Water under the ice
A new study finds that Thwaites Glacier in West Antarctica could add up to 30 millimetres to global sea level by 2100. That is about twice what earlier projections suggested, and it holds even without additional climate warming. The work was published in Nature Communications.
The revision comes from warm seawater. Ocean water can reach several kilometres inland beneath the glacier's floating front, where the ice meets the sea floor and where the bedrock slopes downward. Most ice-sheet models did not include that intrusion, so they understated how fast the glacier can thin.
Why the number doubled
Thwaites already loses more ice each year than snowfall adds. Its front sits on a bed that deepens inland, which means retreat can accelerate once the grounding line moves back. Warm water reaching that zone melts the ice from below and reduces the friction that holds the glacier in place.
Thirty millimetres sounds small next to the several metres of sea level rise that a full collapse of West Antarctica could produce over centuries. But the difference between one and three centimetres by 2100 matters for coastal planning. It changes how often a given flood level is reached during high tides and storms.
What it means for coasts
Coastal cities plan defences on decades-long horizons. Ports, wastewater plants and roads built near present high tide lines may face flooding more often than designs assumed. A higher central estimate pushes those timelines forward.
The study does not forecast a sudden collapse. Thwaites is unlikely to disappear this century. The finding is about rates: how quickly the glacier loses mass, and how much of that loss shows up in tide gauges before 2100.
Antarctic observations remain thin. Few ships reach the Amundsen Sea, and satellite measurements of the sea floor beneath floating ice are indirect. That is why estimates keep shifting, and why each new ocean survey tends to raise rather than lower the projected contribution.