While ice sheets were extensive in the Northern Hemisphere during the Pleistocene glaciations, covering much of North America and Scandinavia, the two remaining continental ice sheets are in Greenland and in Antarctica. Greenland is essentially a single dome reaching above 3 km, while the Antarctic Ice Sheet (AIS) has a more complex form that rises above 4 km and is bordered by two major ice shelves and numerous smaller ones. These ice sheets have existed for millions (tens of millions in the case of Antarctica) of years. Arbitrarily, an ice sheet is defined as glacier ice extending over 50,000 km2 in area.