Ice

The vast majority of the natural ice on Earth is situated at the extreme latitudes; the Greenland ice sheet and sea ice at the North Pole, the Antarctic ice sheet in the South. Sea ice, massive ice sheets, valley and mountain glaciers all combine to form the polar ice caps. Enormous areas of the polar and subpolar regions are underlain by permafrost. Polar ice caps and glaciers contain a large proportion of Earth's freshwater resource. Over 75% of all freshwater, or 2.15% of all water on Earth, presently exists in the form of ice. This proportion was significantly greater during past glacial epochs.

These vast stores of ice are particularly sensitive indicators of climatic change. The rapid retreat of mountain glaciers has been cited as evidence of global warming. If all ice at the poles and in glaciers melted, sea level would rise approximately 260 ft (80 m).

Ice is known to occur extensively on a variety of bodies in space. The origin of water on Earth has been postulated to be a result of collisions with comets and/or meteors containing a significant quantity of ice. The presence of ice has been confirmed at the poles of the Moon and the planet Mars. The existence of ice on Mars may be an indicator of the potential for the existence of life forms in the warmer and wetter past of that planet. The rings of Saturn and even nebulae outside our solar system are believed to contain ice. Europa, a moon of Jupiter, is thought to have a liquid-water ocean beneath a crust of ice. Scientists also assume that ice on such bodies might be utilized to supply the water needs of manned missions to these bodies, as well as being split into its component gases and used for fuel.