The evolution of the Antarctic Ice Sheet from the Last Glacial Maximum to 2100

Abstract

The Antarctic Ice Sheet (AIS) is the largest ice sheet on earth and it is losing mass. In my thesis I study the ice sheet from the Last Glacial Maximum (LGM) to 2100 to assess the contribution of natural processes and of human-induced climate change to the changes in the ice sheet we observe at the present. The research was performed with the ice-dynamical model ANICE. The input of this model consists mainly of an initial ice sheet and an evolution of the climate (temperature and surface mass balance) and the sea level over the modeled period. The climate evolution is acquired by interpolating between the climate states at the LGM and at the present. The two climate states are the output of the regional climate model RACMO2/ANT, which is forced with data from a general circulation model (GCM) or re-analyis data. Re-analysis data are only available for the present-day, so for the climate at the LGM a GCM was used. A suitable GCM was found by comparing the temperature and precipitation output of eighteen different GCMs to observations over the Antarctic region. These observations consist of ice core reconstructions for the past and a reference state of RACMO2/ANT (forced with re-analysis data) for the present. Some GCMs simulate temperature and precipitation significantly better than others in the Antarctic region. The best suited GCM for modeling the Antarctic climate from the LGM to the PD is HadCM3, according to our research. Reconstructions of what the AIS looked like during the LGM are very uncertain. Therefore a spin-up was used where ANICE was run from the last interglacial (120 000 years ago) to provide ANICE with an initial ice sheet at the LGM. The initial ice sheet, climate state and sea level at the last interglacial were assumed to be the same as at the present. ANICE was ran from the last interglacial until the present with different parameter settings to find a model set-up that best fit the observations regarding the present-day ice sheet, LGM ice volume and grounding line retreat during the past 21 000 years. The evolution of the AIS from the LGM to 2100 was studied by running ANICE with different temperature and sea-level reconstructions from the LGM until the present and different temperature and sea-level scenarios for the future. This was done to estimate the range of possible histories and futures of the ice sheet. We found that the ice sheet lost 4.3 +/- 0.5 106 km3 of ice between the LGM and the present, which is equivalent to 10.7 +/- 1.3 m sea-level equivalent. For the evolution of the ice sheet between 2000 and 2100 the history of the ice sheet is very important, i.e., how rapidly it is still losing mass as a response to the long-term changes from the LGM until the present. By 2100, with respect to 2000, a sea-level contribution in the range of -22 to 63 mm was found, depending on the climate scenario and the history of the ice sheet.