Thwaites Glacier

The Thwaites Glacier is a vast ice stream in West Antarctica, roughly the size of Great Britain, that drains approximately 4% of the Antarctic ice sheet into the Amundsen Sea. It is the most unstable large glacier on Earth and the primary structural support for the West Antarctic Ice Sheet (WAIS). Its collapse would destabilise the entire WAIS, raising global sea levels by approximately 65 centimetres directly — and by over 3 metres through the subsequent discharge of interior ice.

Thwaites is a marine ice sheet: its bed lies below sea level and slopes downward toward the interior. This geometry makes it inherently unstable. As the grounding line — where ice transitions from resting on bedrock to floating — retreats, it exposes progressively thicker ice to the ocean. The thicker the ice column, the greater the imbalance between ice weight and buoyant force, and the faster the glacier flows. This positive feedback is the core mechanism of MISI, and Thwaites is the clearest case where it is already active.

The glacier's ice shelf — the floating extension that buttresses the ice stream and slows its flow — has thinned and fractured dramatically over the past two decades. Warm circumpolar deep water is accessing the grounding line through submarine channels carved into the bed, melting the ice from below. Satellite observations show acceleration of ice flow, thinning of the ice stream, and retreat of the grounding line — all consistent with the early stages of irreversible collapse.

The timing is the critical uncertainty. Some models suggest centuries; others, that rapid retreat could unfold within decades if current acceleration continues. The key question is whether the glacier has retreated past a submerged bedrock ridge that currently acts as a pinning point. If the grounding line retreats past this ridge, the geometry becomes self-sustaining: the glacier will continue to retreat even without additional warming. This would be a tipping point crossing — irreversible on human timescales.

Thwaites is sometimes called the 'doomsday glacier'. The label is sensational but not entirely wrong. It is the single most consequential tipping element in the Earth system, and its behaviour over the next few decades will determine whether coastal cities worldwide plan for centimetres or metres of sea level rise. The question is not whether Thwaites will retreat. It is whether we can predict the rate of retreat well enough to inform adaptation decisions — and whether the uncertainty itself is a form of knowledge that should shape policy.