GLACIER

any large mass of perennial ice that originates on land by the recrystallization of snow or other forms of solid precipitation and that shows evidence of past or present flow. A glacier occupying an extensive tract of relatively level land and exhibiting flow from the centre outward is commonly called an ice sheet. Exact limits for the terms large, perennial, and flow cannot be set. Except in size, a small snow patch that persists for more than one season is hydrologically indistinguishable from a true glacier. One international group has recommended that all persisting snow and ice masses larger than 0.1 square kilometre (about 0.04 square mile) be counted as glaciers. any large mass of perennial ice that forms on land through the recrystallization of snow and that moves forward under its own weight. The term ice sheet is commonly applied to a glacier occupying an extensive tract of relatively level land and exhibiting flow from the centre outward. Glaciers occur where snowfall in winter exceeds melting in summer, conditions that presently prevail only in high mountain areas and polar regions. Glaciers occupy only about 11 percent of the Earth's land surface but hold roughly three-fourths of its fresh water. Approximately 99 percent of this glacier ice is concentrated in Antarctica and Greenland, while the remainder is widely scattered on all the continents, except Australia, and on many islands in high latitudes. It is thought that as many as 70,000 to 200,000 glaciers exist throughout the world. Variations in the amount of glacier ice are highly critical to human populations; if all existing glacier ice were to melt, the resulting rise in sea level of about 60 m (197 feet) would submerge virtually every major coastal city in the world. Additional reading W.S.B. Paterson, The Physics of Glaciers, 2nd ed. (1981), is the standard text on glaciers and ice sheets, emphasizing process rather than description. J.T. Andrews, Glacial Systems (1975), is a compact, clear introduction to glaciers and their environment. Michael Hambrey and Jrg Alean, Glaciers (1992); and Robert P. Sharp, Living Ice (1988), are well-illustrated introductions to glaciers and their effect on the landscape. National Research Council (U.S.), Ad Hoc Committee on the Relationship between Land Ice and Sea Level, Glaciers, Ice Sheets, and Sea Level: Effect of a CO2-Induced Climatic Change (1985); and J.T. Houghton, G.J. Jenkins, and J.J. Ephraums (eds.), Climate Change: The IPCC Scientific Assessment (1990), discuss the interaction between climate, glaciers, and sea level. D.J. Drewry (ed.), Antarctica: Glaciological and Geophysical Folio (1983), is a large-format compendium on the Earth's largest ice mass. Fast Glacier Flow: Ice Streams, Surging, and Tidewater Glaciers, Journal of Geophysical Research, part B, Solid Earth and Planets, 92(9):88358841 (1987), is a collection of review papers and scientific contributions from the Chapman Conference on Fast Glacier Flow. Mark F. Meier