While ice sheets were extensive in the Northern Hemisphere during the Pleistocene ­glaciations, covering much of North America and Scandinavia, the two remaining continental ice sheets are in Greenland and in Antarctica. Greenland is essentially a single dome reaching above 3 km, while the Antarctic Ice Sheet has a more complex form that rises above 4 km and is bordered by two major ice shelves and numerous smaller ones. These ice sheets have existed for millions (tens of millions in the case of Antarctica) of years. Arbitrarily, an ice sheet is defined as glacier ice extending over 50,000 km2 in area.

Ice sheet mass balance is determined by the surface mass balance and ice discharge. The surface mass balance is the net outcome of snow accumulation and summer ablation below the equilibrium line altitude. Ice discharge is primarily via a small number of ice streams and major outlet glaciers that calve into the surrounding oceans, directly in Greenland or mainly from ice shelves in Antarctica.