The West Antarctic Ice Sheet (WAIS) is one of three regions making up Antarctica and is more than 4 km thick in places. The long-term stability of the WAIS is of particular concern because it’s a marine-based ice sheet. This means it sits upon bedrock that largely lies below sea level and is in contact with the ocean heat. This makes it vulnerable to rapid and irreversible ice loss. The map below shows the elevation of Antactic bedrock and as we can see, parts of the WAIS are more than 2000m below sea level.
Under the influence of gravity, the ice of the WAIS gradually flows away from its interior toward the coast and into the ocean. Fresh snowfall on the ice sheet replenishes the lost ice. If the ice sheet loses more ice to the ocean than it gains in snow, global sea level rises.
Why is the WAIS a tipping point?
Ice shelves sitting on the ocean surface are subject to melting from above and below by warm air and water, respectively. Because ice shelves float on water, their collapse does not directly cause sea level rise. But thinning and/or collapse of the WAIS’s ice shelves could trigger a positive feedback loop that sees rapid and irreversible loss of land ice into the ocean — which would raise the sea level. This theory is called “marine ice sheet instability” (MISI).
The physical principle that drives the MISI is Archimedes’ principle. Since seawater is denser than ice, marine ice sheets can only remain stable where the ice is thick enough for its mass to exceed the mass of the seawater displaced by the ice. In other words: Wherever ice sits on bedrock below sea level, it is held in place only by the weight of overlying ice. As a marine ice sheet melts, the weight of the overlying ice decreases. If the melting results in thinning that exceeds a critical threshold, the overlying ice may no longer be heavy enough to prevent the submarine ice below it from lifting off the ground, allowing water to penetrate underneath.
Carbon Brief’s article explains very clearly how the whole process works at the WAIS:
The illustration below shows how it works. As an ice shelf thins, more ice lifts off the seafloor and begins to float. This pushes back (see blue arrows) the “grounding line” — the transition point between grounded and floating ice (indicated by dashed lines). Floating ice flows more rapidly than grounded ice and so the rate of ice flow near the grounding line increases (black arrows). Faster flow means thinning, which may in turn cause more ice to lift off and float. And because greater thickness also causes the ice to flow faster, grounding-line retreat into deeper sections of the ice sheet can also produce faster flow.
What makes this a positive feedback loop is the retrograde slope of the WAIS’s bedrock. Not only is much of the bedrock beneath the ice sheet below sea level, large portions of it slope downwards away from the coast. This means that once ice sheet retreat reaches this point, it is self-sustaining.
The rate of ice loss from the WAIS has already tripled from 53 billion tons per year in 1992–97 to 159 billion tons per year in 2012–2017.
Where is the tipping point of the WAIS?
The current best guess is between 1.5–2°C above pre-industrial levels.
What would happen if we crossed the tipping point?
The WAIS contains enough ice to raise global sea level by 3.3 meters. Additionally, the albedo of the Earth would decrease because ice is more reflective than water. This would lead to a further increase in temperature, but this effect is rather negligible.