[Paleopsych] CHE: Hurricanes Have Grown More Intense Since 1970, Researchers Say, and Global Warming Is a Prime Suspect

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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
http://chronicle.com/free/2005/09/2005091605n.htm

    [2]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
    that."

    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 ([3]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.

    More Katrinas

    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."

References

    2. mailto:rich.monastersky at chronicle.com
    3. http://chronicle.com/daily/2005/09/2005090803n.htm