Around the margins of Antarctica, vast plates of ice extend outward from the coastline, floating on the ocean but still connected to the inland ice sheet. These floating ice plates are ice shelves, and they are some of the most distinctive features of the polar regions. They cover approximately 1.5 million square kilometres around Antarctica — an area larger than India — and several smaller ice shelves fringe Greenland and other Arctic islands. Ice shelves are typically a few hundred metres thick at their inland edge, thinning to perhaps 100 metres or less at the seaward front. They project outward into the ocean for tens to hundreds of kilometres, with their outer edges typically marked by tall ice cliffs that calve icebergs into the surrounding sea. Ice shelves are not just scenic features of the polar landscape. They play a critical role in holding back the inland ice sheets, slowing the rate at which inland ice flows into the ocean. The integrity and behaviour of ice shelves is therefore one of the most important factors determining the rate of sea level rise from polar ice loss.
How Ice Shelves Form
Ice shelves form where a glacier or ice sheet flows out from the land onto the surrounding ocean. As the ice reaches the coastline, it continues to flow seaward under its own weight. If the water is deep enough and the ice is thick enough to float, the ice lifts off the seabed and becomes a floating ice shelf. The "grounding line" is the boundary between the grounded (on the seabed) inland ice and the floating ice shelf. Beyond the grounding line, the ice shelf continues to flow seaward, gradually thinning as some of its mass is lost to melting from below (by warm ocean water) and from surface processes. The ice shelf grows by ice flowing into it from the inland ice sheet and shrinks by iceberg calving from its seaward edge and by melt on both surfaces. In equilibrium, the gain and loss are balanced; in non-equilibrium, the shelf grows or shrinks accordingly.
The Major Antarctic Ice Shelves
Antarctica has several major ice shelves. The Ross Ice Shelf is the largest, covering about 500,000 square kilometres — roughly the size of France. It fringes the eastern coast of the Ross Sea on the Pacific side of Antarctica. The Filchner-Ronne Ice Shelf, on the opposite side of the continent in the Weddell Sea, is the second-largest at about 430,000 square kilometres. The Amery Ice Shelf, on the eastern side of Antarctica, is the third-largest at about 60,000 square kilometres. Smaller but still substantial ice shelves include the Larsen Ice Shelf system on the Antarctic Peninsula (which has been substantially disintegrating since the late 1990s), the Wilkins Ice Shelf (also Peninsula), the Pine Island and Thwaites Glacier ice shelves (West Antarctica), and many smaller ice shelves around the continent. The combined area of all Antarctic ice shelves is approximately 1.5 million square kilometres.
Greenland and Other Arctic Ice Shelves
Greenland has smaller ice shelves at the seaward ends of major outlet glaciers — particularly the floating tongues of glaciers like Petermann, Zachariae, and Nioghalvfjerdsfjorden. These ice tongues are typically tens of kilometres long but not the broad continental-scale shelves of Antarctica. Greenland ice shelves have been substantially disintegrating since the early 2000s — the Petermann Glacier lost a 250-square-kilometre section in 2010 and another 130-square-kilometre section in 2012, dramatically shortening its floating tongue. The Jakobshavn Isbræ floating tongue has also retreated substantially. Canada has small ice shelves along the northern coast of Ellesmere Island (the Ward Hunt Ice Shelf and others) that are remnants of larger Holocene ice shelves; most have broken up substantially in the past 25 years.
How Ice Shelves Hold Back the Inland Ice
The critical insight about ice shelves is that they are not just floating extensions — they exert backstress on the inland ice that feeds them. This backstress slows the flow of the inland ice. When an ice shelf is in place and intact, the inland glaciers feeding it flow relatively slowly. When the ice shelf disintegrates or thins substantially, the backstress is reduced or removed, and the inland glaciers accelerate. This is one of the most important mechanisms by which ice shelves regulate the rate of sea level rise. The disintegrated Larsen B Ice Shelf in 2002 was followed by 2-8x acceleration of the glaciers that had previously fed into it — providing one of the clearest demonstrations of this mechanism in action.
