Antarctica's Thwaites Glacier, often called the "Doomsday Glacier," could add up to 30 millimeters (about 1.2 inches) to global sea levels by 2100, roughly double what earlier models projected, according to a new study in Nature Communications. The reason is a part of the glacier that most computer models have treated as a thin line: a band several kilometers wide where warm seawater pushes underneath ice that looks like it is still resting on bedrock.

The finding does not predict a sudden collapse — far from it. The 30-millimeter figure is an upper estimate with an uncertainty of about 6 millimeters either way, and it comes from a scenario that holds the climate at recent conditions rather than warming it further.

For U.S. coastal communities, the number matters because sea-level projections feed into flood maps, infrastructure design, and insurance planning. A single glacier doubling its expected contribution changes the baseline that cities from Miami to San Francisco use when they plan seawalls and drainage for the coming decades.

A Missing Band of Melting Under the Ice

The research team, led by Mattia Poinelli, a former UC Irvine postdoctoral scholar now at UCLA and a research affiliate at NASA's Jet Propulsion Laboratory, focused on the glacier's grounding zone. That is where the ice transitions from sitting on the seafloor to floating on the ocean.

Most ice-sheet models treat this transition as a sharp line. Satellite radar shows it is something else. Data from Finland's ICEYE satellite constellation revealed the grounding zone shifting across several kilometers along the glacier's main trunk, according to UC Irvine. As tides rise and fall, seawater flushes in and out beneath the ice, lifting and lowering it.

"Most projections up to the present have struggled to match the rapidity of the mass loss because of missing physics,"said Eric Rignot, a UC Irvine professor of Earth system science and study co-author.

That missing physics showed up in the record. Conventional models reproduced only about a quarter of the changes actually observed at Thwaites since the 1990s, the researchers reported. When the team added an actively melting grounding zone 4 to 6 kilometers wide, the model matched nearly three decades of observed retreat, thinning and acceleration.

The Ridge Holding Thwaites in Place

The study's most closely watched result involves the Mouginot Ridge, an underwater rise that currently pins part of the glacier in place. In model runs using a grounding zone of the width satellites suggest today, the team found about a 60% chance that Thwaites detaches from that ridge by 2100, with unpinning possible as early as 2038 in some simulations.

"Once that ridge is unpinned, the glacier will continually retreat across a predominantly retrograde seafloor," Poinelli said. A retrograde bed slopes downward inland, so a retreating glacier exposes thicker ice to the ocean as it goes.

The width of the grounding zone drives the result. Simulations with no grounding zone, or a very narrow one, kept the glacier on the ridge through 2100. That sensitivity is the main reason the authors present their findings as a range of probabilities rather than a timetable.

Thwaites as a whole holds enough ice to raise global sea levels by about 65 centimeters (roughly 2 feet), but that would unfold over centuries. The new study addresses only how much it could contribute this century.

Limits of a Single Model

This is a peer-reviewed modeling study calibrated against satellite observations from 1996 to 2023. It is not a direct measurement of melting beneath the ice. The grounding-zone width and melt rate are model inputs constrained by observation, not values drilled and measured on site. The study also used a constant-climate scenario, which the authors describe as a lower-bound baseline that isolates the effect of grounding-zone melting from future ocean warming.

Independent scientists said the work is important but not the final word. Robert Larter, a marine geophysicist at the British Antarctic Survey who was not involved, told CNN that the environment beneath Thwaites is even more complex than the paper describes. "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," Larter said, as reported by CTV News.

Rignot said the next step is collecting observations directly from the grounding line, which is among the hardest places on Earth to reach.

Coastal Planning and the Next Steps

Thirty millimeters sounds small — and on its own, it is. But sea-level rise is cumulative. It adds to thermal expansion of warming oceans, melting from Greenland and mountain glaciers, and land sinking in places like Louisiana and the Chesapeake Bay. Every increment raises the starting point for high tides and storm surge, which is when most coastal flooding damage occurs.

The risk is not evenly shared. Low-lying neighborhoods, coastal roads, older stormwater systems and wetlands that buffer storm surge face the earliest effects. Communities with aging seawalls or no flood insurance coverage face the largest financial burden. Standard homeowners policies generally exclude flood damage, so coastal property owners should know whether they carry separate flood coverage.

The practical takeaway is not alarm. It is that long-range projections are still being revised upward as scientists add processes that older models left out. Readers who live near the coast can check their local flood maps, ask their city or county how sea-level projections are used in planning, and watch for updated federal sea-level guidance as new research is incorporated.

Researchers will continue tracking Thwaites with satellites like ICEYE, and field teams are working to gather measurements beneath the ice. The study's central uncertainty is the true width and melt rate of the grounding zone. Answering that question will determine whether the higher estimate holds.

The bottom line: a new model suggests Thwaites could contribute up to about 1.2 inches of sea-level rise by 2100, twice prior estimates, because seawater reaches farther under the glacier than older models assumed. It is a projection with real uncertainty, not a prediction of imminent collapse.

What Readers Want to Know

What did the new Thwaites study find? A model that includes seawater melting under the glacier's grounding zone projects Thwaites could add up to 30 millimeters (about 1.2 inches) to global sea level by 2100, about double earlier estimates.

Is Thwaites about to collapse? No. The study does not predict a collapse this century. It estimates a 60% chance the glacier detaches from a stabilizing ridge by 2100 under its central assumptions.

What is a grounding zone? It is the band where a glacier shifts from resting on the seafloor to floating. At Thwaites, satellite radar shows it is several kilometers wide and moves with the tides.

How certain is the 30 mm figure? It is an upper estimate of about 30 mm plus or minus 6 mm under a constant-climate scenario. Narrower grounding zones produce smaller results.

How does this affect U.S. coastal residents? Sea-level rise raises the baseline for tides and storm surge. Coastal homeowners should review flood maps and flood insurance, since standard policies usually exclude flood damage.

What happens next? Scientists plan to collect direct observations at the grounding line to measure how wide and how active the melting zone really is.