thumb|upright=1.3|One of Earth's two ice sheets: The [[Antarctic ice sheet covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth. It has an average thickness of over 2 kilometers.]]
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 . 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. This ice layer forms because most of the snow which falls onto the ice sheet never melts, and is instead compressed by the mass of newer snow layers.
Dynamics
Glacial flows
300 px|thumb|Glacial flow rate in the Antarctic ice sheet.
thumb|upright=1.35|The motion of ice in Antarctica
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. This motion is driven by gravity but is controlled by temperature and the strength of individual glacier bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on time scales ranging from hourly (i.e. tidal flows) to the centennial (Milankovich cycles). During larger spring tides, an ice stream will remain almost stationary for hours at a time, before a surge of around a foot in under an hour, just after the peak high tide; a stationary period then takes hold until another surge towards the middle or end of the falling tide. At neap tides, this interaction is less pronounced, and surges instead occur approximately every 12 hours. Lakes of a diameter greater than ~300 m are capable of creating a fluid-filled crevasse to the glacier/bed interface. When these crevasses form, the entirety of the lake's (relatively warm) contents can reach the base of the glacier in as little as 2–18 hours – lubricating the bed and causing the glacier to surge. Water that reaches the bed of a glacier may freeze there, increasing the thickness of the glacier by pushing it up from below.
Boundary conditions
thumb|The collapse of the [[Larsen B ice shelf had profound effects on the velocities of its feeder glaciers.]]
thumb|Accelerated ice flows after the break-up of an ice shelf
As the margins end at the marine boundary, excess ice is discharged through ice streams or outlet glaciers. Then, it either falls directly into the sea or is accumulated atop the floating ice shelves. Those ice shelves then calve icebergs at their periphery if they experience excess of ice. Ice shelves would also experience accelerated calving due to basal melting. In Antarctica, this is driven by heat fed to the shelf by the circumpolar deep water current, which is 3 °C above the ice's melting point.
The presence of ice shelves has a stabilizing influence on the glacier behind them, while an absence of an ice shelf becomes destabilizing. For instance, when Larsen B ice shelf in the Antarctic Peninsula had collapsed over three weeks in February 2002, the four glaciers behind it - Crane Glacier, Green Glacier, Hektoria Glacier and Jorum Glacier - all started to flow at a much faster rate, while the two glaciers (Flask and Leppard) stabilized by the remnants of the ice shelf did not accelerate.
The collapse of the Larsen B shelf was preceded by thinning of just 1 metre per year, while some other Antarctic ice shelves have displayed thinning of tens of metres per year.
Vulnerable locations
thumb|Distribution of meltwater hotspots caused by ice losses in [[Pine Island Bay, the location of both Thwaites (TEIS refers to Thwaites Eastern Ice Shelf) and Pine Island Glaciers.]]
Because the entire West Antarctic Ice Sheet is grounded below the sea level, it would be vulnerable to geologically rapid ice loss in this scenario. In particular, the Thwaites and Pine Island glaciers are most likely to be prone to MISI, and both glaciers have been rapidly thinning and accelerating in recent decades. As a result, sea level rise from the ice sheet could be accelerated by tens of centimeters within the 21st century alone.
The majority of the East Antarctic Ice Sheet would not be affected. Totten Glacier is the largest glacier there which is known to be subject to MISI - yet, its potential contribution to sea level rise is comparable to that of the entire West Antarctic Ice Sheet. Totten Glacier has been losing mass nearly monotonically in recent decades, suggesting rapid retreat is possible in the near future, although the dynamic behavior of Totten Ice Shelf is known to vary on seasonal to interannual timescales. The Wilkes Basin is the only major submarine basin in Antarctica that is not thought to be sensitive to warming.
Marine ice cliff instability
thumb|A collage of footage and animation to explain the changes that are occurring on the West Antarctic Ice Sheet, narrated by glaciologist [[Eric Rignot]]
A related process known as Marine Ice Cliff Instability (MICI) posits that ice cliffs which exceed ~ in above-ground height and are ~ in basal (underground) height are likely to collapse under their own weight once the peripheral ice stabilizing them is gone. This theory had been highly influential - in a 2020 survey of 106 experts, the paper which had advanced this theory was considered more important than even the year 2014 IPCC Fifth Assessment Report. Sea level rise projections which involve MICI are much larger than the others, particularly under high warming rate.
At the same time, this theory has also been highly controversial. It was originally proposed in order to describe how the large sea level rise during the Pliocene and the Last Interglacial could have occurred - yet more recent research found that these sea level rise episodes can be explained without any ice cliff instability taking place. Research in Pine Island Bay in West Antarctica (the location of Thwaites and Pine Island Glacier) had found seabed gouging by ice from the Younger Dryas period which appears consistent with MICI.
thumb|left|If MICI can occur, the structure of the glacier [[embayment (viewed from the top) would do a lot to determine how quickly it may proceed. Bays which are deep or narrow towards the exit would experience much less rapid retreat than the opposite The retreat of Greenland ice sheet's three largest glaciers - Jakobshavn, Helheim, and Kangerdlugssuaq Glacier - did not resemble predictions from ice cliff collapse at least up until the end of 2013, but an event observed at Helheim Glacier in August 2014 may fit the definition. Further, modelling done after the initial hypothesis indicates that ice-cliff instability would require implausibly fast ice shelf collapse (i.e. within an hour for ~-tall cliffs), unless the ice had already been substantially damaged beforehand.
Some scientists - including the originators of the hypothesis, Robert DeConto and David Pollard - have suggested that the best way to resolve the question would be to precisely determine sea level rise during the Last Interglacial. As of 2023, the most recent analysis indicates that the Last Interglacial SLR is unlikely to have been higher than , appear inconsistent with the new paleoclimate data from The Bahamas and the known history of the Greenland Ice Sheet.
Earth's current two ice sheets
Complete melting of global ice sheets and glaciers could take thousands of years, but would produce of sea level rise. (Melting of sea ice and ice shelves does not affect sea level.)