Thwaites 'Doomsday Glacier' May Raise Sea Levels Twice as Much as Predicted, Study Finds

Beneath the Thwaites Glacier in West Antarctica, seawater is moving miles under ice that is still resting on the seafloor. As the tides rise and fall, the water lifts the ice and exposes its underside to warm, pressurized ocean water, again and again. That process, newly built into a computer model by a team led from UC Irvine, may explain why earlier projections have undersold how fast the glacier is changing.

The findings were published in Nature Communications. The model projects that Thwaites could add up to 30 millimeters to global sea levels by 2100, and it puts the chance of the glacier losing a key stabilizing ridge before the century ends at 60%. Eric Rignot, a UC Irvine professor of Earth system science and a co-author, said the new model produces projections twice as large as before. The lead summary of the work goes further: the glacier may be melting twice as much as predicted, even without additional global warming.

The change at the heart of the study is how the model treats the place where ice leaves land and starts to float. Glaciologists have long relied on the idea of a grounding line, which lead author Mattia Poinelli described as a discrete, narrow and mostly stable demarcation. Poinelli was a UC Irvine postdoctoral scholar in Earth system science when the work was done. He said satellite observations now show something different under the world's large ocean-terminating glaciers: a grounding zone, sometimes kilometers wide, where warm ocean water reaches deep under the ice and drives vigorous melting.

Thwaites sits in the Amundsen Sea Embayment, a region responsible for roughly one third of the Antarctic Ice Sheet's total mass loss. In recent decades the glacier has thinned and retreated rapidly, and its ice has flowed faster. Traditional computer models have had trouble reproducing the acceleration that satellites detected.

To test the new approach, the team compared its simulations with more than two decades of observations. It also analyzed synthetic aperture radar measurements from Finland's ICEYE satellite constellation, which showed fluctuations across several kilometers of the grounding zone along the glacier's main trunk. When the model allowed for a broad zone rather than a thin line, it reproduced changes at Thwaites from the 1990s to the present that were four times more pronounced than those captured by conventional models. According to the study, the water gets to the Antarctic coast on ocean currents that climate change has altered.

The melting matters in part because of what it does to the glacier's grip. Poinelli, who is also a NASA JPL research affiliate and now a project scientist at UCLA, said the movement of water beneath the ice weakens its connection to underwater ridges that help hold it in place. With less grip, the ice can flow faster and retreat more quickly.

The ridge at the center of the projection is the Mouginot Ridge, which Poinelli said is temporarily stabilizing the main trunk. Once it is unpinned, he said, the glacier will keep retreating across a predominantly retrograde seafloor toward its next topographic high, the South Ridge. When the model includes a grounding zone 4 to 6 kilometers across, it gives a 60% probability that Thwaites detaches from Mouginot Ridge by the end of the century.

Rignot said that most earlier projections struggled to match the speed of mass loss because of missing physics, and that the new model, with more complete physics, matches the record and then projects larger losses. He said the results point to greater risk of coastal flooding, and greater exposure to it, than expected in the coming decades. Poinelli said that calibrating against historical observations while including key physical processes offers a reliable way to narrow uncertainty in projections. What to watch next is whether further observations of the grounding zone and the Mouginot Ridge bear out the model.