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Antarctic Glaciers Surged After 1995 Ice-Shelf Collapse

John Roach
for National Geographic News
March 6, 2003
 
When a huge floating shelf of ice hinged to the northern end of the
Antarctic Peninsula disintegrated in January 1995, several glaciers that
were backed up into it surged towards the sea, according to a pair of
Argentinean researchers.

The discovery marks the first positive evidence that glacial surge follows an ice shelf collapse. It may lead scientists to revive the previously discarded theory that ice shelves acts as dams that prevent inland glaciers from slipping into the seas.

"We think that our discovery strongly supports the view of the ice shelves as major dams for the inland ice," said Hernán De Angelis, a scientist with the Glaciology Division of the Instituto Antártico Argentino in Buenos Aires.


De Angelis and colleague Pedro Skvarca made their discovery through analysis of airborne mapping data, satellite imagery, and flights over the area of the peninsula affected by the collapse of the northern section of the Larsen Ice Shelf. They report their findings in the March 7 issue of the journal Science.

For more than 30 years, scientists have debated whether or not ice shelves act as barriers for inland ice. The speed at which glaciers move, a rate technically known as glacial flow, is largely determined by the type of surface glaciers rest on, according to recent computer modeling studies. Those studies lead many scientists to discount the theory that ice shelves act as a sort of brake.

The debate is important, say scientists, because if the same disintegration scenario were to play out with major ice shelves such as the Ross and Filchner-Ronne, the entire West Antarctic Ice Sheet could collapse and cause global sea levels to rise upwards of 16 feet (5 meters)—provided ice shelves were the only thing holding the glaciers back from the sea.

"In certain circumstances ice shelves probably do act as a barrier. When removed the glaciers will undergo some change, as described in the paper," said Chris Doake, a scientist with the British Antarctic Survey in Cambridge, England. "But you cannot extrapolate this behavior to the rather different physical conditions seen around much of the West Antarctic Ice Sheet."

Glacial Surge

De Angelis and Skvarca found that five of the six major streams of ice that fed into the northern section of the Larsen Ice Shelf dramatically surged towards the sea and then retreated in the years following the 1995 disintegration of the shelf.

Their evidence for this surging includes several locations where blocks of ice sit stranded about 66 to 131 feet (20 to 40 meters) above glacier surfaces. These so-called ice terraces are formed when a glacier suddenly lowers as a result of a surge.

In addition, maps of the glaciers made in March 2002 show that they have retreated since their surge, an indication that the glaciers dumped much of their ice into the seas, said De Angelis.

"Our discovery is indicating that a strong coupling between ice-stream and ice-shelves exists," he said.

Richard Alley, a geoscientist at Pennsylvania State University in University Park, said that De Angelis and Skvarca's evidence for glacial surge is pretty good given the fact that a long-term record on the rate of flow of these glaciers does not exist. The Argentine scientists simply looked for—and found—telltale signs of glacial surge.

"Nature might have caused a surge naturally at about the time of an ice shelf breakup. But nature is highly unlikely to have caused a bunch of surges at almost the same time unless they are related to the ice shelf breakup," he said. "The authors found such features. So it appears that the ice shelf breakup and ice flow speedup did occur together."

Balance of Forces

De Angelis and Skvarca say their find is significant because it shows that at least for some glaciers, ice shelves do act as barriers that prevent them from spilling into the sea. This finding, they say, is directly linked to the debate over the stability of the West Antarctic Sheet and its sensitivity to ice shelf disintegration.

Ted Scambos, a research scientist at the National Snow and Ice Data Center in Boulder, Colorado, said that much of this debate centers on defining the balance of forces that prevent glaciers from spilling into the ocean. The braking ability of ice shelves is one force. Another is the bed upon which the glacier rests.

"Different characteristics of the bed have a big effect on how a glacier moves and how fast it would move if you remove the back-stress [such as an ice shelf]," he said. "People have also debated how much back-stress is within the glacier itself."

For example, if the glacier is frozen to its bed, the removal of an ice shelf would not have much of an effect on the glacier's velocity, said Alley. But if the glacier rests on a slippery bed of mud, the loss of an ice shelf sheet could cause a glacier to accelerate. There are also cases where the bed is already so slippery that that the glacier is essentially floating on the bed but held back because of other factors such as being frozen to side walls.

"Telling where the ice goes from having a non-slippery to slippery to too-slippery bed is not all that easy," said Allen. "We know a lot, are learning more, but a clear answer is not yet in hand."
 

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