Antarctica's 'Doomsday Glacier' could add double the forecast sea-level rise, new model finds

A new ice-sheet model built around a previously underestimated feature of West Antarctica's geography suggests that Thwaites Glacier could add roughly twice as much to global sea levels by 2100 as earlier projections assumed, even if ocean temperatures rise no further than they already have. The study, led by researchers affiliated with the University of California, Irvine, NASA's Jet Propulsion Laboratory and Dartmouth College, appeared Oct. 12 in the journal Nature Communications and was detailed in a NASA Jet Propulsion Laboratory release on Oct. 20.

Thwaites, an ice stream roughly the size of Florida that already accounts for about 4 percent of the world's current sea-level rise, has long carried the nickname the "Doomsday Glacier" because its eventual collapse could add more than two feet to global seas on its own, and roughly 10 feet if it destabilizes the wider West Antarctic Ice Sheet. The new study does not project that outcome. It instead revises upward, substantially, how much the glacier is likely to contribute within this century.

A band, not a line, beneath the ice

Earlier ice-sheet models generally treated the spot where a glacier lifts off bedrock and begins floating on the ocean, known as the grounding line, as a single, narrow boundary. Satellite radar data gathered in 2023, much of it from Finland's ICEYE constellation, instead showed that boundary flexing by kilometers over each tidal cycle, as seawater intruded repeatedly beneath the grounded ice, a process UC Irvine researchers had already documented in that dataset. The new model incorporates that process, known as tidal pumping, by treating the grounding area as a zone several kilometers wide rather than a line.

Mattia Poinelli, the study's lead author, who conducted the work as a UC Irvine postdoctoral researcher and is now a project scientist at UCLA and a NASA JPL research affiliate, said the data point to a different arrangement beneath Earth's major ocean-terminating glaciers than scientists had assumed, one in which seawater reaches deep underneath the ice and drives vigorous melting from below.

Doubling the sea-level forecast

Tested against nearly three decades of satellite observations of Thwaites's retreat, thinning and flow speed, the grounding-zone version of the model matched the historical record about four times more closely than versions without it, according to NASA's release on the study. Projected forward, it put Thwaites's contribution to global sea-level rise at up to 30 millimeters, or about 1.2 inches, by 2100, with an uncertainty of about 6 millimeters either way, even in scenarios with no additional ocean warming — roughly double the figures from earlier modeling efforts.

The model also pointed to a tipping point. Thwaites's main trunk is currently held back by an underwater rise called the Mouginot Ridge, a few kilometers across and about 950 meters below sea level. In simulations using a grounding zone four to six kilometers wide, the width current satellite data suggest is realistic, the model put the odds at roughly 60 percent that the ice would separate from the ridge by the end of the century, after which retreat would likely accelerate. Eric Rignot, a UC Irvine professor of Earth system science and a senior project scientist at JPL who co-authored the study, said the findings point to greater risk and exposure to coastal flooding than previously anticipated in the coming decades, and that even if nations slow greenhouse-gas emissions sharply, higher seas are already locked in by past warming.

Stakes beyond Antarctica

Thwaites sits on the Amundsen Sea coast of West Antarctica, among the fastest-changing parts of the ice sheet; its flow speed has roughly doubled since the 1990s. Because it drains into open ocean rather than a sheltered bay, and because its bedrock slopes downward away from the coast, glaciologists have long considered it prone to runaway retreat once an anchoring point is breached. Coastal cities and counties from the Gulf Coast to Southeast Asia rely on current sea-level projections, drawn in part from models of individual glaciers like this one, to plan seawalls, drainage upgrades and building codes decades in advance. A doubled estimate from a single glacier, even measured in inches, compounds with contributions from Greenland, other Antarctic basins and ocean thermal expansion.

Scientists weigh in

Researchers not involved in the study offered a mix of praise and caution, according to a CTV/CNN report. Ted Scambos, a senior research scientist at the University of Colorado Boulder's Earth Science and Observation Center, said the work could prove a real gamechanger for sea-level forecasting from Thwaites. Robert Larter, a marine geophysicist with the British Antarctic Survey, was more measured, noting there is a lot still to learn about grounding-zone processes before scientists can make confident predictions about how quickly the ice will be lost, and that conditions beneath Thwaites are likely even more complex than the paper describes.

"A real gamechanger for sea-level forecasting from Thwaites." — Ted Scambos, University of Colorado Boulder, on the new model

No instrument has yet measured the tidal flushing of seawater directly beneath Thwaites's grounding zone; the model's inference rests on how the ice surface flexes, as seen from orbit. The study's authors say they plan to return to the glacier in 2028 to deploy instruments capable of recording that seawater movement directly, work that could narrow the uncertainty in the current estimate. Until then, the new projection will be weighed alongside other modeling approaches, including an earlier NASA-led effort that found some prior methods may have overstated the glacier's ice loss over the coming decades by failing to let ice and ocean models evolve together.