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Will Washington drown not in rhetoric, but water? Scientists have long known that the Atlantic Ocean's sea level appears to be rising faster south of New York than north of it. Some experts speculated that our possibly warming world had raised ocean temperatures: warm water expands more than cold water. Others guessed that the Gulf Stream, the warm ocean current that runs along the East Coast, might have changed its flow. But geophysicist James L. Davis, an associate of the Harvard College Observatory, and physicist Jerry Mitrovica, of the University of Toronto, began to wonder if the answer might not lie in the standard model of Earth's interior.

During the last Ice Age, the enormous weight of miles-thick glacial ice compressed the earth's crust and mantle-the gooey, semisoft material flowing under the crust. Over the centuries, as the ice melted away, the crust and mantle have been slowly "rebounding," like a sponge regaining its shape after having been squashed beneath a dictionary. Consequently, the northernmost parts of North America have risen, while its southern parts have sunk. The continent is tilting like a giant seesaw with a southern list. Thus, at least in part, the rise in sea level south of New York does not mean rising water, but sinking land.

Davis and Mitrovica first studied "glacial rebound" in Scandinavia. There the researchers successfully measured sea- level changes by juxtaposing long-term data from tide gauges with the accepted value of the mantle's viscosity, or flow. (Tide gauges are floats attached to a digitized wire that allows readings to be sent by satellite to a central location.)

Then, back in this country, they tried the same method on the East Coast. "When we applied our theory to the data from tide gauges here, we didn't get a match," says Davis. The researchers began to question the standard model of how the earth's crust has been moving since the melting of the big glaciers. They were especially interested in the viscosity of the mantle, a key factor in glacial rebound.

Applying Occam's razor, Davis and Mitrovica narrowed their parameters to three, representing adjustments to the viscosity of the earth's upper and lower mantles, and to the thickness of the lithosphere (the uppermost portion of the mantle). The data were most sensitive to the viscosity of the lower mantle. Values for the upper mantle and the lithosphere did not fit their tide-gauge readings.

Next came a computer analysis. "The results were baffling," says Davis. The researchers checked their earlier papers for clues. Soon one emerged: the "peripheral bulge"-formed by the crust and mantle rising up where glacial ice had once compressed them. The bulge runs along the East Coast from Hudson Bay to Florida, but it was not where it was thought to be. Its predicted location is sensitive to changes in the lower mantle's viscosity. According to Davis, the data told the researchers that if they increased the standard model's value of the viscosity by a factor of 2.5, that would alter the geographic pattern of the glacial rebound and solve the puzzle. "It completely explains the difference in observed sea-level rise along the East Coast," says Davis.

The adjusted value for the viscosity of the mantle could be applied elsewhere, he adds. But the East Coast is one of the few places in the world that straddle a peripheral bulge. Scandinavia also has a peripheral bulge, but the American one is larger.

The pair's findings suggest that the level of the mid-Atlantic coast is falling as the peripheral bulge slowly settles. This discovery, combined with apparently rising sea levels, means the Atlantic Ocean is encroaching on the Chesapeake Bay region at a rate of about an inch every eight years, and on other sections of the East Coast by about an inch every 25 years. Decades from now, mid-Atlantic areas such as Washington, D.C., could become prone to flooding as a result, says Davis.

"Before, we didn't know what was causing rising oceans; we thought it was human activity or glaciers melting," says Davis. "Our information now tells us we don't have control over areas of global glacial rebound. But global sea-level rise," he adds, "to the extent that it derives from the greenhouse effect, we can do something about."

molly colin

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