1) Changes in future ice sheets, climatic impacts and mass balance modification: answers from two coupled ice sheet-climate models

Three questions are investigated: first, which changes can we expect in the ice sheets of Greenland and Antarctica in response to present and future climate; second, what would be the impact of these changes in the climate system?; and third, what would be the impact of these changes on the local climate over the ice sheets and therefore on their mass balance?

Two fully-coupled ice sheet-climate models are used to address these issues. These two models contain AOGCMs for the representation of ocean and atmospheric components. One of these models is designed for the study of the long-term behaviour of the ice sheets; while the second model has improved physics. Ablation is calculated with an empirical formulation in the first case (degree-day method), and with an energy balance scheme in the second.

The impact of modified freshwater fluxes from the ice sheets on the meridional overturning circulation (MOC) is investigated, as well as the impacts of climate modification by a weakened MOC on the mass balance of the Greenland ice sheet (GrIS). The impact of a substantial reduction of the extent and volume of the GrIS on the local climate of Greenland and the far-field climate of the northern hemisphere is explored. Changes in atmospheric circulation and surface albedo are analyzed, as well as their relevance for northern hemisphere climate. The contribution of the height-feedback to the mass balance of the ice sheets is also investigated.

2) Glacial inception at ~2.75 Ma: what caused the onset?

The climate of the early and mid-Pliocene (5-3 Ma) was 2-3 K warmer than today and the northern hemisphere lacked ice sheets until glacial inception at ~2.75 Ma. Several hypotheses have been proposed to explain this transition, such as higher carbon dioxide levels, long-term variations in orbital forcing, the closure of the Panama seaway, and changes in North Pacific stratification. More recently, evidences of a permanent El Niño before ~2.75 Ma have suggested a tropical cause for the lack of ice sheets. This study presents the mechanism by which a permanent El Niño could prevent the establishment of ice sheets prior to ~2.75 Ma and a comparison with the "glaciation impetus" of other hypotheses.

Host: William Lipscomb, T-03