Larsen Ice Shelf Disintegration
The Larsen Ice Shelf system on the eastern side of the Antarctic Peninsula has been the type case for studying ice shelf disintegration. Larsen A disintegrated in 1995 (a 1,500 km² area collapsed within a few weeks). Larsen B disintegrated in 2002 (a 3,250 km² area collapsed in a few weeks). Larsen C lost the A-68 iceberg in 2017 — about 5,800 km², one of the largest icebergs ever recorded — substantially shortening the shelf, though it has not yet fully collapsed. The disintegrations are believed to have been triggered by surface melt water pooling on the shelf during unusually warm summers — the meltwater drains into existing crevasses, hydraulically wedges them open, and produces the chain reaction of disintegration. The Antarctic Peninsula has warmed approximately 3°C since the 1950s, which has substantially increased surface melting on shelf surfaces previously well below freezing.
Basal Melt
Most ice shelf mass loss does not happen by surface melt and calving but by melt at the underside of the shelf where it contacts ocean water. The temperature of the ocean water beneath ice shelves determines the basal melt rate. Antarctic ice shelves have historically had different basal melt rates in different regions — the Ross and Filchner-Ronne shelves sit in cool waters and have low basal melt rates; the West Antarctic shelves (Pine Island, Thwaites, Larsen, Wilkins) sit in waters that have been warmed by intrusions of relatively warm Circumpolar Deep Water and have much higher basal melt rates. The total basal melt from Antarctic ice shelves is currently about 1,300 gigatonnes per year — comparable to the total mass output of the ice sheet through surface processes. The basal melt is therefore a major term in the Antarctic ice budget.
Marine Ice Sheet Instability
One of the major concerns about Antarctic ice loss is the marine ice sheet instability mechanism. The West Antarctic Ice Sheet sits on bedrock that is below sea level and that slopes inward — meaning the bedrock gets deeper as you move from the coast toward the interior. If the grounding line retreats (as it has been doing for decades at Pine Island and Thwaites), the ice above the new grounding line is now grounded in deeper water. Thicker, deeper-grounded ice has more buoyancy, which can accelerate the retreat in an unstable way. The Pine Island and Thwaites glaciers — currently together responsible for about 5 percent of global sea level rise — are particularly vulnerable to this mechanism. Some scientists have argued that the West Antarctic Ice Sheet may have already entered an irreversible retreat, though the timescale is uncertain (decades to centuries).
Iceberg Calving
The seaward edges of ice shelves periodically calve large icebergs. Most of these calving events are minor and routine — small chunks of ice breaking off and floating away. But major calving events can produce icebergs the size of small countries. The B-15 iceberg calved from the Ross Ice Shelf in 2000 — initially 11,000 square kilometres, larger than Jamaica. The A-68 iceberg from Larsen C in 2017 was 5,800 km². The A-74 iceberg from Brunt Ice Shelf in 2021 was 1,270 km². These mega-icebergs drift in the Southern Ocean for years to decades, gradually breaking up, eventually melting to release fresh water and any contaminants they carry. The largest icebergs are major hazards to shipping and can substantially modify regional oceanography during their lifetimes.
Ice Shelves and Sea Level
The ice itself in an ice shelf is already floating — it has already displaced its own weight of seawater, and its disintegration does not directly contribute to sea level rise. But because ice shelves hold back inland ice, their loss has substantial sea level consequences through the acceleration of the inland glaciers they previously buttressed. The current Antarctic and Greenland ice sheets contain enough water to raise global sea level by approximately 60 metres if they fully melted. The ice shelves themselves contain only a few tens of centimetres of equivalent sea level rise as direct ice content, but they regulate the rate at which the much larger inland ice volumes flow into the ocean. The behaviour of ice shelves over the coming decades will therefore be one of the major determinants of sea level rise.
The Coming Decades
Antarctic ice shelves are at risk of substantial thinning and possible disintegration under continued climate warming. The Antarctic Peninsula has lost most of its smaller ice shelves over the past 30 years. West Antarctic ice shelves are losing mass through basal melt at unprecedented rates. Even the largest shelves (Ross, Filchner-Ronne) are showing signs of stress, though their behaviour is more stable than the smaller and more vulnerable systems. The continued integrity of Antarctic ice shelves is one of the central uncertainties in sea level rise projections, and the dynamics of ice shelf disintegration and ocean-shelf interaction is one of the most active research areas in cryospheric science. The shelves themselves may not seem dramatic — broad, flat plains of floating ice that change slowly — but their behaviour over the coming decades will substantially shape the planet's future.