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Antarctic Ice Sheet

West Antarctic Ice Sheet

The West Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West (or Lesser) Antarctica, the portion of Antarctica west of the Transantarctic Mountains. The WAIS is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.

Description

It is estimated that the volume of the Antarctic ice sheet is about 25.4 million km3, and the WAIS contains just under 10% of this, or 2.2 million km3. The weight of the ice has caused the underlying rock to sink by between 0.5 and 1 kilometres in a process known as isostatic depression.

Under the forces from their own weight, the ice sheets deform and flow. The interior ice flows slowly over rough bedrock. In some circumstances, ice can flow faster in ice streams, separated by slow-flowing ice ridges. The inter-stream ridges are frozen to the bed while the bed beneath the ice streams consists of water-saturated clay. The clay was deposited before the ice sheet occupied the region, when much of West Antarctica was a marine seaway. The rapid ice-stream flow is a non-linear process still not fully understood; streams can start and stop for unclear reasons.

When ice reaches the coast, it will continue to flow outward onto the water. The result is a large, floating shelf of ice affixed to the continent.

Potential collapse of the WAIS

Large parts of the WAIS sit on a reverse-sloping bed below sea level. The reverse slope, and the low isostatic head, means that the ice sheet is theoretically unstable: a small retreat could in theory destabilize the entire WAIS leading to rapid disintegration. Current theoretical models do not include the physics necessary to understand this process, and observations do not provide guidance, so predictions as to its rate of retreat remain uncertain. This has been known for decades.

In January 2006, in a UK government-commissioned report, the head of the British Antarctic Survey, Chris Rapley, warned that this huge west Antarctic ice sheet may be starting to disintegrate, an event that could raise sea levels by at least 5 metres (16 ft). Estimates by others have ranged from 6 to 15 m (20–50 ft). Rapley said a previous Intergovernmental Panel on Climate Change report playing down worries about the ice sheet's stability should be revised. "The last IPCC report characterized Antarctica as a slumbering giant in terms of climate change," he wrote. "I would say it is now an awakened giant. There is real concern." . Note that the IPCC report did not use the words "slumbering giant".

Rapley said, "Parts of the Antarctic ice sheet that rest on bedrock below sea level have begun to discharge ice fast enough to make a significant contribution to sea level rise. Understanding the reason for this change is urgent in order to be able to predict how much ice may ultimately be discharged and over what timescale. Current computer models do not include the effect of liquid water on ice sheet sliding and flow, and so provide only conservative estimates of future behaviour."

James Hansen, a senior NASA scientist who is a leading climate adviser to the US government, said the results were deeply worrying. "Once a sheet starts to disintegrate, it can reach a tipping point beyond which break-up is explosively rapid," he said.

Indications that climate change may be affecting the west Antarctic ice sheet comes from three glaciers, including Pine Island and Thwaites. Data reveal they are losing more ice - mainly through the calving of icebergs - than is being replaced by snowfall. According to a preliminary analysis, the difference between the mass lost and mass replaced is about 60%. The melting of these three glaciers alone is contributing an estimated 0.24 millimetres per year to the rise in the worldwide sea level. There is growing evidence that this trend is accelerating: there has been a 75% increase in Antarctic ice mass loss in the ten years 1996-2006, with glacier acceleration a primary cause.

Polar ice experts from the U.S. and U.K. met at the University of Texas at Austin in March, 2007 for the West Antarctic Links to Sea-Level Estimation (WALSE) Workshop The experts developed a new hypothesis to explain the observed increased melting of the West Antarctic Ice Sheet. They proposed that changes in air circulation patterns brought on by a warming atmosphere has led to increased upwelling of warm water along the coast of Antarctica and that that warm water has increased the melting of the floating edge of the ice sheet. Recently published data collected from satellites support this hypothesis, suggesting that the west Antarctic ice sheet is beginning to show signs of instability.

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