The cryosphere is the whole of ice (solid H2O) on the earth. It consists of ice sheets and glaciers, sea ice (frozen sea water), snow pack and permafrost (Table 4.1-2). Among these, snow pack and sea ice have the largest areas, but their mean residence time is several months and only small parts of them last beyond a summer. Also, their thickness is merely around 1 meter. On the other hand, glaciers consist of ice (solid ice where crystals contact each other) made from perennial snow pack, and flow by their own weight at a speed much slower than that of liquid water. Ice sheets are continental scale glaciers, which presently covers most of Antarctica and most of Greenland with average depth of 2 km. For the Antarctic ice sheet, annual accumulation is approximately 100 mm liquid water equivalent, and therefore the average residence time of ice is twenty thousand years. There is still large uncertainty in the estimation of ice mass in the permafrost.