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Hurricanes
AUGUST 2005
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In Hot Water (continued)
By Chris Carroll
Photographs by Tyrone Turner
Last year's record hurricane season may have been just the beginning. Forecasters predict the Atlantic seaboard could be in for decades of relentless pounding.

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"The whole East Coast was very lucky for the past 30 or 40 years," says William Gray, of Colorado State University, a pioneer in long-range hurricane research and forecasting. Gray had predicted a damaging storm season for last year. "We'd been saying things would change, but nobody expected anything like the 2004 season."
 
The coming hurricane seasons might make last year's estimated 40 billion dollars' worth of U.S. damage look small. Warns Gray: "There's no way—if we see a return to the type of landfall activity we saw in the thirties, forties, and fifties—no way but that economic losses will double, triple, quadruple, or worse."
 
That's because societal risk—roughly defined as the number of people and value of property vulnerable to hurricanes—has exploded. The southeastern coastal population has grown dramatically, with Florida's alone more than tripling since 1960. The economic risk has also multiplied. About 1.1 trillion dollars' worth of at-risk property was insured in 1980; the total now is an estimated 5.5 trillion dollars' worth. And the latest census figures show that between 2000 and 2004, 29 of the 50 fastest-growing U.S. counties were in East Coast and Gulf Coast states.
 
The hurricane glut is happening at the same time sea levels continue to rise—the result of global warming that most scientists blame in part on human activity. A recent study using the latest computer climate models predicts warming of the tropical sea surface will strengthen hurricane winds and rainfall by the end of the 21st century. However, some experts, including Gray, argue that climate change due to human activity will not significantly affect hurricanes.
 
That debate will continue, but many scientists agree that the present hurricane surge is likely part of a 60-to-70-year cycle that changes the strength of ocean currents distributing heat around the globe. Researchers have used tree rings and ice cores to track this variability back hundreds of years. We're now in a fast-flowing mode of this up-and-down cycle, named the Atlantic Multidecadal Oscillation (AMO), during which Atlantic sea-surface temperatures and wind conditions favor hurricane generation. Ten years from now, or perhaps thirty (the timetable is difficult to predict), the cycle should reverse, tending to suppress major hurricanes.
 
Why the variation? "Frankly, no one can say with 100 percent certainty, but it appears to be a natural effect," says Thomas Delworth, a climate modeler at NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. Delworth is part of a major scientific effort to develop accurate computer climate models, and much of his work focuses on thermohaline circulation—that is, the way ocean currents, and consequently such cycles as the AMO, are driven by heat and salinity.
 
Thermohaline circulation runs the Atlantic conveyor belt, part of a global ocean system in which a continuous flow of upper-level water is drawn from the tropical Atlantic north toward the Pole. There the water cools, sinks, and cycles back to the southern oceans in deepwater currents.
 
As the conveyor belt speeds up, tropical surface water is drawn north more quickly, and temperatures in the North Atlantic are as much as 2°F warmer. That's good for hurricanes. "A hurricane is essentially an engine that runs on heat," says Chris Landsea, a meteorologist at NOAA's Hurricane Research Division in Miami. "The warmer the sea-surface temperature [it must be at least 80°F for a hurricane to start] and the more warm, moist air that's available, the stronger a hurricane can become."
 
How and where does the conveyor belt speed up, increasing the overturning circulation of warm and cold water? It's at the point where cold surface water sinks that the acceleration of the Atlantic conveyor belt probably happens, Delworth says. Cold, dry air coming off Canada extracts heat from the water. When these winds blow stronger and colder than average over a number of years, increasingly chilled water sinks faster because it is more dense, intensifying the flow rate. Years of weaker and warmer winds have the opposite effect, slowing the conveyor belt.
 
Climate records indicate a correlation between a pattern of increased cold winds and the 1995 upswing in hurricane formation. "From the late sixties to the mid-nineties, westerly winds strengthened," Delworth says. "The overturning circulation probably increased in that period in response."
 
Increased circulation brings mighty storms—born as air spirals into a low-pressure zone charged with warm, humid air over warmer sea surfaces. The winds meet and ascend, causing clouds to billow upward, further lowering air pressure and causing winds to barrel even faster toward the center. The Earth's rotation lends spin to the gathering cyclone. When water vapor in the ascending clouds cools and falls as rain, the amount of heat energy released dwarfs the amount of electricity consumed daily by all of humanity. The energy warms the eye, further lowering the pressure and strengthening the storm.
 
The cyclone can continue to strengthen if atmospheric currents guide it over warm water, and if it is not destroyed by vertical wind shear—the differential between wind speeds at lower and higher altitudes. Strong wind shear can dissipate a storm, but the warm phase of the AMO tends to weaken vertical wind shear in the Atlantic. 

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