Engaging an Earth System Feedback: The Antarctic Ice Sheet and Sea Level in a Warmer World

A range of evidence indicates that global sea level will be increasingly driven by ice sheet mass loss in the 21st century and beyond. Both the Greenland and Antarctic Ice sheets are loosing mass at an accelerating rate, and both could combine to contribute many meters of sea level rise if polar atmospheric and ocean warming in the 21st century is allowed to exceed the critical threshold somewhere at or below the peak warming of the last interglacial period (the Eemian), ca. 128,000 to 116,000 years ago - a period associated with global sea level rise 4 to 8m above present. In contrast to the greenhouse-gas-induced warming of the last 150 years, the orbitally-driven warming of the Eemian was much more season and latitude dependent. As a result, global annual average warming was small, and the amount of sea level rise above present due to thermal expansion of the global ocean was likely no more than 0.4 ± 0.3 m. A range of evidence also suggests that the contribution of the Greenland Ice Sheet was only 2.5 ± 0.5 m of sea level equivalent. This means that the bulk of Eemian sea level rise may have been due to a large scale wasting of the Antarctic Ice Sheet. Evidence suggests that the critical southern ocean warming threshold required to initiate such Antarctic Ice Sheet mass loss could be surpassed in the mid- to late-21st century. If this is allowed to happen, the Earth could experience sea level rise comparable to the Eemian on top of that due to warming and expansion of the global ocean - i.e., up to a total of 9 or more meters of sea level rise- perhaps at a rate exceeding 1m per century. If polar warming exceeds the Eemian threshold, the planet will enter increasingly uncharted territory, with a possibility of much larger sea level rise, particularly if the slow wasting of the ice sheets translates into a significantly positive earth system feedback that further amplifies global warming over coming centuries.