An ice sheet is a very large mass of glacial ice covering an area of continental scale, generally defined in glaciology as an ice body covering more than 50,000 square kilometres. Ice sheets are among the largest components of the cryosphere and are substantially larger than individual glaciers, ice caps and icefields. Only two modern ice sheets exist on Earth under the conventional geographical definition, the Antarctic Ice Sheet and the Greenland Ice Sheet.
An ice sheet spreads outward under its own weight and gravitational stress, covering underlying mountains, valleys, plateaus and basins rather than being confined to one particular valley. Its margins may feed numerous individual outlet glaciers and floating ice shelves. The ice is produced primarily by accumulated snowfall that survives summer melting, becomes compacted into firn and eventually transforms into dense glacial ice.
The Antarctic Ice Sheet is the largest ice sheet on Earth, covering most of the Antarctic continent and containing by far the greatest volume of land based ice. It has two major sectors, the East Antarctic Ice Sheet and West Antarctic Ice Sheet, separated broadly by the Transantarctic Mountains. The Greenland Ice Sheet covers most of Greenland and is the second largest modern ice sheet.
Both ice sheets contain enormous quantities of freshwater and have major importance for global sea level. If all of the Greenland Ice Sheet melted, global mean sea level would ultimately rise by roughly 7 metres, while complete melting of the Antarctic Ice Sheet would produce a rise of many tens of metres. These values describe theoretical total loss rather than expected short term outcomes.
Ice sheets develop where long term accumulation of snow exceeds the combined losses from melting, sublimation, iceberg calving and other processes. Fresh snow is initially porous and contains considerable air. Repeated snowfall buries older layers, causing pressure and metamorphism to transform snow into firn and eventually glacial ice. Under sufficient thickness, the ice deforms plastically and flows outward from regions of greater thickness toward lower gravitational potential. The surface of an ice sheet can therefore move even though individual pieces of ice may appear stationary over short periods. Ice flow is influenced by thickness, temperature, basal conditions, surface slope, bedrock topography, internal deformation and water at the ice bed.
Ice sheets are different from ice shelves because an ice sheet is primarily grounded on land, whereas an ice shelf is floating ice attached to a grounded ice sheet or glacier and extending over ocean water. Ice sheets commonly feed ice shelves through outlet glaciers, particularly around Antarctica. The grounding line marks the transition between grounded ice and floating ice and is an important feature in studies of ice sheet stability.
Changes near grounding lines can influence how quickly inland ice flows toward the ocean. Floating ice itself does not directly raise sea level when it melts because it already displaces seawater, but the loss of a restraining ice shelf can alter the flow of grounded inland ice and thereby contribute indirectly to sea level rise.
Ice sheets have a major influence on landscapes. Their enormous weight can depress the underlying crust, and the crust can slowly rebound when ice is removed through a process called glacial isostatic adjustment. Moving ice can erode bedrock by abrasion and quarrying, deepen valleys and basins, transport enormous quantities of sediment and reshape continental surfaces. Glacial erosion and deposition have helped create many modern landforms, including drumlins, moraines, eskers, glacial troughs and scoured bedrock surfaces. In Antarctica and Greenland, much of the bed is hidden beneath ice, so scientists use radar, gravity measurements, seismic observations and other techniques to reconstruct the buried landscape.
Ice sheets also function as archives of past climate. Snow accumulating on an ice sheet preserves layers containing dust, volcanic material, chemical substances and trapped atmospheric gases. Ice cores can therefore provide records extending hundreds of thousands of years in Antarctica and more than one hundred thousand years in Greenland. Measurements of oxygen and hydrogen isotopes can provide information about past temperatures, while bubbles containing ancient air preserve samples of atmospheric carbon dioxide, methane and other gases. These records have demonstrated repeated relationships between greenhouse gas concentrations, temperature and major climate changes. Ice cores are consequently important evidence in palaeoclimatology and climate reconstruction.
The mass balance of an ice sheet describes the difference between gains and losses over a specified period. Accumulation includes snowfall and other additions of mass, while ablation includes surface melting, sublimation, iceberg calving and related losses. If accumulation exceeds losses, the ice sheet gains mass overall; if losses exceed accumulation, it loses mass. Ice sheet mass balance is not identical to surface melting because large ice masses can lose mass through processes occurring at their margins. Satellite gravimetry, radar altimetry, GPS measurements, aircraft surveys and surface observations are used to monitor changes in ice thickness, velocity and mass. These measurements are central to understanding present day climate change and future sea level.
An ice sheet is therefore a large scale flowing ice system rather than simply a huge quantity of frozen water lying motionless on land. Its behaviour results from interactions among snowfall, ice deformation, basal sliding, bedrock geometry, ocean conditions, atmospheric temperature and surface energy balance. Modern research focuses particularly on how warming affects accumulation, surface melt, outlet glacier acceleration, ice shelf stability and grounding line migration. Ice sheets respond over timescales ranging from seasons to centuries and millennia, and different regions can respond differently. Their enormous size creates substantial inertia, but that does not make them immune to rapid changes at vulnerable margins.
