In some NASA simulations, Antarctica’s Thwaites Glacier added twice as much sea-level rise as current projections

In some computer simulations run at NASA’s Jet Propulsion Laboratory, Antarctica’s Thwaites Glacier contributed as much as twice the sea-level rise that current projections give by the end of the century. NASA described the result on Oct. 20, 2026, in a science news post about a study published in Nature Communications on Oct. 12. The simulations in question are the ones that include a feature earlier models leave out, and they are not forecasts of what will happen.

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The glacier has drawn attention for years because its retreat could affect how fast oceans rise. The new work does not change that basic concern. It argues that the way existing models treat the edge of the grounded ice may understate how much water the glacier could add, and it puts numbers on the difference. Those numbers come with error bars, and the authors give them, which is why the careful reading of any single value matters.

Grounding zones in the JPL model

According to NASA’s account by James Riordon, the model was developed at JPL with the University of California, Irvine, and Dartmouth College. Existing models treat the grounding line, where ice lifts off the bedrock and begins to float, as a sharp boundary. The new one instead represents grounding zones, places where seawater reaches several miles under ice that is still grounded, lifting it with the tides.

That intrusion matters because it lets warmer water reach the ice and speed melting. NASA said the model matched satellite observations dating to the 1990s four times more closely than models without this factor. In its words, Thwaites could contribute more to sea-level rise by the end of the century than some previous estimates, and in some model simulations the contribution was as much as twice current projections.

The numbers in the Nature Communications paper

The paper, titled “Sea-level contribution from Thwaites Glacier doubles in an observation-constrained icesheet model”, lists Mattia Poinelli, Eric Rignot, Hélène Seroussi, Joshua Cuzzone, Tyler Pelle and Eric Larour as authors. It is catalogued under the DOI 10.1038/s41467-026-77976-1 and appears at the journal’s Nature Communications page. The authors report that the model reproduces about three decades of observed retreat, thinning and acceleration only when grounding zones are 2 to 5 kilometers wide.

Projected to 2100 under constant climate forcing, the glacier in the model detaches from the Mouginot Ridge as early as 2038. The sea-level contribution depends on how wide the grounding zone is assumed to be: 8.3 millimeters with none, 14.3 with a 1-kilometer zone, 19.4 with 3 kilometers and up to 30 millimeters, plus or minus 6, with 5 kilometers. The paper sets those against a mean of 15.2 millimeters, with a spread of 21.8, from the multi-model ensemble known as ISMIP6.

Those figures show why the qualifier matters. The doubling appears when the widest grounding zones are used, not across every run. The no-grounding-zone version, at 8.3 millimeters, lands below the ensemble mean, and the ensemble’s own uncertainty is larger than its mean.

Scale helps here. The 2020 ensemble found that the Antarctic ice sheet as a whole could contribute between minus 7.8 and 30.0 centimeters of sea-level equivalent over 2015 to 2100 under a high-emissions scenario, relative to a constant-climate control. Against that range, the top Thwaites figure of 30 millimeters, or 3 centimeters, is about a tenth of the upper bound for the whole continent. The single glacier is small next to the full range, yet it is large enough that a doubling would be visible in coastal planning.

What the result does and does not say

The ISMIP6 ensemble itself is a published benchmark. Its Antarctic paper in The Cryosphere in 2020, led by Hélène Seroussi of JPL and Sophie Nowicki of NASA Goddard, drew on 13 international modeling groups and found that West Antarctica showed the widest spread among models. Seroussi is also an author of the new paper, so the new model is a refinement from inside the same research community rather than an outside challenge. The earlier paper also stressed that its projections depend on climate forcing, ocean-induced melt rates and how those rates are calibrated, which are the same levers the grounding-zone work adjusts.

The study is a model result, and constant climate forcing is a simplification. It does not say Thwaites will add 30 millimeters, roughly an inch, by 2100, and the researchers do not present a single number as the expected outcome. What the paper argues is that models should include actively melting grounding zones to match what satellites have seen, and that doing so raises the upper end of the range.

The work arrived with press coverage that used the glacier’s “Doomsday” nickname, including a CNN report carried by CTV News on Oct. 15. NASA’s own account is more measured, and the practical consequence is for the modelers. Ice-sheet projections feed coastal planning, and a feature that changes the answer by a factor of two in some runs will need testing by other groups with other models before the range is revised.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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