Letter

West Antarctic Ice Sheet Collapse?

Science  02 May 1997:
Vol. 276, Issue 5313, pp. 661-665
DOI: 10.1126/science.276.5313.661d

Charles R. Bentley (Perspectives, 21 Feb., p. 1077) suggests that a useful estimate of the probability of a West Antarctic ice-sheet collapse in the next 100 years can be obtained by assuming it to be a random event occurring once every 100,000 years. He bases this view on results of a single model experiment that imposed a strong 100,000-year forcing resulting in an asynchronous (but not random) response behavior of occasional total collapse and more frequent partial collapse (1). I strongly disagree with the view that the probability of collapse is either random or quantifiable at present. The ice sheet dominates an environment that itself responds to past and ongoing climate changes in a variety of time scales. Because climate changes are not random, neither can the ice sheet’s response to these changes be random.

Research has uncovered a historical record of the ice sheet that is indicative of instability. Microfossils from beneath the ice sheet indicate that at least once since it formed, the West Antarctic ice sheet disappeared and reformed (2). Analysis of sea-level records, of global isostatic adjustments, and of Antarctic geology defines a retreat of the ice sheet that began 11,000 years ago and contributed from 10 to more than 20 meters to sea level (3). Over half of this rise occurred in the last 7000 years, once the Pleistocene ice sheets had virtually disappeared (4).

Areas studied in detail have typically been found to be changing on shorter time scales (decades to 1000 years). Ice streams in the Ross Embayment are changing width and speed and are migrating farther inland; recent grounding of ice shelves has occurred; and in the northern sector of the ice sheet, one ice stream has accelerated while the floating terminus of another has possibly retreated rapidly (5). The overall picture that emerges is definitely not that of a stable ice sheet.

Strong nonlinearities and thermomechanical feedbacks that govern the ice flow have yet to be incorporated in even the most sophisticated numerical models of the ice sheet, making credible prediction of the West Antarctic’s future beyond current capabilities. Opinions vary widely about the probability of even a partial collapse, but there is general agreement that the probability is not yet quantifiable and is certainly not random. The solution to this situation is better knowledge of the ice-flow physics and of the West Antarctic environment than is currently available (6).

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Response: I agree with Bindschadler that the behavior of the West Antarctic Ice Sheet is physically based and is not inherently random. Furthermore, I emphatically agree with his last sentence. The point of my Perspective was not to imply that physics does not control ice-sheet behavior, or that all the glaciological problems have been solved, but simply to point out that I see no compelling reason, theoretical or observational, to suppose that a massive, order-of-magnitude increase in glacial outflow from West Antarctica, which would be required for there to be a major effect on sea level, is likely to occur any more often than once per 100,000-year glacial cycle, if at all. Nothing in the arguments for larger or smaller ice sheets at various times in the geologic past (1, 2) speaks against these points—indeed, Hall and Denton (2) specifically refer to the grounding line in the Ross Sea “retreating slowly to its present position” over the last 7000 years or so. The small-scale, short-term changes to which Bindschadler refers (3) are essentially the same ones I cited in arguing that minor variability has not led to the major response of the ice sheet that would be needed to affect sea level significantly (those changes are, in themselves, too small by at least two orders of magnitude to have any serious effect). The point often lost, I believe, and the one I wish to emphasize, is that a major catastrophe would be required in West Antarctica to have even a modest effect on world-wide sea level. There is no convincing evidence that such a catastrophe is impending, or that there is anything special about the present time relative to a hypothetical collapse cycle, the only existing model for which (4) is pseudorandom.

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