Storm Warning
Smithsonian Magazine
September, 2006, Sep2006, Vol. 37, Issue 6
by Madeleine J. Nash
Scientists have yet to nail down the cause of the multi-decadal oscillation, but these striking ups and downs in surface temperatures appear to correlate--somehow--with hurricane activity. "You can't just heat up the ocean by 1 degree Celsius and Pow! Pow! Pow! get more hurricanes," says Goldenberg. More critical, he thinks, are atmospheric changes--more or less wind shear, for example--that accompany these temperature shifts, but what comes first? "We still don't know which is the chicken and which is the egg," he says. "The ocean tends to warm when the trade winds get weaker, and the trade winds can get weaker if the ocean warms. Will we lock it down? Maybe someday."
After leaving Goldenberg's office, I drive across town to the National Hurricane Center, a low-lying bunker whose roof bristles with satellite dishes and antennae. Inside, as computer monitors rerun satellite images of Katrina's savage waltz toward the Gulf Coast, top National Oceanic and Atmospheric Administration officials have gathered to announce the agency's best estimate of how many tropical storms and hurricanes are likely to form in 2006. It's not an encouraging forecast: eight to ten hurricanes, fewer than last year, but four to six of them Category 3s or higher. (Last year there were seven.) The predictions are based, in large part, on the multi-decadal oscillation. "The researchers are telling us that we're in a very active period for major hurricanes," says Max Mayfield, the center's director, "one that will probably last at least 10 to 20 more years."
FROM HIS 16TH-FLOOR OFFICE on the Massachusetts Institute of Technology campus, meteorologist Kerry Emanuel commands a crow's-nest view of the esplanade along the Charles River, the dividing line between Boston and Cambridge. In 1985, he remembers, the windows wept with spray blown up from the river by Hurricane Gloria, a moderately strong storm that, nonetheless, made a mess of the Northeast. A painting by a Haitian artist that shows people and animals drowning in a storm surge hangs on a wall near his desk.
Last year, right after Katrina hit, Emanuel found himself in the media spotlight. A few weeks earlier he had published evidence in the journal Nature that hurricanes in both the North Atlantic and the western basin of the North Pacific had undergone a startling increase in power over the past half century The increase showed up in both the duration of the storms and their peak wind speeds. The cause, Emanuel suggested, was a rise in tropical sea surface temperatures due, at least in part, to the atmospheric buildup of carbon dioxide and other heat-trapping gases caused by the burning of fossil fuels.
Even scientists who would expect hurricanes to intensify in response to greenhouse warming were stunned by Emanuel's suggestion that global warming has already had a profound effect. Computer simulations of a warming world, notes climate modeler Thomas Knutson of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, suggest that by the end of this century, peak sustained wind speeds could increase by around 7 percent, enough to push some Category 4 hurricanes into Category 5 territory But Knutson, along with many others, did not think that the intensity rise would be detectable so soon--or that it might be five or more times larger than he and his colleagues anticipated. "These are huge changes," Knutson says of Emanuel's results. "If true, they may have serious implications. First we need to find out if they're true."
Emanuel's paper raised the ante in what has grown into an extremely intense debate over the sensitivity of the earth's most violent storms to gases spewed into the atmosphere by human beings. In the months since the dispute began, dozens of other studies have been reported, some of which support Emanuel's conclusions, others of which call them into question. The debate has grown so impassioned that some former colleagues now scarcely speak to one another.
As Emanuel sees it, sea surface temperatures are important because they tweak a fundamental dynamic that controls hurricane intensity. After all, storm clouds form because the ocean's heat warms the overlying air and pumps it full of moisture. And the warmer the air is, the more vigorous its rise. For their part, Emanuel's critics, Goldenberg and Landsea among them, don't utterly discount ocean warmth. They just put far more emphasis on other factors like wind shear as the main determinants of storm intensity.
Sorting out the differences between the two camps is not easy Goldenberg and Landsea, for example, grant that greenhouse gases may be contributing to a slight long-term rise in sea surface temperatures. They just don't think the effect is significant enough to trump the natural swings of the Atlantic multi-decadal oscillation. "It's not simply, yes or no, is global warming having an effect?" says Landsea, the science and operations officer for the National Hurricane Center. "It's how much of an effect is it having?"
Emanuel, while respectful of Landsea, is not backing down. In fact, he has now stirred up a second storm. "If you'd asked me a year ago," Emanuel says, "I would have probably told you that a lot of the variability in hurricane activity was due to the Atlantic multi-decadal oscillation. I've now come to the conclusion that the oscillation either doesn't exist at all or, if it does, has no perceptible influence on the temperature of the tropical Atlantic in the late summer and fall"--that is, in hurricane season.
Emanuel says that much of the cooling in the tropical North Atlantic in the 1970s can be traced to atmospheric pollutants, specifically to a haze of sulfurous droplets spewed out by volcanoes and industrial smokestacks. Global climate modelers have recognized for years that this haze in the atmosphere acts as a sunshade that cools the earth's surface below. Emanuel says that now that this form of air pollution is on the wane (and this is a good thing for all sorts of reasons having nothing to do with hurricanes), the warming influence of greenhouse gas pollution, and its effect on hurricanes, is growing ever more pronounced. "We will have some quiet [hurricane] years," he says. "But unless we have a really big volcanic eruption, we'll never see another quiet decade in the Atlantic in our lifetime or that of our children."
Is such a grim prediction warranted? Scientists on the periphery of the debate aren't yet sure. For now, says meteorologist Hugh Willoughby of Florida International University, the points of agreement among experts are more important than the differences. Whether a natural oscillation or greenhouse warming is to blame, the odds of a major hurricane striking the U.S. coastline are higher than they have been for more than a generation. And the dangers such storms pose are higher than ever...
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