The West Antarctic Ice Sheet (WAIS) is a massive body of ice covering West Antarctica, separated from the larger East Antarctic Ice Sheet by the Transantarctic Mountains. It is classified as a marine ice sheet because the majority of its ice is grounded on bedrock and sediment that lies well below sea level. The WAIS first formed around 27 million years ago, likely fed by advancing glaciers from the older East Antarctic Ice Sheet. Its unique foundation below sea level makes it inherently vulnerable and unstable, a characteristic noted by glaciologist John Mercer in 1968.
The mechanism of its instability is tied to the Marine Ice Sheet Instability (MISI) hypothesis, which suggests that the ice sheet's retreat can become self-sustaining because the bedrock slopes downward toward the continent's interior. This means that as the ice retreats, the grounding line—the point where the ice lifts off the seabed to float—moves into deeper water, allowing more warm ocean water to melt the ice from below. The ice flow is currently constrained by large floating ice shelves, such as the Ross Ice Shelf and the Ronne Ice Shelf, which act as buttresses. Key components connected to the WAIS's stability are the rapidly changing Thwaites Glacier and Pine Island Glacier, which drain into the Amundsen Sea.
The WAIS has been losing mass at an accelerating rate, with a substantial retreat of its coastal glaciers observed since at least the 1990s. Between 1992 and 2017, the WAIS contributed approximately 7.6 mm to global sea-level rise. Recent satellite data shows that in parts of West Antarctica, the grounding line has moved back by over 40 km. The complete collapse of the WAIS, which some models project could occur by 2300 under current emissions, would raise global sea levels by about 3.3 meters.