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both the ice shelf and the glacier are thinning rapidly, and thinning is mostly of dynamic origin.

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Waterfall #2. Ice surface conditions on Larsen-B during the warmest recorded summer. A water-fall cascades over the ~ 30 m high Larsen B ice-front, draining meltwater off the ice shelf observed during February 2002. (Photo: Pedro Skvarca)

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Antarctic Ice Shelfs
The Antarctic ice shelf consists of glacial ice hundreds to thousands of feet thick floating on the sea. As the ice streams move outward, icebergs break off which may be hundreds of feet above water.








Ice shelf buttressing of Antarctic glaciers
The mechanism by which for global warming can influence the rate at which glaciers grain the Antarctic ice sheet is though to be related to the thinning of buttressing ice in ice shelves surrounding the ice sheet. This effect is shown in the graphic below



The West Antarctic Ice Sheet is surrounded by ices shelves hundreds of meters thick which may be be holding back the Antarctic ice sheet glaciers. In a 2004 Scicene article. Richard A. Kerr discusses the buttressing effect.

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These latest results from West Antarctica confirm an unsettling view of glacier behavior. For 30 years, glaciologists have debated whether one part of a glacier can "feel" what's happening in a distant part of the same glacier. At the coastal end of the Pine Island Glacier, for example, warmer water seems to be melting the underside of the glacier's floating ice shelf (Science, 24 July 1998, pp. 499 and 549), pushing landward the point at which the advancing glacier floats off the sea floor.
If an ice shelf pinned against an embayment's shore and floor helps slow a glacier's flow--as was hypothesized in the 1970s--and if changes at the coast could make themselves felt far up the glacier, then the Pine Island Glacier's so-called grounding line retreat would accelerate glacier flow well upstream. The researchers think that's what they're seeing. "I'm convinced the glacier feels what is happening a long way away," says Thomas. Similar accelerations struck after two other floating ice tongues recently broke up in West Antarctica and Greenland (Science, 30 August 2002, p. 1494).

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Potential breakup of the West Antarctic Ice Sheet
In the Scicene article, glaciologist Robert Thomas summarizes evidence for this 'buttressing effect' and it's potential to lead to the breakup of the West Antarctic Ice Sheet.

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Perhaps half the present increase in global sea level of ~1.8 mm/year is caused by melting of terrestrial ice (1). During the 1990s, nonpolar glaciers accounted for an estimated 0.4 mm/year (2) and Greenland for ~0.15 mm/year (3). Although data from Antarctica are still sparse, they suggest a net loss from West Antarctica equivalent to ~0.2 mm/year and approximate balance in East Antarctica, where uncertainty remains large (4). Substantial grounding line retreat (5, 6), thinning (7), and acceleration (8) have been observed on glaciers flowing into the Amundsen Sea, with small ice shelves now but larger ones in the past (9). These glaciers flow into ice shelves over beds well below sea level, and sustained thinning would allow them to float free from bedrock, potentially easing resistive forces acting on upstream ice and thereby leading to further glacier acceleration.

The extent to which ice shelves affect the dynamics of tributary glaciers remains an unresolved controversy within glaciology. Early suggestions that ice-shelf weakening would result in increased discharge from the ice sheet (10–12) require ice-shelf "back forces"to affect glacier dynamics over long distances. If correct, this implies that "marine ice sheets"with beds deep below sea level may be vulnerable to rapid collapse if their deep beds extend to the coast and if buttressing ice shelves are removed

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The danger here is that the West Antarctic Ice Sheet lies on bedrock well below sea level. Rapidly flowing glaciers such as the Pine Island Glacier could lead to the intrusion of the sea into the interior of the ice sheet by floating away ice at the periphery. Indeed the grounding line where the floating Glacial tounge contacts bedrock for Pine Island Glacier has retreated by roughly 5 km. The Image below shows the bedrock elevation of Antarctica. Any areas below sea level (green- blue) have the potential to 'float away' You can see that most of the West Antarctic Ice Sheet is below sea level. (from the Bedmap project



A graphic in an article in Nature shows the outline Antarctic as it built up the current icecap. Note the areas below sea level.




Edited by blt (02/18/08 12:52 AM)

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