The stakes could hardly be higher. Every projection of how much sea level will rise this century depends on initialising a computational ice-flow model so that its simulated velocities match the velocities observed from space. The inversion step that performs this initialisation dates back to Douglas MacAyeal’s pioneering control-methods work in 1992, and it now sits inside virtually every major ice-sheet model, including Úa, BISICLES, ISSM, MALI, STREAMICE, Elmer/Ice and WAVI. Yet the design choices vary wildly between these codes. Úa, WAVI and MALI perform a joint inversion for ice stiffness and basal slipperiness over any domain containing grounded ice. STREAMICE and ISSM invert solely for basal slipperiness over grounded ice, on the belief that viscosity cannot be extracted simultaneously. The deep-learning model IGM restricts its inversion to a linear combination of the two fields amid claims that simultaneous inference leads to non-unique solutions. Until now, there has been little theoretical work to arbitrate between these positions.

Schelpe and Gudmundsson approached the problem by linearising the shallow ice stream approximation, the momentum-conservation framework used in vertically integrated ice-sheet models, into a set of transfer functions. These functions describe, in Fourier space, how small perturbations in each hidden field are transmitted to the surface velocity fields. The key insight comes from inspecting where viscosity actually appears in the governing equations: it multiplies only the derivatives of the horizontal velocity fields. In the absence of velocity gradients, the viscosity has no visible effect on the surface to first order. Horizontal gradients of the velocity field are therefore essential for retrieving the viscosity field in any ice-flow model, a point the authors stress as fundamental.

To build a background state with such gradients, the authors considered an idealised but physically meaningful scenario: a uniform slab of ice flowing down an inclined plane, laterally confined within a channel whose side walls exert drag, much like an Antarctic ice stream. In this domain, lateral drag, which is proportional to viscosity, competes with basal drag, which is proportional to slipperiness, to determine the flow. The velocity profile has a simple analytical solution, a hyperbolic cosine shape that slows toward the channel margins, and from it the authors derived four transfer functions: two describing how viscosity perturbations reach the along-flow and transverse surface velocities, and two doing the same for slipperiness perturbations.

The frequency profiles of these transfer functions turn out to be distinct. The viscosity transfer functions drop to zero wherever the background velocity gradients vanish, and they are maximal at the lateral edges of the channel, where horizontal stresses in the ice are greatest. The slipperiness transfer functions, by contrast, are maximal at the centre of the flow, where the velocity is greatest. At very short wavelengths, all transfer functions tend to zero, meaning fine-scale features in either field simply cannot be retrieved through inverse methods because they leave no visible imprint on the surface. Crucially, the two sets of functions are not linear scalings of one another in general, so the hidden fields have distinct frequency profiles and do not mix. In principle, the spatial distribution of viscosity can be extracted uniquely at the same time as the slipperiness distribution.

The authors then framed the inversion as a Gaussian maximum a-posteriori estimation with explicitly stated priors, a Bayesian formulation equivalent to the velocity-misfit minimisation and Tikhonov regularisation used in operational ice-sheet models. In this framework, the retrieved estimate has one component coming from the priors, scaled by the errors in that estimate, and another component that updates the priors using the observed surface measurements, scaled by their measurement errors. The prior covariance matrix, which can be chosen to mimic Tikhonov regularisation, suppresses high-frequency components in the retrieval, a smoothing that proves essential because fine-scale perturbations are poorly transmitted to the surface.

The synthetic experiments revealed a sharp dependence on measurement accuracy and position along the flow. With almost perfect data, errors of ten to the minus four percent, both fields could be perfectly retrieved regardless of position. But even with very small data errors of 0.005 percent, the viscosity retrieval near the centre of the flow became poor, contaminated by significant high-frequency noise, because surface measurements there do not constrain spurious fluctuations from appearing. Applying a prior improved the retrieval dramatically. With realistic errors of one percent, regularisation became essential for retrieving both fields anywhere along the flow. Slipperiness was then reasonably well retrieved all along the flow, while the viscosity retrieval struggled even at the lateral margins. Near the centre of the flow, the trade-off between suppressing noise and recovering the true signal proved impossible to satisfy, and the underlying viscosity signal could not be extracted from the noise.

Importantly, the authors found no evidence of mixing between the viscosity and slipperiness fields. In regions of small background strain rates, the surface data simply provides little additional information about ice viscosity beyond what the prior already encodes, but this does not translate into less accurate estimates of basal slipperiness in those regions. To confirm that the analytical conclusions survive non-linear finite-amplitude effects, the authors repeated the experiments with a full numerical inversion in the Úa ice-sheet model, using adjoint gradients and a direct-adjoint Hessian. With perfect data, the numerical inversion achieved near-perfect retrieval of both fields, and with noise and regularisation it followed the same pattern: viscosity best resolved toward the lateral margins, slipperiness best toward the centre, and no mixing between the fields. In regions of poor transference, the numerical inversion stayed close to the initial field values rather than producing noisy estimates.

