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Glaciers Quicken Pace to Sea
A number of massive glaciers in the West Antarctic are sliding into the ocean at an accelerating rate and raising sea levels, according to new data released Thursday.
The new study, published Thursday in the journal Science, found that six glaciers flowing into the Amundsen Sea have quickened their march into the ocean over the past 15 years, and the pace has accelerated recently. The fastest of these, the Pine Island Glacier, is ripping along at a six-yards-a-day pace 25 percent faster than it was moving in the 1970s making it one of the fastest-moving glaciers on Earth.
"What we're seeing here is a comparative gallop," said Eric Rignot, a glaciologist at NASA's Jet Propulsion Laboratory who worked on the study.1
Ice-penetrating radar onboard research aircraft discovered that these glaciers were, on average, 430 yards thicker than previously thought, dramatically increasing the volumes of ice flowing into the seas. Should all six glaciers completely slide into the ocean and melt, sea levels would rise worldwide by more than three feet, Rignot said.
"That amount of fresh water is enough to disturb the global ocean-current circulation," he said.
The Antarctic continent measures 5.4 million square miles nearly 1.5 times the size of the United States and 98 percent of it is covered in ice year-round. This ice is nearly three miles thick in places and locks up more than two-thirds of the planet's fresh water.
Vast floating ice shelves fringe half of the continent and comprise 11 percent of its total area. Ice shelves are the long fingernails of glaciers, averaging 500 yards in thickness. The sea gradually melts the bottom of these shelves, thinning them until storms or waves break off pieces, calving icebergs.
Glaciologist Robert Thomas of EG&G Technical Services at NASA's Wallops Flight Facility in Virginia has long believed that the ice shelves act like a cork in a bottle, greatly slowing glaciers' procession to the sea. However, in this area the bottom of the ice shelves are melting rapidly, becoming thinner at a rate of 10 to 15 feet each year since the early 1990s.
The "corks" have been loosened, allowing the glaciers to flow more quickly, Thomas said. "The climate is warming up in this region, and many ice shelves are thinning and some are breaking up," he said.
Most surprising is that while warm coastal water thins the floating ice shelf, the main trunk of Pine Island Glacier is also thinning by four feet a year, as far as 185 miles inland.
"These thinning rates are double those seen in the 1990s and extend much further inland," Thomas said.
If this continues, within five years at least 270 square miles of very thick ice from Pine Island Glacier will be floating in the ocean. And that will further accelerate the flow of the rest of the glacier. "It could double its current speed within five years," he said.
Glaciers flowing into another part of West Antarctica that lost their ice shelf in 2002 are indeed flowing faster, according to another study released this week. Not long after much of the Larsen B Ice Shelf broke up in the Weddell Sea, nearby glaciers began to flow up to eight times faster than before, said Ted Scambos, a glacier expert who headed the study at the University of Colorado's National Snow and Ice Data Center.
The speed of change was surprising and strongly supports the idea that ice shelves act as brakes on glacier movement, Scambos said.
The West Antarctic region, and in particular its far northern tip just south of Chile and Argentina, has seen a rise in mean annual temperatures of up to 4.5 degrees Fahrenheit in the past 60 years -- faster than almost any region in the world. In the past 30 years, ice shelves in the region have decreased by more than 5,200 square miles.
However, there are far larger and more important ice shelves. The Ross Ice Shelf, the main outlet for the West Antarctic Ice Sheet, bottles up several large glaciers; sea levels could rise by 16 feet if they melted completely.
"Ice-shelf thinning could be happening elsewhere in the Antarctic, but we just don't know," Scambos said.
It's a difficult place to do research, and there is very little data on how much the oceans around the frozen continent may be warming or currents changing. What is certain is that this new evidence means current predictions, which estimate that global warming will cause global sea levels to rise 10 to 36 inches by the year 2100, will have to be revised upward, Thomas said.
"It is cause for concern and that we need to pay much more attention to what's happening in the Antarctic," said Rignot. "But it's not necessary to start running for the hills yet."
Ice velocities derived from five
Landsat 7 images acquired between January 2000 and February 2003 show a two- to
six-fold increase in centerline speed of four glaciers flowing into the
now-collapsed section of the Larsen B Ice Shelf. Satellite laser altimetry from
ICESat indicates the surface of Hektoria Glacier lowered by up to 38 ± 6 m in a
six-month period beginning one year after the break-up in March 2002. Smaller
elevation losses are observed for Crane and Jorum glaciers over a later 5-month
period. Two glaciers south of the collapse area, Flask and Leppard, show little
change in speed or elevation. Seasonal variations in speed preceding the large
post-collapse velocity increases suggest that both summer melt percolation and
changes in the stress field due to shelf removal play a major role in glacier
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