Ice sheet

An ice sheet is defined by the International Glaciological Society as a mass of glacial ice exceeding 50,000 square kilometers, a threshold that separates it from smaller ice caps and glaciers. The distinction is not arbitrary: at this scale, ice flow is dominated by internal deformation and basal sliding, and the ice surface forms a broad dome that drives flow outward in all directions. Ice sheets bury entire mountain ranges and continental shelves, as seen in Antarctica where the Transantarctic Mountains are almost completely covered. In contrast, ice caps are smaller and often rest on highlands, while valley glaciers are confined by topography. The only current ice sheets are the Antarctic Ice Sheet and the Greenland Ice Sheet, but during the last glacial maximum, ice sheets covered large parts of North America (Laurentide) and Eurasia (Fennoscandian).

Ice sheets are layered systems: the upper firn compacts into ice, while the deeper ice flows under its own weight. Flow is concentrated in fast-moving ice streams—like the Siple Coast streams in West Antarctica—that can move hundreds of meters per year, discharging ice into the ocean. Basal conditions are crucial: where the bed is wet and soft, sliding accelerates; where it is frozen, ice moves slowly. Subglacial lakes, such as Lake Vostok in East Antarctica, exist beneath kilometers of ice and host unique microbial ecosystems. Ice sheets also interact with the ocean through ice shelves—floating extensions that buttress the grounded ice. The loss of ice shelves, as seen with Larsen B in 2002, can trigger rapid glacier acceleration and drawdown.

Ice sheets are the largest potential contributors to future sea-level rise. The Antarctic Ice Sheet holds enough ice to raise global sea levels by about 58 meters, and Greenland by about 7 meters. Since the 1990s, both have lost mass at an accelerating rate, driven by warming ocean waters melting ice shelves from below and by surface meltwater lubricating the bed. The Intergovernmental Panel on Climate Change (IPCC) projects that under high-emission scenarios, ice-sheet contributions could add up to 0.5 meters by 2100, with larger long-term losses possible. Ice sheets also influence global climate through albedo feedback, freshwater input to oceans, and changes in atmospheric circulation. Paleoclimate records from ice cores, such as those from EPICA Dome C, show that ice sheets have undergone dramatic collapses in the past, providing analogues for future behavior.

Beyond the headline facts, ice sheets harbor surprising complexities. The Antarctic Ice Sheet is not a single mass but a mosaic of the East Antarctic Ice Sheet (EAIS), West Antarctic Ice Sheet (WAIS), and Antarctic Peninsula, each with distinct histories. The EAIS is the oldest, with ice exceeding 1 million years in places, while WAIS is considered unstable because much of its bed lies below sea level. Subglacial topography includes the Gamburtsev Mountains, a mountain range the size of the Alps buried under East Antarctic ice. Ice sheets also record past atmospheres: the Dome C ice core extends back 800,000 years, capturing greenhouse gas concentrations. In Greenland, Camp Century, a Cold War military base, was built inside the ice sheet and later abandoned, leaving buried waste that could emerge with melting. Ice sheets even host active subglacial hydrology, with rivers and lakes that drain episodically, as observed in Antarctica's Whillans Ice Stream.

Firn

Partially compacted snow that is intermediate between snow and glacial ice.

Ice stream

A fast-moving corridor of ice within an ice sheet, flowing faster than the surrounding ice.

Ice shelf

A floating extension of an ice sheet that spreads over the ocean, often hundreds of meters thick.

Subglacial lake

A lake of liquid water beneath an ice sheet, formed by geothermal heat and pressure.

Buttressing

The mechanical support that ice shelves provide to grounded ice, slowing its flow into the ocean.