The study carries a clear recommendation for the modelling community: always invert for basal slipperiness and viscosity together over grounded ice areas in ice-sheet models, and explicitly recognise prior knowledge of the hidden fields through appropriate priors and regularisation. Models that ignore spatial variation in viscosity during inversion are, whether they admit it or not, prescribing a prior on the viscosity field with one hundred percent confidence. Making that choice explicit, by including viscosity as an unknown field with strong confidence in its prior, or equivalently large regularisation parameters, is the honest approach. The authors also caution that their experiments assumed linear bed sliding and rheology, perfect knowledge of surface elevation, and uncorrelated measurement errors; relaxing these assumptions, or exploring non-linear flow laws with exponents greater than one, could make separating the effects of slipperiness and viscosity harder, although they expect non-linearity would likely differentiate the surface transference of the two fields even further.

For a field in which the degree of regularisation chosen by the modeller has been identified as one of the largest contributors to uncertainty in Antarctic sea level rise projections, the theoretical clarity offered by this study is a significant step. It suggests that the long-running disagreement between modelling groups is not a dispute about mathematical possibility but about how much confidence to place in priors where the ice itself refuses to cooperate. Where lateral drag bends the flow and velocity gradients are strong, the glaciers give up both of their secrets. Where the flow is uniform and wide, the viscosity remains hidden, and only an explicit, well-chosen prior can fill the gap.

Subject of Research: Earth Sciences

Article Title: On the theoretical limitations of joint inversion for basal slipperiness and effective viscosity in ice-flow models

Article References: Schelpe, C. A. O., & Gudmundsson, G. H. (2026). On the theoretical limitations of joint inversion for basal slipperiness and effective viscosity in ice-flow models. The Cryosphere, 20(10), 6003-6020. https://doi.org/10.5194/tc-20-6003-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-6003-2026

Keywords: glaciology, ice-sheet models, inversion methods, sea level rise, The Cryosphere, theoretical, limitations, joint, inversion, basal, slipperiness, effective

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 reason previous studies could not answer this question is technical as much as scientific. Most global climate models do not include an interactive Antarctic Ice Sheet; instead, they prescribe where and how much ice flows into the ocean, which means they cannot capture the slow, centennial-to-millennial feedbacks between ocean temperatures and ice dynamics. Höse and her team broke new ground by coupling the CM2Mc Earth system model, developed at the Geophysical Fluid Dynamics Laboratory and modified at Potsdam, to the Parallel Ice Sheet Model, or PISM, which simulates the entire Antarctic Ice Sheet at a horizontal resolution of 16 kilometers. The two models exchange information through the Potsdam Ice-shelf Cavity model, known as PICO, which calculates how fast ice melts from below based on the temperature and salinity of water entering 19 Antarctic drainage basins. Crucially, the meltwater and heat released by the ice sheet are fed back into the ocean model, closing the loop between the two components.

To force an AMOC collapse, the team followed the protocol of the North Atlantic Hosing Model Intercomparison Project, adding artificial freshwater at a rate of 0.3 Sverdrups, roughly 300,000 cubic meters per second, to the surface of the North Atlantic and Arctic Oceans. This deluge of freshwater stratifies the subpolar North Atlantic, suppressing the deep convection that drives the overturning circulation. Within 100 years, the simulated AMOC weakened from about 21.5 Sverdrups to just 5 Sverdrups, a state the researchers treat as a collapse. They then let the coupled model run for another 1,400 years, long enough for the deep ocean and the ice sheet to respond on their own sluggish timescales, while a parallel control run without freshwater forcing provided a baseline against which every anomaly could be measured.

The immediate consequences of the collapse matched what earlier modeling work had established. Northern Hemisphere surface temperatures dropped sharply, by up to 9 degrees Celsius over the North Atlantic, while the Southern Hemisphere warmed as the oceanic heat transport reversed its imbalance. The Intertropical Convergence Zone shifted southward, the mid-latitude westerly winds over the Southern Ocean strengthened, and the Antarctic Circumpolar Current intensified by roughly 20 Sverdrups. Heat accumulated in the subsurface South Atlantic north of 40 degrees south, exactly as the bipolar seesaw theory, developed to explain the antiphased temperature oscillations recorded in Greenland and Antarctic ice cores, would predict. On the surface, then, everything pointed toward trouble for Antarctica.

