Past sea level highstands and the future evolution of the Antarctic ice sheet
On 31 March this year, a research paper was published in Nature in which geological reconstructions of sea level highstands are combined with projections of future sea level change. In this study, for the first time an ice sheet – climate model successfully reproduces sea level highstands during warm periods in the past. Temperatures were likely 1-3 degrees higher than today, and sea level 6-9 meter above present-day levels during the previous interglacial (115 -130 thousand years ago) and possibly up to 20 meter during the Mid-Pliocene warm period about 3 million years ago. In order to match these geological reconstructions, the authors introduced two new mechanisms in their Antarctic ice sheet model, which allow for a rapid decay of ice shelves and a destabilization of resulting ice cliffs. The rapid decay of ice shelves seems to be in line with observations over the last decades, while the ice cliff instability mechanism cannot be validated by modern observations from Antarctica.
Nevertheless, the model is applied to explore the contribution of the Antarctic ice sheet to sea level in response to warmer conditions in the near future. The results suggest a contribution of up to 1 m in 2100, and possibly more than 15 meters in 2500. These numbers are higher to much higher than previous estimates, and point to the fact that Antarctica may well become the dominant contributor to sea level rise on time scales of centuries. But the uncertainties are still very large, particularly because paleo-data provide limited constraints on the rate of sea level rise; in other words, the geological data at hand can indicate how high sea level has been, but provide little constraints on how fast sea level has risen. This is a very important aspect if one is interested in sea level rise at time scales of centuries or less. While there is a tendency of upward adjustment of the on-going mass loss from the Antarctic ice sheet due to slightly higher rates estimated from satellite observations, these short time series cannot be used to confirm the upper range of the modeled values in this paper. The lower values of a meter in 2100 cannot be ruled out, given all the uncertainties in models and present-day observations.
In summary, contemporary observations provide evidence for a rapid decay of small floating ice shelves, but not for a rapid decay of the major (grounded) Antarctic ice sheet itself. The new paper sets the scene for further studies on the contribution of the Antarctic ice sheet to sea level change by putting forward possible mechanisms that can explain high sea level stands in the past, which may also imply rapid sea-level rise in the future. The least that can be concluded is that the potential contribution from the Antarctic ice sheet to sea level may be dramatic and deserves the highest priority from the glaciological research community.