Aside from such short-term events as storms, anomalous wind and ocean patterns caused a sustained and unexpected rise in sea levels on the U.S. East Coast through much of summer 2009, according to a September 2009 report.
Photograph by Mari Darr-Welch/AP
Photograph by Mari Darr-Welch/AP
In an article written by Brian Handwerk for National Geographic News; Handwerk reports: "Sea levels rose as much as 2 feet (60 centimeters) higher than predicted this summer along the U.S. East Coast, surprising scientists who forecast such periodic fluctuations. In a related article written for the National Geographic News by Christine Dell'Amore; she reports that: "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.
Above, an iceberg melts off Ammassalik Island in eastern Greenland on July 19, 2007.
"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."
Vulnerable
"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.
(Explore an interactive map of global warming's effects.)
"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.
Uncertainties
"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.
"The immediate cause of the unexpected rise has now been solved, U.S. officials say in a new report (hint: it wasn't global warming). But the underlying reason remains a mystery.
"Usually, predicting seasonal tides and sea levels is a pretty cut-and-dried process, governed by the known movements and gravitational influences of astronomical bodies like the moon, said Rich Edwing, deputy director for the Center for Operational Oceanographic Products and Services at the U.S. National Oceanic and Atmospheric Administration (NOAA).
"But NOAA's phones began ringing this summer when East Coast residents reported higher than predicted water levels, much like those associated with short-term weather events like tropical storms. But these high seas persisted for weeks, throughout June and July.
"The startling rise caused only minor coastal flooding—but major head scratching among scientists.
Gulf Stream Mysteriously Slowed
"Now a new report has identified the two major factors behind the high sea levels—a weakened Gulf Stream and steady winds from the northeastern Atlantic.
"The Gulf Stream is a northward-flowing superhighway of ocean water off the U.S. East Coast. Running at full steam, the powerful current pulls water into its "orbit" and away from the East Coast.
"But this summer, for reasons unknown, "the Gulf Stream slowed down," Edwing said, sending water toward the coasts—and sea levels shooting upward.
"Adding to the sustained surge, autumn winds from the northeastern Atlantic arrived a few months early, pushing even more water coastward.
Beaches "Eaten Up"
"The higher waters caused inconveniences for some anglers and boaters and rearranged a bit of shoreline.
"A couple of sand beaches we'd normally fish from were eaten up. And the volume of water was higher than it normally would be," said Paulie Apostolides, owner of Paulie's Tackle in Montauk (map) on New York State's Long Island.
"Even before the new report, released by NOAA on September 2, Apostolides said many local fishers had already attributed the sea level rise to the "ferocious" winds from the northeast.
"But the underlying puzzle remains.
"Why did the Gulf Stream slow down? Why did the fall wind pattern appear earlier?" NOAA's Edwing said. "We don't have those answers."
SOURCES AND RELATED WEB SITES
* NOAA Tides and Currents
* Paulie's Tackle of Montauk
* National Center for Atmospheric Research: Aixue Hu
* Geophysical Research Letters
* University of Colorado: Waleed Abdalati
Articles Originally appeared in the September 10, 2009 and May 28, 2009 issues of National Geographic News
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