Marine Ice Sheet Instability

Marine ice sheet instability (MISI) is a dynamical feedback mechanism that can cause rapid, self-sustaining retreat of glaciers whose beds lie below sea level. It is one of the most important sources of irreducible uncertainty in projections of future sea level rise.

The Mechanism

A marine ice sheet is one whose bed slopes inward, deepening toward the interior. When such a glacier loses mass at its marine terminus — through melting, iceberg calving, or retreat of the grounding line — the thinning reduces the back-stress that holds the glacier in place. The glacier accelerates, thins further, and the grounding line retreats into deeper water. Because the bed deepens inland, the retreat exposes taller ice columns to flotation, which further reduces back-stress and drives additional retreat. The feedback is self-sustaining: once initiated, it can continue without additional climate forcing.

The mechanism was first described by Johannes Weertman in 1974 and later formalised by Richard Alley and others. It is distinct from simple melting because the driver is not thermal energy but geometric instability: the shape of the bed and the physics of flotation combine to make the glacier's position unstable.

Why It Matters

MISI is the dominant concern for the West Antarctic Ice Sheet, where the bed lies below sea level across vast sectors. It is also relevant for parts of the Greenland Ice Sheet, particularly the outlet glaciers that terminate in deep fjords. The instability means that these ice sheets may be committed to substantial retreat even if temperatures stabilise — the response is a function of geometry, not just forcing.

The timescale of MISI-driven retreat is poorly constrained. Some models suggest centuries; others suggest that rapid retreat could occur within decades if buttressing ice shelves collapse. The uncertainty is not about whether the mechanism exists — it is well-established in glacier physics — but about the rate at which it operates in specific geometries.

Marine ice sheet instability is not a prediction of catastrophe. It is a prediction that certain geometries are unstable, and that the instability, once triggered, cannot be reversed by restoring the original climate. It is a lesson in the difference between reversible and irreversible dynamics — a distinction that climate policy has not yet internalised.