West Antarctic Ice Sheet
The Western Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West (or Lesser) Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains which lies in the Western Hemisphere. 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.
It is estimated that the volume of the Antarctic ice sheet is about 25.4 million km3 (6.1 million cu mi), and the WAIS contains just under 10% of this, or 2.2 million km3 (530,000 cu mi). The weight of the ice has caused the underlying rock to sink by between 0.5 and 1 kilometres (0.31–0.62 mi) in a process known as isostatic depression.
Under the force of its own weight, the ice sheet deforms and flows. 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 sediments. Many of these sediments were deposited before the ice sheet occupied the region, when much of West Antarctica was covered by the ocean. 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 either calves or continues to flow outward onto the water. The result is a large, floating ice shelf affixed to the continent.
Indications that the West Antarctic Ice Sheet is losing mass at an increasing rate come from the Amundsen Sea sector, and three glaciers in particular: the Pine Island, Thwaites and Smith Glaciers. Data reveals they are losing more ice 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 (0.0094 in) 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. As of November 2012 the total mass loss from the West Antarctica is estimated at 118 ± 9 Gt/y mainly from the Amundsen Sea coast.
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