Antarctica is fringed by floating shelves of ice, fed by the continental glaciers, continuously spreading outward under their own weight as they are melted by the ocean from below. As ice shelves grow, their thin edges become less able to withstand the stresses of ocean circulation and tides. A crack then forms, propagates across the shelf, and detaches an iceberg, returning the ice shelf to stability. This process of iceberg calving, along with basal melt, is the mechanism by which snowfall over the Antarctic interior is balanced by ice loss to the ocean. The periodic calving of large icebergs, therefore, represents a normal pruning of the ice margin; a process that would occur under any climate scenario.
Besides just passively draining ice to the ocean, however, ice shelves also play a critical role in regulating the flow of the glaciers that feed them. As ice shelves spread outward, they may contact islands and other obstructions that resist their flow. This resistance is transferred back to the glaciers, slowing their flow and causing them to thicken. Thus, ice shelves can also be viewed as dams, regulating the outflow of the Earth’s largest freshwater reservoir. Removal of these dams and the buttressing they provide would cause the glaciers behind them to flow faster, analogous to the opening of a spillway. Additionally, since the Antarctica’s bedrock is isostatically depressed and, therefore, increases in depth toward the interior, this process can initiate runaway glacier retreat through a feedback loop with outflow, a process termed the marine ice sheet instability. The implication is that accelerated ice shelf calving, if occurring where significant buttressing stress is generated, may trigger a sustained period of rapid deglaciation. Evidence of such events exist in the glacial sedimentological record of the Antarctic continental shelf.
With this understanding of the functions of ice shelves we assess their current state, near-future prognosis and implications for ice sheet stability. Ice shelves are now observed with increasing detail from space, with an effective record spanning approximately two decades. These observations reveal processes both expected, such as the recent calving of the large A68 iceberg from the Larsen C ice shelf, and unexpected, including the meltwater-induced 2002 collapse of the Larsen B and the interior “inside-out” rifting of the Pine Island Glacier. Observations of ice shelf dynamics are coupled with improved mapping of ice sheet bedrock topography, allowing us to pinpoint the glaciers with the greatest potential for instability and construct numerical ice flow models to predict their behavior. Most elusive, however, is the deep ocean, for which remote sensing provides much less data and for which predictions of future change remains the most uncertain. On timescales relevant to society, it is likely the ocean and its interaction with ice shelves that will determine the pace of Antarctica’s continued deglaciation.
