Ice sheet margins and ice shelves

A possible consequence of climatic warming is an increase in ice sheet melting rates and, ultimately, a perceptible rise in sea level. This would occur very slowly unless thinning at the ice sheet margins triggered a major increase in ice drainage rates. The ice sheet in West Antarctica is the one most susceptible to this type of collapse because it rests on bedrock well below sea level. The most probable outlet for rapid ice discharge would be through the area covered today by the Ross Ice Shelf, which would have to be severely weakened first. Currently, the ice shelf appears to be either in equilibrium or actually growing thicker. Most of the wastage from the ice shelf is by occasional calving of large tabular icebergs. Weakening could be achieved most rapidly if summer temperatures were to rise to several degrees above the melting point for two or three months each year. Then, widespread surface meltwater would fill crevasses and force them to overdeepen. This would cause accelerated iceberg calving and explosive fragmentation of the resulting icebergs-conditions similar to those found in the Arctic today. Such accelerated iceberg calving could also threaten portions of the ice sheet in East Antarctica. Here, although bedrock is below sea level, the ice sheet appears to be protected by coastal bedrock sills which are too shallow for a tabular iceberg to pass over them, but would not prevent the discharge of small iceberg fragments. The most vulnerable portion of the ice sheet is probably that drained by Pine Island and Thwaites glaciers in West Antarctica. These glaciers are not protected by large ice shelves and, although there is no evidence for present day retreat, they may be the first glaciers to respond to climatic warming. However, even an order of magnitude increase in the activity of one of these glaciers would cause a sea level increase of less than 1 mm yr−l. The measurements necessary to monitor ice sheet extent and elevation, summer melt zones and sea ice cover can all be obtained from a satellite in near-polar orbit using well-proven techniques. The establishment of such a program should take highest priority in future research.