The prescience of paleoclimatology and the future of the Antarctic ice sheet

The emerging view that the West Antarctic ice sheet is in the early stage of collapse owes as much to paleoclimatology as to contemporary observations.

several meters higher than present during the last interglacial period (~132-116 thousand years ago), and it was soon recognized that the Antarctic ice sheet must have been considerably smaller 12 . Sediment cores drilled beneath the present-day ice sheet have since shown that the West Antarctic ice sheet collapsed at least once in the last million years 13 . Sea level during the last interglacial rose rapidly to between 5 and 9 m higher than present, a finding that is difficult to explain without a major contribution from Antarctica 14 . Such evidence remains the strongest indication of ice sheet sensitivity to climate change.
Modern numerical ice-sheet modeling is also informed by paleoclimate evidence. Simulations of the future behavior of the ice sheet rely on estimates of past ice sheet configuration and past climate forcing (i.e., temperature and snow accumulation rate) for initialization and validation. Difficulties in matching the output of model simulations to geological constraints on paleo-ice sheet configuration and sea level have demanded the implementation of new model physics: a more-efficient mechanism for ice-shelf calving than had been conventionally assumed 15 . Simulations with this model physics cause West Antarctic ice sheet collapse during the last interglacial, provided that strong warming of the Southern Ocean is assumed as a boundary condition 16 . Evidence for such warming can indeed be found in the paleoclimate record 17 . The same model physics, under projected anthropogenic climate forcing, leads to rapid deglaciation of West Antarctica 16 . Thus, evidence of the past response of the ice sheet to paleoclimate informs our understanding of its future.
Paleoclimate data also provide context for the ice sheet changes being observed today (Fig. 1). Increased delivery of warm ocean water to the margin of the ice sheet, implicated in the current phase of retreat, is the consequence of changing ocean circulation, driven by changes in the atmosphere. Investigation of this atmosphere-ocean forcing relies heavily on climate reanalysis data, which are reliable in the Antarctic region only since 1979; there is only one weather station in West Antarctica (Byrd Station) that provides continuous meteorological observations further back in time, and that begins only in the 1957 International Geophysical Year. Ice cores, on the other hand, provide information stretching back thousands of years. Annually-resolved ice cores show that the major El Niño event of 1939-1942 had a significant impact on West Antarctic climate 19 leading to the suggestion that this event was an important forcing of glacier retreat in the Amundsen Sea 20 . Sediment cores 21 collected from beneath the Pine Island Glacier ice shelf have since shown that flooding of the continental shelf by warm Circumpolar Deep Water began around the 1940s. Data like these will be critical for determining whenand ifclimate changes in Antarctica reach the point where, as Mercer put it, "a dangerous trend is under way." To date, a definitive answer is available only for the Antarctic Peninsula, but the result from an ice core there is dramatic: summer melt now exceeds anything in the past millennium 22 .
Paleoclimate data, of course, have limitations compared with instrumental climate observations and modern geophysical imaging of ice sheet structure. Yet, the spatial coverage of ice cores is far greater than that of weather stations, and there is potential to use such records even more effectively, through the blending of proxy data with information from climate models to produce longer-term climate reanalysis products 23 . For such techniques to yield reliable information about the conditions most relevant to the ice dynamics, additional records will be needed from regions closer to the center of action. Cores from the numerous ice domes along the Amundsen and Bellingshausen Sea coasts of West Antarctica would be particularly valuable in this respect, though the logistical difficulties of working in this remote region have thus far prevented initiatives to obtain such cores from moving forward.
Somewhat remarkably, it still remains to be resolved whether the West Antarctic ice sheet collapsed during the last interglacial period. This uncertainty presents an important challenge that can only be answered definitively with paleoclimate and paleoglaciological data. One approach is to obtain spatial reconstructions of climate using deep ice core records. A significantly smaller ice sheet would be associated with changes in weather patterns which should be detectable in ice cores from strategically-located sites such as Hercules Dome, inland of the Transantarctic Mountains in East Antarctica, adjacent to the West Antarctic ice sheet. Climate model results suggest that the climate at Hercules Dome would be especially sensitive to atmospheric circulation changes associated with icesheet collapse 24 . Another promising idea is to use borehole drilling to obtain samples of bedrock from beneath the current ice sheet, and then use cosmogenic nuclide measurements to determine bedrock exposure history, yielding information about times when the ice sheet was smaller 25 .
Some researchers have concluded that the West Antarctic ice sheet is already in the early stages of an irreversible collapse 26,27 . Certainly, the current retreat of the margins of the Antarctic ice sheet is remarkable, and expert opinion on whether such trends are worrisome has probably moved somewhat from "uncertain" towards "likely" in the last few years. Observations from paleoclimate have played a powerful role in this emerging view, by providing critical boundary conditions for ice sheet models and elucidating the conditions under which ice sheet collapse may have occurred in the past. Paleoclimate data also provide an important role in balancing the interpretation of modern observations against the longer-term context in which ongoing ice sheet changes are occurring. Such complementary information will continue to be necessary as the ice sheet continues to change in a warming world.