RAPID SEA-LEVEL RISE FROM A WEST ANTARCTIC ICE-SHEET COLLAPSE - A SHORT-TERM PERSPECTIVE

Will worldwide sea level soon rise rapidly because of a shrinkage of t he West Antarctic Ice sheet (WAIS)? Here I give a personal perspective of that probability. The crucial question is not whether large change s in ice mass can occur, but how likely it is that a large, rapid chan ge, say a several-fold increase in the 20th-century rate of about 2 mm a(-1) will occur in the next century or two from a West Antarctic cau se. Twenty years ago Weertman proposed that a marine ice sheet is inhe rently unstable. But Weertman's analysis was based on a simple model o f a marine ice sheet that did not include fast-flowing, wet-based ice streams, which are now known to dominate the grounded ice sheet. Moder n analyses do not definitively determine just how ice streams affect t he stability of the WAIS, but it can at least be said that there is no compelling theoretical reason to expect a rapid rise in sea level fro m the WAIS triggered by ice-shelf thinning. Of the three main ice-drai nage systems in the WAIS, the one that flows into Pine Island Bay migh t be a particularly likely site for accelerated now since there is no ice shelf to restrain the inflowing ice streams, yet measurements show that this system is not significantly out of mass balance. If the ''R oss Embayment'' system, which has undergone several sudden glacial reo rganizations in the last thousand years, were unstable one might expec t a history of large changes in the total outflow of ice into the Ross Ice Shelf, yet the total outflow in the ''Ross Embayment'' has remain ed relatively unchanged despite the large internal perturbations, a fa ct that points to a stable, not an unstable, system. Study of the thir d major drainage from the WAIS, into the Ronne Ice Shelf, also suggest s that there is no gross discordance between the present velocity. vec tors and flow tracers in the ice shelf, although the evidence is limit ed. In the light of the evidence for recent stability, it is difficult to see how climate warming (whether anthropogenic or natural) could t rigger a collapse of the WAIS in the next century or two. Thus, I beli eve that a rapid rise in sea level in the next century or two from a W est Antarctic cause could only occur if a natural (not induced) collap se of the WAIS were imminent. Based on a concept of pseudo-random coll apse once per major glacial cycle, I estimate the chances of that to b e on the order of one in a thousand.