The preceding sections suggested that Antarctica's potential to protect us from the hazard of sea-level rise is considerably less than its potential to make the problem much worse. Predicting the future of the great ice sheets is likely to become increasingly important as global climate change is finally accepted at a governmental level. The present state of change in the Antarctic and Greenland ice sheets was succinctly summarized by Rignot & Thomas (2002): 'As measurements become more precise and more widespread, it is becoming increasingly apparent that change on relatively short time-scales is commonplace: stoppage of huge glaciers, acceleration of others, appreciable thickening and far more rapid thinning of large sectors of ice sheet, rapid breakup of vast areas of ice shelf and acceleration of tributary glaciers, surface melt-induced acceleration of ice sheet flow [although probably only in Greenland], and vigorous bottom melting near grounding lines. These observations run counter to much of the accepted wisdom regarding ice sheets, which, lacking modern observational capabilities, was largely based on 'steady-state' assumptions.'

My opinion is similar, if less succinctly stated. There is now a growing rait of evidence that large changes are afoot in the major glaciers in the Amundsen Sea sector of West Antarctica. Despite this evidence and the long-lived speculation that this portion of the West Antarctic Ice Sheet could be particularly prone to collapse, however, it is too early to predict disaster. Indeed, the overall opinion of a panel of experts brought together to assess the risk of collapse of the West Antarctic Ice Sheet was that there remains a 5% probability that this sheet will make a 1 cmyr-1 contribution to sea-level rise, and a 30% probability of a 2 mm yr-1 contribution over the next 200yr (Vaughan & Spouge, 2002). As with all assessments of risk, though, the likelihood of a particular hazard occurring must be balanced by its potential to cause harm, and even a relatively small addition to predicted sea-level rise could have huge social and economic consequences—the lack of certainty over the likelihood of collapse cannot be taken as good reason to reduce the relatively modest efforts in trying to make reliable and defensible predictions of change in general. It is also the case that we cannot afford to focus all our effort on this one area, because observational evidence of change elsewhere in the Antarctic Ice Sheet is also building; we have observed local changes on the Antarctic Peninsula that are probably a direct and immediate response to the recent rapid regional atmospheric warming and we have strong evidence that ongoing Holocene icesheet thinning may be persistent across much of West Antarctica. Finally we should bear in mind that the stagnation of Ice Stream C demonstrates the potential for highly non-linear and rapid change in the ice streams which are so significant in controlling the rate of Antarctic ice loss.

Without ruling out surprises, there are five mechanisms of change that may become important in the Antarctic Ice Sheet over the predictable future.

1 In the warmest areas, on the Antarctic Peninsula and perhaps in some coastal areas of East and West Antarctica, contemporary climate warming may cause increased melting and promote immediate glacial retreat. This is likely to be particularly noticeable where ice shelves are driven to retreat.

2 Subtle changes in the temperature of ocean water in contact with the ice sheet around the grounding line may drive increased melt and grounding-line retreat. We must work to understand if such changes have the potential to interact with glacier acceleration to amplify retreat or even to lead to collapse of whole drainage basins.

3 There is a high likelihood that climate change will cause increased precipitation over much of the interior of the Antarctic Ice Sheet. This could cause thickening of the ice sheet and act to compensate to some extent for other sources of sea-level rise.

4 A continued retreat in response to long-term Holocene warming may continue to cause continued background thinning of the ice sheet, especially in West Antarctica.

5 Internal instability, or non-steady behaviour of ice streams and outlet glaciers, may cause rapid, hard to predict changes, especially in West Antarctica.

Each of these mechanisms could have a substantial impact on the ice sheet and hence on future sea levels.

The clear task facing researchers is to begin to understand each of these mechanisms, their likely longevity and predictability. Given the theoretical and proven potential of ice sheets to exhibit non-linear responses, if several of these effects operate together we may find the pattern a complex one to unravel, let alone predict with confidence. We will require much more data describing contemporary and past changes and an improvement in our understanding of the basic mechanics of glacial flow and restraint even to attempt the task.

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