Snow and ice phenomena, commonly referred to under the generic heading of the cryosphere, include snow cover and avalanches, sea and freshwater ice, glaciers, ice sheets, and ground ice. Seasonally, snow and ice are prominent features of the terrestrial environment. Together they occupy 15% of the earth's surface in January, at a time when Antarctic sea ice is near its minimum extent. In July, when northern hemisphere snow cover is largely absent, the total area is just under 9%. In contrast, permanent ice is relatively insignificant in extent. Table 1 shows that the Antarctic and Greenland ice sheets plus smaller ice caps and glaciers account for just under 10% of the land surface. In volumetric terms, however, these ice masses contain the equivalent of a 77-m rise in world sea level. During the Last Glacial Maximum, about 18,000 years ago, when ice covered nearly all of Canada and much of northwestern Europe, world sea level was lowered by at least 120 m, and perhaps as much as 180 m, because of the increased volume of water locked up in the augmented ice masses.