The key to the finding lies in a zone of the glacier that is only a few kilometers wide but exerts an outsized influence on its fate: the grounding zone. This is the region where the ice sheet, resting on the seafloor, transitions to floating ice. Traditional models treat the grounding line as a sharp, discrete boundary between grounded and floating ice. Yet recent high-resolution satellite observations from the US-Finnish ICEYE synthetic aperture radar constellation have revealed something different in reality. As tides rise and fall, the grounding line migrates back and forth, and seawater of warm, modified Circumpolar Deep Water origin penetrates several kilometers beneath what is nominally grounded ice, episodically lifting it and actively melting it from below.

To test whether this dynamic grounding zone matters for the glacier’s long-term evolution, the researchers used the Ice-sheet and Sea-level System Model (ISSM), a high-resolution model in which the mesh resolves features as small as 500 meters near the grounding line. They implemented an actively melting grounding zone in which basal melt decreases linearly from a maximum at the grounding line to zero at a specified intrusion distance, building on earlier numerical frameworks. They then ran an ensemble of 105 simulations spanning melt rates from 10 to 150 meters per year and grounding-zone lengths from zero to six kilometers, and asked a simple question: which combination of parameters can reproduce nearly three decades of observed retreat, thinning, and acceleration of Thwaites Glacier between 1996 and 2023?

The answer was unambiguous. Simulations without a grounding zone, or with an intrusion of only about one kilometer, systematically failed to match the observational record. They underestimated the glacier’s ice discharge and thinning between 2004 and 2022, missed the observed acceleration, and required implausibly high submarine melt rates of around 150 meters per year to even approximate the early retreat. In contrast, simulations with grounding zones two to five kilometers wide, combined with melt rates of roughly 90 to 120 meters per year — values consistent with independent satellite estimates and high-resolution ocean modeling — successfully reproduced the concurrent retreat of both the fast-flowing Thwaites Main Trunk and the slower Thwaites Eastern Ice Shelf, along with the roughly 32 meters of grounded-ice thinning observed over the period.

The team formalized this comparison using a Bayesian calibration framework, drawing 10,000 random samples of melt rate and grounding-zone length and weighting them by how well each reproduced four independent observables: grounding-line retreat, ice discharge, thickness change, and glacier speed. The resulting posterior distribution revealed a well-constrained region of parameter space centered on grounding-zone lengths of two to five kilometers and melt rates of 90 to 120 meters per year. Strikingly, simulations lacking a grounding zone were four times less likely to reproduce the historical trend than simulations with a kilometer-scale intrusion. The analysis also exposed a trade-off between the two parameters — longer intrusions can compensate for somewhat lower melt rates — underscoring the value of calibrating against multiple variables simultaneously rather than a single metric.

With the model validated against the historical record, the researchers extended the simulations to 2100 under a deliberately conservative, lower-bound forcing scenario: ocean and atmospheric conditions held constant at their 1996–2023 averages. Even under this no-extra-warming assumption, the projections diverged sharply from conventional results, and the reason traces back to the seafloor topography beneath the glacier. Thwaites’ main trunk currently rests against the Mouginot Ridge, a bathymetric high a few kilometers across, lying roughly 950 meters below sea level, that acts as a pinning point slowing the retreat. The simulations show that whether the glacier detaches from this stabilizing ridge is governed almost entirely by the length of the grounding zone.

Simulations without a grounding zone, or with a one-kilometer intrusion, never retreat beyond the Mouginot Ridge by 2100, regardless of the melt rate imposed. But simulations with two- to three-kilometer grounding zones show about a 45 percent probability of unpinning by century’s end, with average unpinning dates of 2088 and 2065 respectively. For grounding-zone lengths of four to six kilometers — consistent with the most recent satellite-inferred extents — the probability of crossing the ridge rises to roughly 60 percent, and at five to six kilometers the glacier can unpin as early as 2038. Once free of the ridge, the grounding line retreats across predominantly retrograde, deepening seafloor toward the next major topographic barrier, the South Ridge, with no indication of slowdown or stabilizing feedback anywhere in the ensemble.

The sea-level consequences follow directly. With no grounding zone represented, the model yields a contribution of just 8.3 ± 3.0 millimeters by 2100, below the mean of the ISMIP6 intercomparison ensemble. A narrow one-kilometer grounding zone brings the estimate to 14.3 ± 3.1 millimeters, comparable to the ISMIP6 mean of 15.2 ± 21.8 millimeters. But a three-kilometer grounding zone — the maximum of the Bayesian posterior — produces 19.4 ± 5.2 millimeters, a 130 percent increase over the ISMIP6 mean, and a five-kilometer zone, matching the latest observations, yields 30.1 ± 5.7 millimeters, twice the multi-model average. Sensitivity tests revealed that the projections are far more sensitive to grounding-zone extent than to ocean temperature: each additional kilometer of intrusion changes the projected sea-level contribution by about 22 percent on average, whereas a plausible 0.5 to 0.7 degree Celsius of future ocean warming would raise ice loss by only about 15 percent. Simply introducing a grounding zone at all increases the projected contribution by 51 percent relative to traditional formulations.

The study is not without caveats, which the authors lay out carefully. The grounding-zone length is held fixed in time, whereas in reality the intrusion extent likely varies with bed slope, tidal amplitude, and subglacial water pressure; the team interprets it as an effective, time-averaged scale that captures decadal behavior. The ice front is fixed, so calving and the possible loss of pinning points that buttress the floating ice shelf are excluded by design, allowing the isolated effect of grounding-zone melting to be quantified. The melt parameterization is simplified rather than a fully coupled ocean model, and basal friction is not reduced by seawater lubrication, which may lead the model to underestimate thinning and speed-up. Bathymetric uncertainties also required a localized correction of up to 100 meters to the seafloor elevation beneath floating ice, within the error bounds of gravity inversion methods, to achieve consistency with the observed 1996 grounding line.

Even so, the implications for sea-level science are difficult to overstate. The work demonstrates that a process operating over just a few kilometers — invisible to coarse models and long treated as a detail — fundamentally changes both how well models can reproduce the past and how much ice loss they project for the future. The authors argue that actively melting grounding zones should be systematically incorporated into regional and continental-scale ice-sheet models, including international intercomparison exercises such as ISMIP6, and that expanded in-situ measurements near grounding zones are urgently needed to constrain heat delivery to the ice-shelf cavities. For a glacier holding the equivalent of 65 centimeters of global sea-level rise, the message is clear: the most important action at Thwaites may be happening in the narrow, tidally flooded strip of seafloor where ocean and ice meet, and until models capture it, the world’s best projections of Antarctic sea-level contribution may be undercounting the risk.

Subject of Research: The role of kilometer-scale seawater intrusions at the grounding zone of Thwaites Glacier in driving ice retreat and sea-level rise

Article Title: Sea-level contribution from Thwaites Glacier doubles in an observation-constrained ice-sheet model

Article References: Sea-level contribution from Thwaites Glacier doubles in an observation-constrained ice-sheet model. (n.d.). https://doi.org/10.1038/s41467-026-77976-1

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77976-1

Keywords: Thwaites Glacier, sea-level rise, grounding zone, Antarctica, ice-sheet modeling, seawater intrusion, ISSM, Bayesian calibration, Mouginot Ridge, ocean-driven melting, West Antarctic Ice Sheet, ISMIP6