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viernes, 29 de mayo de 2009

Greenland Ice Melt May Threaten Northeast

The Greenland Ice Sheet is melting -- and that could spell trouble for the northeast coast of the United States and eastern Canada.

Sea level in those regions might rise by as much as a foot more than current projections, according to a new study. Possible consequences include flooding and damage to both cities and ecosystems.

While experts say it's too soon to know how big the effects will be, lead researcher Aixue Hu said it's probably worth bracing for the worst-case scenario.

"In some sense, I think we should be alarmed and prepared that something could happen in the future," said Hu, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. "Right now, we're not sure which scenario is going to happen. Hopefully, the worst case will not happen." The Greenland Ice Sheet, a large mass of ice that covers most of Greenland, has been melting at an increasing rate since the early 1990s. In recent years, melting has accelerated at a rate of 7 percent each year. Scientists estimate that the ice sheet won’t disappear completely for another 3,000 years, but effects of melting could happen far sooner than that.

Every drop of water that melts from the ice flows into the ocean, raising sea levels. The input of relatively light freshwater also alters circulation patterns that cycle water between Greenland and northern North America, which could lead to more water on the North American end.

Without factoring in the melting of Greenland ice, warmer surface waters are already expected to shift currents, sending about 8 inches of water above the average global rise to the Northeast by 2100, Hu said.

To find out what melting in Greenland would add to the equation, he and colleagues used a computer model to consider three scenarios, in which melting increased at a rate of 7 percent, 3 percent, or 1 percent each year. The team projected out to the year 2100. Given the best-case scenario, the researchers report today in the journal Geophysical Research Letters, the east coasts of Canada and New England would get another four inches of water. In the worst-case scenario, sea level would rise by an extra foot in coastal areas from New York to Halifax.

"In the East Coast region, there are many big cities," Hu said. "If sea levels rise, it means some parts of land will submerge under water."

Water could surge over levies and flood estuaries, where organisms depend on a careful balance between salt and freshwater.

Other researchers warn against jumping to conclusions. For now, Greenland's melting contributes less than half a millimeter of water to the oceans each year, said Mike Winton, an oceanographer at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J. That's a tiny amount.

Whether acceleration of melting from the Greenland Ice Sheet will continue, he added, is a big question mark. The Arctic has experienced wide swings in climate variability.

"I don't think we need to get particularly alarmed about this particular phenomenon," Winton said. "It's just part of the mix."

More important will be figuring out how sea level rise is going to vary from one region to the next around the world -- and what we're going to do about it, said glaciologist Eric Rignot.

It can take 30 or 40 years for polar ice to respond to our efforts to reduce emissions of carbon dioxide and other greenhouse gases, said Rignot, of the Jet Propulsion Laboratory in Pasadena, Calif. That means we need to start thinking about the distant future today.

"Even the most conservative people agree that Greenland is doomed," Rignot said. "Sea level is not going to stop rising after 2100. Instead of being scared, we need to take it seriously and plan ahead."

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