3.3b. The West Antarctic Ice Sheet

In its 2001 report, the IPCC described the West Antarctic Ice Sheet (WAIS) (Figure 8) as a “slumbering giant”. It is now described by Chris Rapley, Director of the British Antarctic Survey, as “an awakened giant”. The entire Antarctic Ice Sheet (west and east) has a volume of 25 million km³, or nearly 9 times bigger than the Greenland Ice Sheet. About 10 percent of this volume makes up the West Antarctic Ice Sheet, much of which rests on bedrock below sea-level. Because of this, melting of some of the WAIS would not contribute to sea-level rise – as ice occupies a greater volume than liquid water. Nevertheless, complete collapse and melting of the WAIS would result in a rise in average global sea-level of about 5 m.

Figure 8: Composite satellite image of Antarctica. The West Antarctic Ice Sheet occupies the left half of the image, west of the obvious Trans-Antarctic Mountains. Courtesy: NASA.

As long ago as 1978, a US geologist speculated that human-induced global warming could cause the melting and disintegration of the WAIS. He foresaw this taking the form of a progressive southward wave of ice-shelf break-up along the coasts of the Antarctic Peninsula, followed by glacier acceleration and penetration of collapse into the interior of the ice sheet. The course of events over the last decade seems to be following at least the early stages of this trend. Over the last 50 years, 13,500 km², of the floating ice shelves that buttress the WAIS glaciers along the Antarctic Peninsula have been lost (Figure 9a and b) – an area larger than the Lebanon or Jamaica. This includes the Larsen-A shelf, which foundered in 1995, and the Luxembourg-sized (3,250 km²) Larsen-B ice shelf, which broke off and disintegrated over a period of just a month in 2002. Ice-shelf loss is now allowing the glaciers that were previously held back to speed up, with 5 out of 6 glaciers that fed the Larsen-A shelf, accelerating after its loss, some by up to four times their previous speed.

Figure 9: Original positions of the Larsen-A and Larsen-B Ice Shelves (a) and the break-up of the Larsen-B Ice Shelf in 2002 (b). The Larsen-A shelf disintegrated in 1995, while the Larsen-C shelf is currently intact to the south of Larsen-B. Courtesy: NASA.
(a)


(b)

Loss of the huge Larsen-B ice shelf is of particular concern as it seems that the event is unprecedented since the end of the last Ice Age. If the pattern of break-up continues as expected, the next shelf to break off should be the larger Larsen-C, immediately to the south. As the ice shelves are floating, they will not add to rising sea-levels. Their loss could, however, make it easier for the interior glaciers that are lodged on land to surge seawards, breaking up and melting and making a significant contribution to sea-level rise (Figure 10). As in Greenland, meltwater is lubricating the bases of glaciers and increasing their rates of seaward sliding. Around the Amundsen Sea, on the western side of the Antarctic Peninsula, the lubricated Pine Island and Thwaites glaciers are now discharging ice into the sea three times as fast as 10 years ago – a total volume of 110 km³ every year. If they melted in their entirety, these two glaciers alone could raise global sea-levels by over a metre. In total, Antarctica lost about 150 km³ a year between 2002 and 2005, most from the WAIS. At present, this contributes around half a millimetre a year to sea-level rise; a contribution that is likely to rise significantly along with global temperatures.

Figure 10: The Antarctic ice shelves float and their break up and melting will not contribute to sea-level rise. Their loss, however, could cause the interior glaciers that rest on land to surge seawards. Courtesy: USGS.




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