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Giant Antarctic Ice Shelves Shatter and Break Away

BOULDER, Colorado, March 19, 2002 (ENS) - The accelerating break up of Antarctic ice shelves has reached a new peak, with the dramatic loss of two huge pieces on separate sides of the continent.

On the rapidly warming Antarctic Peninsula, the northern section of the Larsen B ice shelf has collapsed, reducing it to a size not seen for some 12,000 years. The large floating ice mass on the eastern side of the Antarctic Peninsula has shattered and separated from the continent in the largest single event in a 30 year series of ice shelf retreats on the peninsula.

At the same time on the coast of West Antarctica, a monster iceberg has broken off the Thwaites Glacier, increasing concerns such events may lead to much bigger losses of stored ice.

Larsen B

A British research vessel, the RRS James Clark Ross, was in the area of the Larsen B ice shelf just as the breakup occurred and provided images from the ocean surface. March 8, 2002 (Photo by Keith Nicholls courtesy British Antarctic Survey)
The two events were observed over the past month by British and American authorities using satellite and ship based data.

Recent satellite imagery analyzed at the National Snow and Ice Data Center (NSIDC)at the University of Colorado at Boulder revealed that the shattered Larsen B ice shelf has formed a plume of thousands of icebergs adrift in the Weddell Sea, east of the Antarctic Peninsula.

A total of about 3,250 square kilometers (1,250 square miles) of shelf area has disintegrated in a 35 day period beginning on January 31.

"This breakup gave us the information we need to reassess the stability of ice shelves around the rest of the Antarctic continent," said NSIDC glaciologist Ted Scambos. "They are closer to the limit than we thought."

Both atmospheric and ocean warming are behind the ice breakages, according to Professor Bill Budd of Australia's Antarctic Co-operative Research Centre. "The atmospheric warming is contributing to more melt on the ice surface, and that can lead to the extension of crevasses through the ice shelf, thus breaking it up," said Budd.

"We also have evidence of warming of the ocean below floating ice which shows that even one or two tenths of a degree make a significant difference," Budd said. "It's a double whammy, so to speak."

The Larsen ice shelves that line the east coast of the Antarctic Peninsula are being closely monitored by the British Antarctic Survey, which found the region had warmed 2.5 degrees in just 50 years.

The Larsen A shelf broke off in 1995, allowing ships to navigate where they had never before been able to go. Then in 1998 researchers forecast Larsen B would be next to go.

ice

View of the broken Larsen B ice shelf March 12, 2002 (Photo courtesy Pedro Skvarca, Instituto Antártico Argentino)
Scientists worldwide have monitored the Larsen B shelf since November 2001, when a researcher at the Instituto Antártico Argentino warned the community of an impending breakup in the wake of warm spring temperatures and a dramatic 20 percent increase in the ice shelf's flow rate.

International cooperation between Argentinian, American, British, Austrian and German scientists has resulted in detailed information on the breakup from field observations, shipboard studies and a variety of satellite sensors.

Scambos and colleagues Mark Fahnestock at the University of Maryland and Christine Hulbe of Portland State University have theorized that once melt water appears on the surface, the rate of ice disintegration increases. They say melt water ponding on the surface in late summer magnifies fracturing by filling smaller cracks. From there, Scambos said, the weight of the water drives the cracks through the ice, making it shatter.

Scientists say the breakup is not over. The nearest shelf south, Larsen C, is next on the list, Scambos said. "The Larsen C is very near its stability limit, and may start to recede in coming decades if the warming trend continues," he said.

More importantly, he said, regions of the giant Ross Ice Shelf "are just a few degrees Celsius away from being overtaken by the same processes that have destroyed the Larsen."

Scambos

Ted Scambos takes a location reading on the West Antarctic ice sheet during a 1994 expedition. (Photo courtesy University of Washington)
If the Ross ice shelf breaks away it has the potential to release ice equal to a five metre (19 foot) sea level rise, Scambos said.

While the breakup of floating ice shelves such as the Larsen B will not raise sea level, scientists are concerned when glaciers lose too much of their ocean frontage.

When Iceberg B-22 calved off the Thwaites Glacier Tongue on March 11, it took about 5,538 square kilometers of ice with it.

Such enormous ice barriers act as buttresses, or braking systems for glaciers. Once gone, the glaciers can run out faster. Dr. Neal Young from the Australian Antarctic Co-operative Research Centre said ice flows increased two or three times after the loss of Larsen A.

The loss of B-22 has taken the Thwaites Glacier back further than had ever been observed, and follows only months after the Pine Island Glacier yielded a similar berg.

"On top of what we know of the thinning ice sheet leading into these glaciers," said Dr. Young, it is something we need to look at carefully, to determine whether they are part of a long term trend which could affect sea level."

{Published in cooperation with The Antarctican}



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