Ice sheets and glaciers form during long periods from annual snow accumulation exceeding ablation (see Paterson 1999). Ice sheets are large (area more than 50,000 km2) continental ice masses, and in the present Earth such are only the Antarctic and Greenland ice massifs. Smaller long-term accumulations of ice on land are counted as glaciers. The largest of them is the Southern Patagonia ice field in Argentina and Chile, covering an area of 13,000 km2 (Rignot et al. 2003), which is just 0.75 % of the area of Greenland ice sheet.