Science and Implementation Plan: Chapter 1

Marine Ice Sheet Instability and Sea Level

There is a class of ice sheets believed to be inherently unstable and prone to collapse. Termed "marine ice sheets," they are characterized by being grounded on beds well below sea level. They go afloat around their peripheries at the grounding line and feed large floating ice shelves which continue to thin as ice moves towards the margin. This type of ice sheet was far more prevalent during the Last Glacial Maximum (LGM), when many populated the higher latitudes of the Northern Hemisphere. Geologic evidence abounds that these Northern Hemisphere ice sheets disappeared catastrophically during the climatic transition to the current interglacial (warm period).

In the absence of ice-sheet collapse, rates of sea-level rise have been very modest (1.5 to 2 mm/a) for the past 4000 years. However, with increasing clarity, marine records of the period since the LGM are showing intervals accompanying ice-sheet collapse when rates of sea level rise were nearly two orders of magnitude higher than the present rate. While all Northern Hemisphere marine ice sheets have disappeared, the largest marine ice sheet to have ever existed, the West Antarctic ice sheet, remains. It contains enough grounded ice to raise global sea level 5 meters if it collapsed. Thus, the potential for a final episode of rapid sea-level rise exists.

The instability at issue arises from the situation where a submarine, subglacial bed becomes progressively deeper inland from the grounding line. If ice thins at the grounding line thins, the grounding line would have to retreat inland. Unless the ice at the new grounding line can be supported, for example by a "buttressing" effect of the floating ice shelf beyond the grounding line, it will thin more rapidly in the deeper water, causing even further grounding-line retreat. This process of retreat will continue at an ever-increasing rate, progressively dispersing more and more ice into the ocean. This process is referred to as "marine-ice-sheet collapse."

Ice streams-large river-like currents of ice flowing through the ice sheets at speeds one to two orders of magnitude faster than the general ice flow-greatly increase the potential swiftness of ice-sheet collapse by rapidly transporting ice from the interior of the ice sheet to the margin. Understanding the nature of ice-stream formation and evolution is central to understanding the nature of ice-sheet collapse. Any process that initiates a retreat of the grounding line could be a possible trigger of collapse. Possible triggers include rising sea level and thinning ice shelves caused by increased sub-shelf melting or enhanced calving. Possible safety latches are increased ice-shelf grounding caused by increased advection from ice streams, ice-shelf basal freezing caused by altered sub-shelf circulation, and changes in either sub-ice-stream lubrication or basal till dilatancy, increasing the resistance to ice flow.

Although the concept of marine-ice-sheet instability and collapse was originally theoretical, results of recent WAIS field studies support the assumptions and reasoning on which it is based. The deep, basin-shaped bedrock floor of the West Antarctic ice sheet has the geometry appropriate for instability. Marine diatoms recovered from beneath the ice indicate that it has disappeared at least once since it formed-some believe this episode occurred during the last interglacial period, the Eemian, about 125,000 years ago. More certain is the fact that during the last glacial maximum, this ice sheet was much larger, extending in places to the edge of the continental shelf. In the last 18,000 year, it has retreated to a position well back of that maximal stand, but its current rate of change is unknown. Whether the West Antarctic ice sheet is continuing to retreat, has reached some state of equilibrium, or is readvancing is not known.

Measurements of net mass balance in separate regions of the ice sheet indicate a complex pattern of growth and wastage, but only a portion of the total ice sheet has been measured. Direct indications of currently unstable behavior have been recently discovered in the form of rapid changes taking place along the Siple Coast (see Map). Recent measurements and numerical modeling of the ocean circulation on the continental shelf indicate relatively high melting rates near the grounding lines of the large ice shelves. Ice stream B is losing mass at a high rate, while the adjacent ice stream C is gaining mass. Ice streams D and E appear nearly in balance, but a large bulge has been observed moving rapidly down the smaller ice stream F. The current absence of substantial ice shelves that could buttress West Antarctic ice drainage along its northern coast could cause a forthcoming collapse of that part of the ice sheet. In this area, an acceleration of Thwaites Glacier has already been measured.

A closely related concept, the potential for unstable retreat of tidewater glaciers, has been verified by the onset of a dramatic retreat of Columbia Glacier, Alaska, which has continued for the last 15 years. A final, disturbing extension of West Antarctic collapse is that a substantial portion of the much-larger East Antarctic ice sheet would likely drain through the gap in the Transantarctic Mountains now occupied by the West Antarctic ice sheet. This would only increase the eventual magnitude of the change in sea level, further exacerbating the calamity.