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: Poinelli, M., Rignot, E., Seroussi, H., Cuzzone, J., Pelle, T., & Larour, E. (2026). Sea-level contribution from Thwaites Glacier doubles in an observation-constrained ice-sheet model. Nature Communications, 17(1), Article 10364. 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

The Antarctic coastal zone is far more than a simple geographic boundary marking the edge of a continent. According to the review, it is a tightly coupled physical system in which grounded ice, floating ice shelves, the ocean beneath them, the overlying atmosphere, and the bedrock below all interact continuously. At the heart of this system is the grounding zone, the critical line where ice resting on the bedrock lifts off and begins to float. Processes occurring near this transition can regulate the discharge of ice into the ocean or, under certain conditions, trigger self-reinforcing feedbacks that accelerate ice loss. Because these feedbacks can amplify small perturbations into large and potentially irreversible changes, the coastal zone effectively acts as a control point for the entire ice sheet’s contribution to global sea level.

Observations accumulated over recent decades show that Antarctica’s mass loss has increased, and the review emphasizes that the most rapid changes are driven not by surface melting alone but by interactions between the ice and the surrounding ocean, and by the flow of ice at the margins of the ice sheet. This finding places the coastal zone squarely at the center of the sea level problem. Yet despite its outsized importance, key conditions along the coast remain poorly mapped, leaving scientists unable to constrain the very processes that matter most for projecting how quickly ice will be lost in the coming centuries.

Among the most persistent gaps identified in the review are direct observations of the bed topography beneath coastal ice and of the sub-ice shelf cavities where ocean water circulates under floating ice. These hidden landscapes and water volumes are not scientific curiosities; they are the boundary conditions that determine how ice flows, where warm water can reach vulnerable ice, and how quickly grounding lines can retreat. Ice sheet models are highly sensitive to conditions at the coast, which means that even the most sophisticated computer models can produce misleading results when the underlying data is missing or poorly constrained. In effect, the models are being asked to predict the behavior of a system whose geometry is only partially known.

Satellite observations, for all their transformative power in measuring ice motion and surface change, cannot fill this gap. As first author Kenichi Matsuoka of the Norwegian Polar Institute, who chairs the RINGS Action Group, explains, satellites cannot observe the bedrock beneath the ice from space. Direct, targeted field observations remain the only way to map the subglacial topography and ocean cavity geometry that models require. Without them, both satellite-based estimates of ice loss into the ocean and model-based predictions of future change rest on incomplete foundations, propagating uncertainty into every downstream projection of global sea level.

The consequences of this uncertainty are not academic. Sea level rise projections feed directly into coastal planning, infrastructure investment, and climate adaptation decisions for hundreds of millions of people living in low-lying regions worldwide. When Antarctic coastal conditions are poorly constrained, projections must span wider ranges of possible outcomes, complicating risk assessments and potentially leading to costly over- or under-preparation. Sharpening the observational picture of the Antarctic coast is therefore one of the most direct ways to narrow the range of future sea level scenarios and give decision-makers more actionable information.

The scale of the challenge is matched by the scale of the response the review proposes. Because no single nation possesses the logistics, funding, or expertise to achieve comprehensive observational coverage of Antarctica’s vast and remote coastline, the authors argue that international coordination is essential. Uncoordinated surveys, they warn, risk leaving critical gaps or duplicating effort, wasting scarce resources in one of the most expensive environments on Earth to work in. The paper therefore provides an evidence-based framework designed to support the coordination of RINGS activities under SCAR, with logistical knowledge supplied through the group’s endorsement by the Council of Managers of National Antarctic Programs (COMNAP), the body that coordinates the operational support of national Antarctic programs.

This coordination framework is intended to do more than organize individual surveys. By helping national programs plan efficient, complementary campaigns and share the resulting data, the effort aims to build comprehensive pan-Antarctic datasets that can directly improve sea level projections. Matsuoka notes that such internationally coordinated efforts can also serve as a steppingstone toward the next International Polar Year in 2032-33, the kind of concentrated, multinational polar science campaign that historically has produced breakthroughs in understanding the cryosphere. In this sense, the review functions both as a scientific synthesis and as a strategic roadmap for the next decade of Antarctic research.

The collaborative breadth of the project underscores how central Antarctica has become to the global climate science agenda. Contributing institutions range from the British Antarctic Survey and NASA’s Goddard Space Flight Center to the Norwegian Polar Institute, the Alfred Wegener Institute, the Polar Research Institute of China, and universities across Australia, Japan, India, South America, and Europe. The authors declare no conflicts of interest relevant to the study, and the work was conducted as a systematic, community-led review rather than the product of any single research group, reflecting a growing consensus that the sea level problem can only be solved collectively.

What emerges from the review is a clear-eyed assessment of where the science stands and what it lacks. The physics of ice-ocean interaction at the Antarctic margin is increasingly well understood in principle, and models have grown steadily more capable. But capability without data is a ceiling, not a solution. The review’s central message is that the fastest route to better sea level projections runs directly through the grounding zones and sub-ice shelf cavities that no satellite can see. Filling those blind spots with coordinated, targeted observations, the authors argue, is the key missing piece in humanity’s attempt to anticipate one of the defining consequences of a warming planet.

Subject of Research: Antarctic coastal zone observations and their role in constraining ice loss and global sea level rise projections

Article Title: New scientific review identifies Antarctic coastal blind spots limiting sea level projections

Article References: New scientific review identifies Antarctic coastal blind spots limiting sea level projections. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Antarctica, sea level rise, ice sheet, grounding zone, ice shelves, bed topography, ice-ocean interaction, SCAR, RINGS Action Group, climate change, polar research, ice sheet modeling