The West Antarctic Ice Sheet (WAIS) is the sector of the Antarctic Ice Sheet covering West Antarctica. The WAIS is a marine ice sheet, as the majority of its ice is grounded well below sea level, on rock and sediment that would be the ocean floor if the ice were to retreat. The ice sheet first formed about 27 million years ago, likely fed partially by advancing glaciers of the older East Antarctic Ice Sheet . The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.
As a smaller part of Antarctica, WAIS is also more strongly affected by climate change. There has been warming over the ice sheet since the 1950s, and a substantial retreat of its coastal glaciers since at least the 1990s. Estimates suggest it added around to the global sea level rise between 1992 and 2017, and has been losing ice in the 2010s at a rate equivalent to of annual sea level rise. While some of its losses are offset by the growth of the East Antarctic Ice Sheet, Antarctica as a whole will most likely lose enough ice by 2100 to add to sea levels. Further, marine ice sheet instability may increase this amount by tens of centimeters, particularly under high warming. Fresh meltwater from WAIS also contributes to ocean stratification and dilutes the formation of salty Antarctic bottom water, which destabilizes Southern Ocean overturning circulation.
In the long term, the West Antarctic Ice Sheet is likely to disappear due to the warming which has already occurred. Paleoclimate evidence suggests that this has already happened during the Eemian period, when the global temperatures were similar to the early 21st century. It is believed that the loss of the ice sheet would take place between 2,000 and 13,000 years in the future, although several centuries of high emissions may shorten this to 500 years. of sea level rise would occur if the ice sheet collapses but leaves ice caps on the mountains behind. Total sea level rise from West Antarctica increases to if they melt as well, but this would require a higher level of warming. Isostatic rebound of ice-free land may also add around to the global sea levels over another 1,000 years.
The preservation of WAIS may require a persistent reduction of global temperatures to below the preindustrial level, or to below the temperature of 2020. Because the collapse of the ice sheet would be preceded by the loss of Thwaites Glacier and Pine Island Glacier, some have instead proposed interventions to preserve them. In theory, adding thousands of gigatonnes of artificially created snow could stabilize them, but it would be extraordinarily difficult and may not account for the ongoing acceleration of ocean warming in the area. Others suggest that building obstacles to warm water flows beneath glaciers would be able to delay the disappearance of the ice sheet by many centuries, but it would still require one of the largest civil engineering interventions in history.
Description
The total volume of the entire Antarctic ice sheet is estimated at 26.92 million km3 (6.46 million cu mi), while the WAIS contains about 2.1 million km3 (530,000 cu mi) in ice that is above the sea level, and ~1 million km3 (240,000 cu mi) in ice that is below it. The weight of the ice has caused the underlying rock to sink by between in a process known as isostatic depression.
Under the force of its own weight, the ice sheet deforms and flows slowly over rough bedrock. Ice ridges are the areas where ice sheet movement is slow because it is frozen to the bed, while ice streams flow much faster because there is liquid water in the sediments beneath them. Those are either the marine sediments which used to cover the ocean floor before the ice sheet froze above them, or they have been created due to erosion from the constant friction of ice against the bedrock. The water in these sediments stays liquid because the Earth's crust below the ice streams is thin and conducts heat from geothermal activity, and because the friction also generates heat, particularly at the margins between ice streams and ice ridges.
When ice reaches the coast, it either calves or continues to flow outward onto the water. The result is a large, floating ice shelf affixed to the continent. These ice shelves restrain the flow of ice into the ocean for as long as they are present.
West Antarctic Rift System
The West Antarctic Rift System (WARS) is one of the major active continental rifts on Earth. It is believed to have a major influence on ice flows in West Antarctica. In western Marie Byrd Land, active glaciers flow through fault-bounded valleys (grabens) of the WARS. Sub-ice volcanism has been detected and is known to influence ice flows. In 2017, geologists from Edinburgh University discovered 91 volcanoes located two kilometres below the icy surface, making it the largest volcanic region on Earth.
Fast-moving ice streams in the Siple Coast adjacent to the east edge of the Ross Ice Shelf are influenced by the lubrication provided by water-saturated till within fault-bounded grabens within the rift, which would act to accelerate ice-sheet disintegration at more intense levels of climate change.
