Thwaites Glacier melt could double sea level forecasts, new model shows

Antarctica's Thwaites Glacier — the Florida-sized mass of ice whose collapse would reshape coastlines from Miami to Manila — is melting far more aggressively than existing models have projected, according to a study published in the journal Nature Communications. The culprit is a physical process that most forecasting models have simply left out.

Glaciologists at the University of California, Irvine and NASA's Jet Propulsion Laboratory built a new ice sheet model that accounts for a 'grounding zone' — a region several kilometers wide where seawater constantly flushes in and out with the tides beneath the glacier. When they ran projections using that more complete physics, Thwaites' contribution to global sea level rise by 2100 came out at up to 30 millimeters (about 1.2 inches) — roughly double previous estimates — even without any additional climate warming.

Glaciologists have long based their assumptions of ice sheet dynamics on the 'grounding line,' a discrete, narrow and mostly stable demarcation where ice leaves the land and begins floating on water. We have come to acknowledge a different arrangement beneath Earth's major ocean-terminating glaciers. Satellite observations demonstrate the existence of a 'grounding zone,' sometimes kilometers wide, where warm ocean water is reaching deep underneath the ice sheets and causing vigorous melting.— Mattia Poinelli, lead author and project scientist at UCLA

The distinction matters enormously. Traditional models treat the boundary between grounded and floating ice as a fixed, narrow line. But satellite data from the Finnish ICEYE constellation — using synthetic aperture radar — shows that seawater actually penetrates several kilometers beneath the grounded ice, lifting it during tidal cycles in a process known as 'tidal pumping.' That pressurized, relatively warm seawater is being pushed to the Antarctic coastline by currents altered by climate change, and it is driving vigorous melting from below.

When the team calibrated their model against more than two decades of satellite observations, it reconstructed historical glacier change from the 1990s to the present that was four times more pronounced than traditional models had shown — finally closing the long-standing gap between what models predicted and what satellites actually observed.

Most projections to date have struggled to match the rapidity of the mass loss because of missing physics; our new model with more complete physics is able to match the record and then produces projections that are twice as large as before. These results predict greater risk of and exposure to coastal flooding than anticipated in the coming decades.— Eric Rignot, professor of Earth system science at UC Irvine and report co-author

Thwaites sits in the Amundsen Sea Embayment sector of West Antarctica, a region that accounts for roughly one-third of the mass loss from the entire Antarctic Ice Sheet. The glacier has been retreating for decades and already contributes around 4% to current global sea level rise. Its full collapse would raise seas by more than 2 feet on its own; because it also acts as a buttress holding back the broader West Antarctic Ice Sheet, its loss could ultimately drive around 10 feet of sea level rise — enough to devastate low-lying coastal communities worldwide.

A central finding of the new study is what happens to the Mouginot Ridge, a large underwater ridge that currently 'pins' the glacier, holding back faster flow into the ocean. The grounding zone melting processes are accelerating the glacier's detachment from that ridge. Computer simulations factoring in a 4–6 kilometer grounding zone find a 60% probability that the Mouginot Ridge will be unpinned by the end of the century — with unpinning possible as early as 2038, according to the Nature Communications paper. Once that happens, the glacier is likely to retreat rapidly down sloping bedrock toward the next major stabilizing feature, the South Ridge.

A 1.2-inch rise in global sea level may sound modest in isolation, but scientists note that even small increments significantly amplify the reach and frequency of coastal flooding events for millions of people. The land beneath Thwaites slopes downward, meaning that as the glacier retreats, progressively more ice is exposed to warm ocean water — a self-reinforcing dynamic.

Outside researchers welcomed the findings but urged caution. Ted Scambos, a senior research scientist for the Earth Science and Observation Center at the University of Colorado Boulder, called the work potentially 'a real gamechanger for sea level forecasting from Thwaites,' according to CNN. But Robert Larter, a marine geophysicist at the British Antarctic Survey, noted that the environment beneath Thwaites is even more complex than the paper describes and that more research is needed before confident predictions can be made.

The reality is that there is a lot we still need to find out about grounding zone processes to be able to make confident predictions about how fast ice will be lost.— Robert Larter, marine geophysicist, British Antarctic Survey

The next step, Rignot said, is getting direct observational data from beneath the glacier itself. The team is planning a field expedition in 2028 to deploy instruments capable of documenting the tidal flushing of seawater in and out of the grounding zone — data that would either confirm or refine the model's projections.

We have been aware of the impacts of climate change on our planet's sensitive ice sheets for decades, but a mounting realization gained in recent years of what is really happening on the undersides of glaciers like Thwaites is leading to an even greater sense of alarm.— Eric Rignot, professor of Earth system science at UC Irvine

Why it matters — If the new model's physics are confirmed by direct observation, sea level rise projections used by coastal planners, insurers, and governments worldwide may need to be significantly revised upward — with implications for hundreds of millions of people living near coastlines.

Reported by cnn.com, phys.org, nature.com