The concern around the vulnerability of the WAIS [West Antarctic Ice Sheet] principally lies in something called “marine ice sheet instability” (MISI) – “marine” because the base of the ice sheet is below sea level and “instability” for the fact that, once it starts, the retreat is self-sustaining.

Ice sheets can be thought of as huge freshwater reservoirs. Snow accumulates in the cold interior, slowly compacts to become glacier ice and then begins to flow like a very thick fluid back toward the ocean.

In some places, the ice reaches the coast and floats on the ocean surface, forming an ice shelf. The boundary between ice resting on the land surface (or the sea floor in the case of a marine ice sheet) is called the “grounding line”. The grounding line is where water stored in the ice sheet returns to the ocean. And when it moves seaward, we say the ice sheet has a positive “mass balance” – that is to say, it is gaining more ice mass than it is losing back to the sea.

But when the grounding line retreats, the balance is negative. A negative ice sheet balance means a positive contribution to the ocean and, thus, to global sea level.

Instability
This basic picture of ice sheet mass balance is all you need to understand why glaciologists are concerned about MISI.

Changes to the ice shelf on the floating side of the grounding line – such as thinning – can cause ice on the grounded side to lift off from the seafloor. As this ice floats, the grounding line will retreat. Because the ice flows more rapidly when it is floating than it does when grounded, the rate of ice flow near the grounding line will increase. Stretching caused by the faster flow becomes a new source of thinning near the grounding line.

This is illustrated in the figure below. As the newly floating ice flows and thins more quickly, it can cause more ice to lift off and float, driving the grounding line back.

In addition, the areas of the ice sheet at risk of MISI have a reverse, or “retrograde” gradient, which means it gets deeper further inland. As the grounding line retreats further into thicker parts of the ice sheet, the flow speeds up, further increasing ice loss. The reverse gradient makes this process self-sustaining as a positive feedback loop – this is what makes MISI an instability.

Illustration of Marine Ice Sheet Instability, or MISI. Thinning of the buttressing ice shelf leads to acceleration of the ice sheet flow and thinning of the marine-terminated ice margin. Because bedrock under the ice sheet is sloping towards ice sheet interior, thinning of the ice causes retreat of the grounding line followed by an increase of the seaward ice flux, further thinning of the ice margin, and further retreat of the grounding line. Credit: IPCC SROCC (2019) Fig CB8.1a
Illustration of Marine Ice Sheet Instability, or MISI. Thinning of the buttressing ice shelf leads to acceleration of the ice sheet flow and thinning of the marine-terminated ice margin. Because bedrock under the ice sheet is sloping towards ice sheet interior, thinning of the ice causes retreat of the grounding line followed by an increase of the seaward ice flux, further thinning of the ice margin, and further retreat of the grounding line. Credit: IPCC SROCC (2019) Fig CB8.1a
It is not clear yet if the MISI threshold has been crossed anywhere in Antarctica. We do know that grounding lines are retreating along the Amundsen Sea coastline – most spectacularly on the Thwaites Glacier. And the driver for the retreat appears to be relatively warm ocean water – about 2C warmer than the historical average – flowing toward the grounding line and causing stronger than usual melting.


If the instability has not started and if the ocean warming stops, then the grounding line should find a new balancing point at a new location. But if it has started, then the retreat will continue no matter what happens next.