Ice sheet
In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi). The two currently existing ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice sheets are the largest glacial bodies on Earth, distinguished from smaller ice caps or alpine glaciers. Ice sheets can have multiple ice domes, the topographic highs from which ice flows outwards, and are typically drained by ice streams and outlet glaciers around their periphery.
Although the surface is cold, the base of an ice sheet is generally warmer due to geothermal heat. In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels within the ice sheet called ice streams.
Even stable ice sheets are continually in motion as the ice gradually flows outward from the central plateau, which is the tallest point of the ice sheet, and towards the margins. The ice sheet slope is low around the plateau but increases steeply at the margins.
Increasing global air temperatures due to climate change take around 10,000 years to directly propagate through the ice before they influence bed temperatures, but may have an effect through increased surface melting, producing more supraglacial lakes. These lakes may feed warm water to glacial bases and facilitate glacial motion.
The growth of ice sheets is a signature of glacial periods, which occur during ice ages. There have been many different ice sheets over Earth's history. At the maximum of the Last Glacial Period, the historic Laurentide and Cordilleran ice sheets merged with the Greenland Ice Sheet to cover much of North America. In the same period, the Weichselian Ice Sheet covered Northern Europe and the Patagonian Ice Sheet covered southern South America. The retreat and disappearance of these ice sheets marked the beginning of the current Holocene interglacial period, and the ongoing retreat and future fate of the Greenland and Antarctic ice sheets plays a major role in climate change and sea level rise.