Rapid sea-level rise from a West Antarctic ice-sheet collapse: a short-term perspective

Abstract

Abstract Will worldwide sea level soon rise rapidly because of a shrinkage of the West Antarctic ice sheet (WAIS)? Here I give a personal perspective of that probability. The crucial question is not whether large changes in ice mass can occur, but how likely it is that a large, rapid change, 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 cause. Twenty years ago Weertman proposed that a marine ice sheet is inherently unstable. But Weertman’s analysis was based on a simple model of a marine ice sheet that did not include fast-flowing, wet-based ice streams, which are now known to dominate the grounded ice sheet. Modern analyses do not definitively determine just how ice streams affect the 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 from the WAIS triggered by ice-shelf thinning. Of the three main ice-drainage systems in the WAIS, the one that flows into Pine Island Bay might be a particularly likely site for accelerated flow 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 “Ross Embayment” system, which has undergone several sudden glacial reorganizations in the last thousand years, were unstable one might expect 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 remained relatively unchanged despite the large internal perturbations, a fact that, points to a stable, not an unstable, system. Study of the third major drainage from the WAIS, into the Ronne Ice Shelf, also suggests that there is no gross discordance between the present velocity vectors and flow tracers in the ice shelf, although the evidence is limited. In the light of the evidence for recent stability, it is difficult to see how climate warming (whether anthropogenic or natural) could trigger a collapse of the WAIS in the next century or two. Thus, I believe that a rapid rise in sea level in the next century or two from a West Antarctic cause could only occur if a natural (not induced) collapse of the WAIS were imminent. Based on a concept of pseudo-random collapse once per major glacial cycle, I estimate the chances of that to be on the order of one in a thousand.