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viernes, 29 de mayo de 2009

Boston "Directly in Path" of Sea Level Rise

By 2100 visitors to Boston could be parking their boats, not their cars, in Harvard Yard.

Major cities in the northeastern U.S. and eastern Canada "are directly in the path of the greatest rise" in sea level if Greenland continues to melt due to global warming, a new study says.

Scientists believe that the influx of fresh water from the disintegrating ice sheets will disrupt a circulation pattern in the Atlantic Ocean, causing seas to expand.

The new projections call for an extra 4 to 12 inches (10.2 to 30.5 centimeters) on top of the rise of 8 inches (20.3 centimeters) previously estimated in the journal Nature Geoscience in March.

That previous study found that, if global warming continues, sea levels around New York City would rise by twice as much as in other parts of the United States within this century.

In the new study, researchers considered three scenarios: that Greenland's present melt rate of 7 percent would continue, or a drop to either one or 3 percent a year—viewed by many as more likely, as the rate is actually expected to slow in coming decades.

"We hope the high end wouldn't happen," said study lead author Aixue Hu of the National Center for Atmospheric Research in Boulder, Colorado.

But "we should be aware that there's a potential the melt of the Greenland ice sheet could be faster than we expected."


Of the three scenarios, the two lower melt rates are more realistic, according to computer models of future ice sheet melting, Hu said.

A 3 percent melt rate would mean an extra 12 inches (30.5 centimeters) on top of the predicted global sea level rise of 21 inches (54 centimeters), and a one percent rate would mean an extra 8 inches (20.3 centimeters) for the region. But if the current 7 percent rate were to persist, up to 20 inches (50 centimeters) of extra water would inundate cities such as New York and Halifax, Nova Scotia.

Still, Waleed Abdalati, a glaciologist at the University of Colorado, Boulder, said that the 7 percent scenario is really a worst case, particularly because "melt rate" is sometimes a misnomer, he said.

When glaciers break off into the ocean, they don't immediately melt, so scientists can't really say that all of Greenland's ice loss is due to melting, he explained.

But the finding is still important, he added, because "a few inches [of sea-level rise], depending on the time frame on which this occurs, makes a significant amount of difference," especially when it impacts heavily populated coastal areas.

What's more, previous studies had not taken into account how Greenland's melt might interact with an oceanic "conveyor belt" in the Atlantic Ocean, which transports water north from the tropics.

Normally in the belt tropical water gets cooler and becomes a deep layer of dense, cold water in the North Atlantic.

But the freshwater flow from Greenland slows down this conveyor belt and prevents the deep, dense water from accumulating. This would make deep water warmer and less dense, causing surface waters to expand throughout the North Atlantic.

It's for this reason that the northeast coast of North America is particularly vulnerable to rising seas, said Hu, whose research will appear May 29 in the journal Geophysical Research Letters.


Such predictions, and the study of ice sheets in general, is often plagued with uncertainties.

For instance, scientists still don't understand ice sheet dynamics, such as how fast an unstable ice sheet will melt.

And data from the Intergovernmental Panel on Climate Change (IPCC)—which many researchers depend on as the gold standard in climate change predictions—can become easily outdated.

The 2007 IPCC assessment, for example, projected a sea-level rise of up to 23 inches (59 centimeters) this century. But the rapid decline of the world's ice sheets has led many researchers to believe the rise will be even greater.

"The more we know," said the University of Colorado's Abdalati, "the more we're finding things are more severe than we thought."

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