An ice sheet, also known as a continental glacier, is a continuous mass of glacial ice that is greater than 50,000 square kilometers in area and can have a thickness of as much as 3.2 kilometers. Ice sheets form from the accumulation and compacting of snow that has fallen over thousands of years without melting, even seasonally. Ice caps differ from ice sheets in that they cover less than 50,000 square kilometers, lie on top of mountains, and usually feed a series of glaciers around their edges. Polar ice caps are high-latitude regions covered in ice and are not strictly ice caps but rather ice sheets because they exceed 50,000 square kilometers in area.
The only currently existing ice sheets are in Antarctica and Greenland, although during the last glacial period ice sheets also covered much of North America, northern Europe and southern South America. The Antarctic ice sheet is the largest by far, with an area of nearly 14 million square kilometers and containing about 90 percent of the earth's total ice mass. These two ice sheets contain about 99 percent of the earth's fresh water.
The Greenland ice sheet, which currently covers about 82 percent of Greenland's land area, has been melting at an accelerating pace as a result of global warming and has experienced an annual net loss every year since 1998. If it were to completely melt, the oceans would rise by more than six meters, flooding numerous coastal cities and other low-lying areas. If the entire Antarctic ice sheet melted, the oceans would rise by about 60 meters. In addition to other catastrophic effects, the resulting huge increase in fresh water into the oceans would also reduce their salinity, which would decimate some corals and other marine organisms that could not adjust to it.