History Like the other ice sheets, West Antarctic Ice Sheet had undergone significant changes in size during its history. Until around 400,000 years ago, the state of WAIS was largely governed by the effects of solar variation on heat content of the Southern Ocean, and it waxed and waned in accordance with a 41,000-year-long cycle. Around 80,000 years ago, its size was comparable to now, but then it grew substantially larger, until its extent reached the margins of Antarctica's continental shelves during the Last Glacial Maximum ~30,000 years ago. It then shrunk to around its preindustrial state some 3,000 years ago. It also at times shrunk to a point where only minor and isolated ice caps remained, such as during the Marine isotope stage 31 ~1.07 million years ago, or the Eemian period ~130,000 years ago.
Climate change
Observations
West Antarctica has experienced statistically significant warming in recent decades, although there is some uncertainty about its magnitude. In 2015, the warming of the WAIS between 1976 and 2012 was calculated as a range between per decade and per decade. In 2009, the warming of the region since 1957 was estimated as exceeding per decade. This warming is strongest in the Antarctic Peninsula. In 2012, research found that the West Antarctic ice sheet had warmed by since 1958 – around per decade, which was almost double the 2009 estimate. In 2022, Central WAIS warming between 1959 and 2000 was estimated at per decade, with this change conclusively attributed to increases in greenhouse gas concentrations.
The continually increasing ocean heat content forces the melting and retreat of ice sheet's coastal glaciers. Normally, glacier mass balance offsets coastal losses through gains from snowfall at the surface, but between 1996 and 2006, Antarctic ice mass loss had already increased by 75%. Between 2005 and 2010, WAIS melting was thought to have added to global sea levels every year. Around 2012, the total mass loss from the West Antarctic Ice Sheet was estimated at . Subsequent satellite observations revealed that the West Antarctic ice loss increased from in 1992 to in 2017, resulting in of Antarctica sea level rise. By 2023, became the average annual rate of mass loss since 2002, equivalent to of annual sea level rise.
Coastal glaciers are typically buttressed by ice shelves, which are massive blocks of floating ice next to a glacier. Yet, the ice shelves melt relatively quickly, as they are constantly in contact with the warming ocean water. Glacier retreat accelerates substantially once they collapse and stop providing structural support to the glacier, and once warm water can flow to the glacier unimpeded. Most ice losses occur at the Amundsen Sea Embayment and its three most vulnerable glaciers – Thwaites Glacier, Pine Island Glacier and Smith Glacier. Around 2005, they were thought to lose 60% more mass than what they have gained, and to contribute about per year to global sea level rise.Jenny Hogan, "Antarctic ice sheet is an 'awakened giant'", New Scientist, February 2, 2005.
Of the three, Thwaites Glacier is the best-known, to the point of being nicknamed the "Doomsday Glacier" by some in the press, although many scientists consider it alarmist and inaccurate. The reason for concern about Thwaites is because it had been experiencing substantial mass loss since at least the early 1990s, while its local seabed topography provides no obstacles to rapid retreat, with its most vulnerable parts located below the sea level. Further, it had been shown in 2021 that the Thwaites Ice Shelf, which restrains the eastern portion of the Thwaites Glacier, could start to collapse within five years. The glacier would start to see major losses "within decades" after the ice shelf's failure, and its annual contribution to sea level rise would increase from the current 4% to 5%, although it would still take centuries to disappear entirely.
Projected 21st century ice loss
As the West Antarctic Ice Sheet loses ice due to the warming ocean water melting its coastal glaciers, it inevitably contributes to sea level rise. However, projections are complicated by additional processes that are difficult to model, such as meltwater from the ice sheet itself changing local circulation due to being warmer and fresher than the ocean water. Another complicated process is hydrofracturing, where meltwater collecting atop the ice sheet may pool into fractures and force them open, further damaging its integrity. Climate change alters winds above Antarctica, which can also affect surface current circulation, but the importance of this process has been disputed.
Most importantly, the WAIS has a complex topography which magnifies its vulnerability. The grounding lines of its glaciers are below the sea level by several hundred metres or more, and the bed only deepens upstream. This means that as the ice sheet loses mass to melting, an increasing fraction of its height becomes exposed to warm water flows that are no longer displaced by its mass. This hypothesis is known as marine ice sheet instability (MISI) and it has the potential to greatly accelerate ice losses. The lack of knowledge about its specifics introduces substantial uncertainty into projections of 21st century sea level rise. WAIS could be even more vulnerable under the so-called marine ice cliff instability hypothesis (MICI). It suggests that when a glacier's ice shelf melts, it would not just retreat faster, but rapidly collapse under its own weight if the height of its cliffs was greater than . This particular process has never been observed and was even ruled out by some of the more detailed modelling, but it still adds to the uncertainty in sea level projections.