The West Antarctic Ice Sheet (WAIS) is often described as the world's most unstable large ice sheet. As Hillenbrand et al. (2002) report, "it was speculated, from observed fast grounding-line retreat and thinning of a glacier in Pine Island Bay (Rignot, 1998; Shepherd et al., 2001), from the timing of late Pleistocene-Holocene deglaciation in the Ross Sea (Bindschadler, 1998; Conway et al., 1999), and from predicted activity of ice-stream drainage in response to presumed future global warming (Oppenheimer, 1998), that the WAIS may disappear in the future, causing the sea-level to rise at a rate of 1 to 10 mm/year (Bindschadler, 1998; Oppenheimer, 1998)." Hence, it is important to keep an eye on what is written about the subject in the scientific literature, which is what we do as ever more pertinent papers are published, the findings of a number of which we review in this Summary as they pertain to potential WAIS collapse and disintegration.
Cofaigh et al. (2001) analyzed five sediment cores from the continental rise west of the Antarctic Peninsula and six from the Weddell and Scotia Seas for their ice rafted debris (IRD) content, in an attempt to see if there are Antarctic analogues of the Heinrich layers of the North Atlantic Ocean, which testify of the repeated collapse of the eastern margin of the Laurentide Ice Sheet and the concomitant massive discharge of icebergs. If such IRD layers exist around Antarctica, the researchers reasoned, they would be evidence of "periodic, widespread catastrophic collapse of basins within the Antarctic Ice Sheet," which could obviously occur again. However, after carefully studying their data, they concluded that "the ice sheet over the Antarctic Peninsula did not undergo widespread catastrophic collapse along its western margin during the late Quaternary," and they say this evidence "argues against pervasive, rapid ice-sheet collapse around the Weddell embayment over the last few glacial cycles." And if there was no dramatic break-up of the WAIS over the last few glacial cycles, there's a very good chance there will be none before the current interglacial ends, especially since the data of Petit et al. (1999) indicate that the peak temperatures of each of the previous four intergalcials were warmer than the peak temperature of the current interglacial ... and by an average of more than 2°C.
Hillenbrand et al. (2002) studied the nature and history of glaciomarine deposits contained in sediment cores recovered from the West Antarctic continental margin in the Amundsen Sea to "test hypotheses of past disintegration of the WAIS." In doing so, they found that all proxies regarded as sensitive to a WAIS collapse changed markedly during the global climatic cycles of the past 1.8 million years, but that they "do not confirm a complete disintegration of the WAIS during the Pleistocene" at a place where "dramatic environmental changes linked to such an event should be documented." In fact, they say their results "suggest relative stability rather than instability of the WAIS during the Pleistocene climatic cycles," and they note that this conclusion is "consistent with only a minor reduction of the WAIS during the last interglacial period," citing the work of Huybrechts (1990), Cuffey and Marshall (2000) and Huybrechts (2002).
In another paper we have reviewed that addresses the subject of possible WAIS collapse, O'Neill and Oppenheimer (2002) say the ice sheet "may have disintegrated in the past during periods only modestly warmer (~2°C global mean) than today," and they thus claim that setting "a limit of 2°C above the 1990 global average temperature" -- above which the mean temperature of the globe should not be allowed to rise -- "is justified." If the truth be told, however, a 2°C warming of the globe would likely have little to no impact on the stability of the WAIS.
As we have already noted, for example, the average Antarctic peak temperature of all four of the world's prior interglacials was at least 2°C greater than the Antarctic peak temperature of the current interglacial; yet, in the words of the scientists who developed the pertinent temperature record (Petit et al., 1999), the evidence contained in the core "makes it unlikely that the West Antarctic ice sheet collapsed during the past 420,000 years," which is pretty much the same conclusion that was drawn by Cofaigh et al. In addition, we know from the Vostok ice core record that the peak Antarctic temperature of the most recent prior interglacial was fully 3°C warmer than the peak Antarctic temperature of the interglacial in which we presently live, yet the WAIS still did not disintegrate during that prior time of elevated warmth. Furthermore, we know that throughout the long central portion of the current interglacial (when the most recent peak Antarctic temperature was reached), it was much warmer than it was in 1990 (for references, see our Editorial of 26 June 2002), which is the year from which O'Neill and Oppenheimer's critical 2°C warming increment is measured; and this fact raises the 3°C temperature elevation of the last interglacial relative to the global temperature of 1990 to something on the order of 4 or 5°C, for which, again, there was no evidence of even a partial WAIS disintegration.
Seven years later, Naish et al. (2009) wrote that "an understanding of the behavior of the marine-based West Antarctic ice sheet during the 'warmer-than-present' early-Pliocene epoch (~5-3 Myr ago) is needed to better constrain the possible range of ice-sheet behavior in the context of future global warming," and they thus undertook a project to hopefully provide such understanding. As they describe it, they derived "a marine glacial record from the upper 600 meters of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL program," which demonstrated the "well-dated ~40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the earth's axial tilt (obliquity) during the Pliocene." More specifically, they state that their data "provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ~3°C warmer than today and atmospheric CO2 concentration was as high as ~400 ppm," the latter of which numbers is only about 3% greater than what it is today.
