In 1990, application of various temperature scenarios suggested that ice retreat around the Antarctic Peninsula and the east Antarctic coast would be pronounced while the Ross and Ronne- Filchner Ice Shelves might experience only a small retreat [34]. In the same year, NASA scientists speculated that fast-flowing ice streams on the Antarctic Ross Ice shelf may indicate ice loss from the West Antarctic Ice Sheet. A report of a January 1990 workshop on the problem concluded that "the presence of five active ice streams feeding ice from the interior of West Antarctica into the Ronne Ice Shelf and separate peripheral ice shelves on the northern boundary of the ice sheet, may be manifestations of collapse already underway." [35].
A University of Chicago study of the behaviour of the West Antarctic Ice Sheet (WAIS) in the past, suggests that sporadic or even chaotic collapse of the ice sheet has occurred several times during the past million years. Evidence from fossil beach and coral reef terraces suggests that sea level was up to six metres higher than present levels when this happened, most recently during the interglacial warm phase 120,000 years ago. The erratic behaviour of the ice sheet is due to the way the ice sheet slips over the rocks underneath it, in particular glacial deposits. The study concluded that the hypothesis that WAIS snow accumulation rates increase when the climate warms does not contradict the hypothesis that future climate warming could cause the WAIS to collapse. Said Macayeal: "My simulations incorporate the feedback between accumulation rate and climate warming, yet they display ice-sheet collapse or shrinkage at times when model climate forcing is warm. What is most important in determining the possibility of WAIS collapse in the near future is the distribution of deformable subglacial till." [36]
Recent measurements at Pine Island Glacier increase fears of ice sheet collapse. Ice from glaciers feeds directly from the West Antarctic Ice Sheet into Pine Island Bay, a region where there is no substantial ice shelf. These glaciers are therefore likely to be the weakest points in the potentially unstable ice sheet [37]. Jenkins et al [38] find in a recent study that the base of Pine Island Glacier is melting at a rate in excess of ten metres per year - more than an order of magnitude higher than has been estimated for the larger Antarctic ice shelves. Their oceanographic measurements show relatively warm salty water near the sea floor and fresher outflows nearer the surface, which may be caused by meltwater from the glacier. They suggest that the West Antarctic Ice Sheet could be especially vulnerable if global warming increases deep ocean temperatures.
35. Bindschadler, R.A., "SeaRISE: A multidisciplinary research initiative to predict rapid changes in global sea level caused by collapse of marine ice sheets", Results of a workshop held in College Park, Maryland, January 23-25, 1990, NASA. [Return]
36. Macayeal, R., "Irregular Oscillations Of The West Antarctic Ice Sheet", Nature, v. 359, p. 29-32, 3 September 1992. [Return]
37. Hughes, T. J. (1981), "The weak underbelly of the West Antarctic Ice Sheet", Journal of Glaciology, Vol. 27, No. 97, 518-525. [Return]
38. Jenkins, A., Jacobs, S.S., Keys, J.R. (1995) "Is this little PIG in hot water?", {Antarctic Journal of the United States} (in press). [Return]