A Storm Over Warming
Are hurricanes linked to global warming?
It's been a blessedly calm storm season thus far-a striking contrast to last year, when 11 named storms had formed by the end of August. But among those who study hurricanes, things haven't been so calm. Twelve months after Katrina churned up public fears about the possible effects of global warming on hurricanes, the debate is splintering the scientific community. And it's more than an academic dispute, because scientists do agree on this: Stronger-than-normal hurricanes will be bludgeoning America's coastlines for years to come.
Like all matters involving global warming, the wrangle has gone political; the winning viewpoint will carry implications for public policy and the economy. "Global warming is real," says Chris Landsea of the National Oceanic and Atmospheric Administration. "The question is: What kind of impact does it have on hurricanes?" The answer is, well, cloudy.
There are basically two camps: those who blame warming for a heightened ferocity of storms and those who see only a peak in a natural cycle called the Atlantic Multidecadal Oscillation. NOAA has regularly come down on the side of natural variability-a storm pattern that spans several decades. NOAA scientists note that high hurricane activity took place from the 1920s to the 1960s, slowed down in the '70s and '80s, and then took off again after 1994. The upswing could continue for several decades. While the cause of the cycle is poorly understood, NOAA argues that it comes from conditions that power hurricanes, such as warm water, low wind shear, and favorable easterly winds. "We see absolutely no indication," said NOAA scientist Gerry Bell in a November press conference, "that greenhouse warming is causing any of it."
But several studies make a different case. Last August, Kerry Emanuel of the Massachusetts Institute of Technology correlated strengthening hurricanes with rising sea-surface temperatures, which are probably due to global warming, he said. A month later, Peter Webster and Judith Curry of Georgia Tech issued a report bolstering Emanuel. They found a doubling worldwide in Category 4 and 5 storms over the past 35 years, while noting that global sea-surface temperatures have increased by about 1 degree. "If [increased storm intensity] was due to global warming, wouldn't we find it everywhere?" Webster says. "Well, we've found increases everywhere." In the tit for tat that followed, Landsea, the NOAA scientist, published a report criticizing Webster's data as unreliable. "Some or perhaps all of [Webster's] trend is spurious," Landsea says. Webster counters that even factoring in uncertainties, the trend is undeniable.
Cooling. Emanuel of MIT has followed with a study this summer that blames aerosols and other pollutants-which can have a cooling effect on the atmosphere-for the downswing in hurricane activity in the '70s and '80s, rather than the variability cited by NOAA.
Escalating the debate are charges that NOAA has squelched scientists who speak out about global warming. Jerry Mahlman, the former director of NOAA's laboratory in Princeton, N.J., says his former colleagues have been "under pressure to minimize the effects of global warming." With the debate raging late last year, NOAA scientist Tom Knutson, who believes global warming may be exacerbating the multidecadal cycle, says he was prevented from doing television interviews on the subject by public affairs officers. In November, a NOAA report asserted a "consensus" among its scientists that recent hurricane activity resulted primarily from natural variability, not global warming. After protests from inside and out, NOAA attached a footnote saying the report "does not necessarily represent the views of all NOAA scientists." All this generated unwelcome publicity, prompting the NOAA administrator, Vice Adm. Conrad Lautenbacher, to issue a memo denying censorship had occurred. Knutson says he has not been prevented from doing interviews since. And the NOAA controversy has quieted.
But the debate rages on. This spring, Emanuel and Webster joined a press conference hosted by the liberal National Environmental Trust to assail NOAA's stance on global warming. The conservative Competitive Enterprise Institute, meanwhile, calls links between warming and hurricanes premature and inconclusive at best. This July, Curry testified before Congress that an independent assessment, perhaps by the National Research Council, would help settle the debate. Congress, to date, hasn't followed up. Until it does-or some other definitive data emerge-this storm seems sure to continue.
This story appears in the September 5, 2006 print edition of U.S. News & World Report.
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