An important implication of this last observation is that the much greater periodic warmth of the early-Pliocene was clearly not the primary result of periodic changes in the air's CO2 concentration; and the 56 researchers tacitly acknowledge that fact by attributing the variable warmth to periodic changes in the planet's axial tilt that produced 40,000-year cycles of insolation. Setting aside that fact, the next most important inquiry into the matter would be how long it took for such warmth to bring about a total collapse of the WAIS, such as Al Gore and James Hansen suggest could occur "the day after tomorrow."
For the answer to this question, we look to the companion paper of Pollard and DeConto (2009), who state that projections of future WAIS behavior "have been hampered by limited understanding of past variations and their underlying mechanisms." With the findings of Naish et al. (2009), however, they gained important new knowledge that helped them frame a greatly-improved "ice sheet/ice shelf model capable of high-resolution nesting with a new treatment of grounding-line dynamics and ice-shelf buttressing to simulate Antarctic ice sheet variations over the past five million years." So what did they learn?
The two researchers report that they modeled WAIS variations ranging "from full glacial extents with grounding lines near the continental shelf break, intermediate states similar to modern, and brief but dramatic retreats, leaving only small, isolated ice caps on West Antarctic islands." And they say their work suggests that "the WAIS will begin [italics added] to collapse when nearby ocean temperatures warm by roughly 5°C." And how long would it take to complete the process?
In a News & Views story on Pollard and DeConto's findings, Huybrechts (2009) states that "the amount of nearby ocean warming required to generate enough sub-ice-shelf melting to initiate a significant retreat of the West Antarctic ice sheet ... may well take several centuries to develop." And once started, he says that the transition time for a total collapse of the West Antarctic ice sheet would range from "one thousand to several thousand years," which time period, in his words, "is nowhere near the century timescales for West Antarctic ice-sheet decay based on simple marine ice-sheet models," such as have been employed in the past.
Once again, therefore, the specter of 21st-century sea level rise being measured in meters -- as hyped by Al Gore and James Hansen -- can be seen to be receding ever further into the distance of unreality. What is more, and in spite of the current interglacial's current relative coolness, the Vostok ice core data indicate that the current interglacial has been by far the longest stable warm period of the entire 420,000-year record, which suggests we are probably long overdue for the next ice age to begin, and that we may not have the "5 to 50 centuries" that O'Neill and Oppenheimer suggest could be needed to bring about the WAIS disintegration subsequent to the attainment of whatever temperature in excess of 4 or 5°C above the current global mean would be needed to initiate the process.
In conclusion, therefore, it would appear that the climate-alarmist vision of impending WAIS collapse and disintegration is nothing more than an ill-founded hallucination.
Bindschadler, R. 1998. Future of the West Antarctic Ice Sheet. Science 282: 428-429.
Cofaigh, C.O., Dowdeswell, J.A. and Pudsey, C.J. 2001. Late Quaternary iceberg rafting along the Antarctic Peninsula continental rise in the Weddell and Scotia Seas. Quaternary Research 56: 308-321.
Conway, H., Hall, B.L., Denton, G.H., Gades, A.M. and Waddington, E.D. 1999. Past and future grounding-line retreat of the West Antarctic Ice Sheet. Science 286: 280-283.
Cuffey, K.M. and Marshall, S.J. 2000. Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet. Nature 404: 591-594.
Hillenbrand, C-D., Futterer, D.K., Grobe, H. and Frederichs, T. 2002. No evidence for a Pleistocene collapse of the West Antarctic Ice Sheet from continental margin sediments recovered in the Amundsen Sea. Geo-Marine Letters 22: 51-59.
Huybrechts, P. 1990. The Antarctic Ice Sheet during the last glacial-interglacial cycle: a three-dimensional experiment. Annals of Glaciology 14: 115-119.
Huybrechts, P. 2002. Sea-level changes at the LGM from ice-dynamic reconstructions of the Greenland and Antarctic ice sheets during the glacial cycles. Quaternary Science Reviews 21: 203-231.
Huybrechts, P. 2009. West-side story of Antarctic ice. Nature 458: 295-296.
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Pollard, D. and DeConto, R.M. 2009. Modelling West Antarctic ice sheet growth and collapse through the past five million years. Nature 458: 329-332.
Rignot, E.J. 1998. Fast recession of a West Antarctic glacier. Science 281: 549-551.
Shepherd, A., Wingham, D.J., Mansley, J.A.D. and Corr, H.F.J. 2001. Inland thinning of Pine Island Glacier, West Antarctica. Science 291: 862-864.Last updated 23 June 2010