An ice sheet is a large permanent layer of ice covering a continental shelf, defined as having an extent greater than 50,000 km2 (19,305 mi2). An ice sheet is larger than a glacier or ice shelf. There are two ice sheets in the world today: the Antarctic ice sheet (which contains 61% of fresh water on the planet) and the Greenland ice sheet (containing 7%). Only about 32% of the world's fresh water is found in streams, lakes, and aquifers — the rest can be found in the ice sheets.
Ice sheets form when snow falls on ground with a subzero temperature and doesn't melt, even seasonally. Over thousands of years, the snow builds up and compacts into ice, forming sheets averaging 1 mi (1.6 km) thick, or up to 2 mi (3.2 km) at maximum. In some areas of the West Antarctic ice sheet, the base is as far as 1.5 mi (2.4 km) below sea level, comparable to the depth of some areas of ocean. If the entire Antarctic ice sheet or the Greenland ice sheet were to melt, the world's seas would rise by about 60 meters or 7 meters, respectively. The possibility of the ice sheets melting due to global warming has been a cause of concern worldwide in recent decades.
Land covered by ice sheets takes on a characteristic jagged appearance due to extensive glacial weathering. Terrain of this type can be seen in Patagonia (far southern South America), Norway, northern Canada, and Siberia. All of these areas were covered by ice sheets during the last Ice Age, more than 10,000 years ago. Only when the ice sheets receded could mankind colonize these areas.
Ice sheets are an environment largely sterile of life. Despite being made of nothing but frozen water, ice sheets tend to be very dry, providing little moisture for life to thrive on. They cover the earth, preventing the buildup of nutrient-rich soil. The only life forms that really make their home on the ice sheets are microbes that live on rocks peeking out from the ice, blown there by the wind. The tips of hills or mountains sticking out of ice sheets are called nunataks.
The Earth did not always have ice sheets around the poles. In fact, such a circumstance is relatively atypical. For most of the Earth's history, the climate was warm enough that ice sheets did not form around the poles, and forests stretched from pole to pole. Dinosaur fossils have even been found less than 10 degrees latitude away from the South Pole.