulak / The Chronicle