West Antarctic Ice Sheet

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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.

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[edit] Description

A satellite composite image of Antarctica
A satellite composite image of Antarctica

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.[1] This enormous weight has caused the underlying rock to sink by between 0.5 and 1 kilometres[2] in a process known as isostatic depression.

Under the massive forces of their own weight, the ice sheets deform and drag themselves outward. The interior ice flows slowly over rough bedrock. Away from the interior, the ice is channelled into ice streams, transporting ice from the center of the continent to the sea. These streams are 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 due to the water-saturated clay.

When ice streams finally reach the coast and push out across the ocean, they pass over rocky terrain, anchoring themselves to the irregular rocks to form a hinge. The ice will continue to grow outward onto the water. The result is a large, floating shelf of ice affixed to the continent. [3]

[edit] Potential collapse of the WAIS

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." [4]

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." [5]

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. [6]

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[4]. 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[7].

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.[8] Recently published data collected from satellites support this hypothesis, suggesting that the west Antarctic ice sheet is beginning to show signs of instability.[9]

[edit] Impact of Greenland ice sheet melting

While models predict significant melting of the Greenland ice sheet as summer temperatures in the Arctic rise by between 3 and 5 ºC (5.4–9 ºF), most models suggest that the ice sheets of Antarctica will remain more stable. However, historical data shows that the last time that Greenland became this warm, the sea level rise generated by meltwater destabilised the Antarctic ice. That means that the models of sea-level rise used to predict an increase of up to 1 m (3 ft) by 2100 may have significantly underestimated its ultimate extent, which could be as great as 6 m (20 ft). [10] [11]

This conclusion emerged from a study that used data from ancient coral reefs, ice cores and other natural records to reconstruct the climate during the last gap between Ice Ages, between 129,000 and 116,000 years ago. Scientists used computer models to show that meltwater from Greenland raised the sea level by up to 3.4 m (11 ft), but coral records showed that the total global rise was between 4 and 6 m (13–20 ft). The most likely explanation is the melting of Antarctic ice sheets: as sea levels rose, the floating ice shelves off the coast of the continent would have become more likely to break up. That in turn would have allowed glaciers to dump more ice from the continent itself into the sea. The base of the West Antarctic ice sheet lies below sea level, which allows ice to escape to the sea easily.[10] [11]

[edit] Satellite Record

In contrast to work done since the last period where cooling was topical, the 1970's, satellite imagery has not conclusively verified the cryosphere is warming. On the contrary, the slow accumulation of ice around the Amundsen-Scott South Pole Station appears indicative, at least for Antarctica, about what is generally occurring. The global satellite record since 1979 shows a constant area of ice until the mid 90's where a slow decline starts until a sharp drop in 2006 and 2007, but then a sharp correction above the average area in late 2007 and 2008. Further examination shows most of this variation is in the Arctic for global ice area and the Antarctic ice area has been trending up slowly since the satellite record starts.[12] This has been confirmed in recent observations of the Earth's rotation speeding up which can only be explained by a growing concentration of mass at the poles.[13]

Primarily, what the satellite record shows is Antarctica, given its size and being surrounded by oceans is relatively stable while the Arctic ice area varies wildly. Prior to 1910, anecdotal evidence from ships logs showed the minimum ice pack in the Arctic in 2007 was a more common occurrence. [14]

[edit] References

  1. ^ Lythe, Matthew B. & Vaughan, David G. (Jun 2001), “BEDMAP: A new ice thickness and subglacial topographic model of Antarctica”, Journal of Geophysical Research 106 (B6): 11335–11352, DOI 10.1029/2000JB900449 
  2. ^ Anderson, John B. (1999), Antarctic marine geology, Cambridge University Press, p. 59, ISBN 0521593174, <http://books.google.com/books?id=f9YqF73oe4IC> 
  3. ^ Ice Shelves, Antarctic and Southern Ocean Coalition,
  4. ^ a b Jenny Hogan, "Antarctic ice sheet is an 'awakened giant'", New Scientist, February 2, 2005
  5. ^ "West Antarctic ice sheet: Waking the sleeping giant?", Symposium, February 19, 2006
  6. ^ Jonathan Leake and Jonathan Milne, "Focus: The climate of fear", The Sunday Times - Britain, February 19, 2006
  7. ^ [Eric Rignot, Jonathan L. Bamber, Michiel R. van den Broeke, Curt Davis, Yonghong Li, Willem Jan van de Berg & Erik van Meijgaard. Recent Antarctic ice mass loss from radar interferometry and regional climate modelling, Nature Geoscience, Published online: 13 January 2008 doi:10.1038/ngeo102, http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo102.html]
  8. ^ Statement: Thinning of West Antarctic Ice Sheet Demands Improved Monitoring to Reduce Uncertainty over Potential Sea-Level Rise (March 28, 2007)
  9. ^ Kaufman, Mark (2008) "Escalating Ice Loss Found in Antarctica: Sheets Melting in an Area Once Thought to Be Unaffected by Global Warming" Washington Post (January 14) p. A01 online
  10. ^ a b "London 'under water by 2100' as Antarctica crumbles into the sea", The Times (UK), March 24, 2006
  11. ^ a b Otto-Bliesner, Bette L.; Marshall, Shawn J.; Overpeck, Jonathan T.; Miller, Giffford H. & Hu, Aixue (24 March 2006), “Simulating Arctic climate warmth and icefield retreat in the last interglaciation”, Science 311 (5768): 1751–1753, DOI 10.1126/science.1120808 
  12. ^ The Cryosphere Today,http://arctic.atmos.uiuc.edu/cryosphere/
  13. ^ Ameling, David J. The Ice Caps are Growing, April,2008: published April 14,2008 http://www.icecap.us
  14. ^ Overland, J.E., and Wood, K., 2003. Accounts from 19th-century Canadian Arctic Explorers’ Logs Reflect Present Climate Conditions, EOS Transactions of the American Geophysical Union, 84.

[edit] See also

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