But the surface is not where the ice sheet lives. What matters for basal melting, the erosion of ice shelves from below, is the temperature of water at depths of 500 to 1,000 meters, near the cavities beneath floating ice. And here the simulation delivered its first counterintuitive finding: for the first eight centuries after the AMOC collapse, subsurface temperatures along most of the Antarctic margin did not warm at all. In fact, they cooled slightly, by around 0.15 degrees Celsius on average, because residual North Atlantic Deep Water, chilled by the collapsed overturning, was carried southward and upwelled near the continent before being swept around the coast by the Circumpolar Current. As a result, basal melt rates actually declined, by up to 100 gigatonnes per year in some periods, and the total Antarctic ice volume barely changed, remaining within the range of natural variability seen in the control run.

The story took another turn around model year 830, when the Southern Ocean underwent a dramatic regime shift. Deep convection, the vertical mixing of the water column, intensified in the Ross Sea and later spread to coastal regions east of the Weddell Sea, releasing centuries of accumulated heat from the deep ocean to the surface. Antarctic Bottom Water formation surged by more than 10 Sverdrups, maximum sea ice extent abruptly shrank by about 40 percent, and sea surface temperatures around the continent rose roughly four times more than they had in the immediate aftermath of the AMOC collapse. Yet paradoxically, this convective heat release cooled the subsurface waters at ice-shelf cavity depths, producing average negative temperature anomalies of 1.4 degrees Celsius along the continental slope. Basal melting dropped further, with a mean decline of 160 gigatonnes per year, and ice shelves grew thicker.

Herein lies the elegant compensating mechanism that kept the ice sheet stable. As reduced basal melting allowed the ice shelves to expand, their calving fronts extended into deeper water and became more prone to fracturing, so the calving flux increased by an average of 170 gigatonnes per year, almost exactly offsetting the reduction in melt. The net change in sea-level-rise potential after 1,500 years was a decrease of only about 2.5 centimeters, a trivial amount compared with the several meters locked in the West Antarctic Ice Sheet. The authors conclude that the widely assumed positive feedback, in which an AMOC shutdown destabilizes Antarctica through surface warming, is not supported when the full ocean-ice interaction is simulated: it is the subsurface temperature at cavity depths, not the sea surface temperature, that governs the ice sheet’s fate.

The researchers are careful to spell out the limitations of their approach. The coupling between the climate and ice-sheet models is restricted to the ocean-ice interface, so atmospheric changes over Antarctica, including warming of 0.5 to 5 degrees Celsius and increased coastal precipitation, were not transmitted to the ice sheet surface. The artificial freshwater hosing, maintained continuously for 1,500 years, freshened the global ocean without compensation, an amount of water roughly seven times the current freshwater flux from Greenland, and the coarse ocean model does not resolve the Antarctic continental shelf, so bottom water forms through open-ocean convection rather than shelf processes, a known bias shared by about 80 percent of CMIP6 models. The timing of the convective regime shift, and therefore of the subsurface cooling, may also be model-specific, and the authors emphasize that repeating the experiment with other coupled models is essential before general conclusions can be drawn.

Nevertheless, the study marks a milestone: the first freshwater hosing experiment ever conducted with an interactively coupled climate-ice sheet model capable of resolving the millennial response of Antarctica. Its message cuts both ways for the climate conversation. On one hand, it suggests that a collapsing AMOC would not, at least through the ocean pathway alone, immediately push the West Antarctic Ice Sheet over its tipping point, which previous work places at an oceanic warming of about 1.5 degrees Celsius above preindustrial levels. On the other hand, it warns that simple assumptions about tipping cascades, in which one tipping element triggers another through a single variable such as sea surface temperature, may be dangerously oversimplified in a system where deep convection can invert the sign of a temperature anomaly within a few centuries. As freshwater pours from Greenland and Antarctic ice shelves melt at accelerating rates, understanding these cross-hemispheric connections has never mattered more, and this first coupled glimpse suggests the true dynamics are far stranger than the seesaw metaphor implies.

Subject of Research: The impact of an AMOC collapse on the stability of the Antarctic Ice Sheet simulated with a coupled climate-ice sheet model

Article Title: Simulating the impact of an AMOC weakening on the Antarctic Ice Sheet using a coupled climate and ice-sheet model

Article References: Höse, A., Kreuzer, M., Huiskamp, W., Petri, S., & Feulner, G. (2026). Simulating the impact of an AMOC weakening on the Antarctic Ice Sheet using a coupled climate and ice-sheet model. Earth System Dynamics, 17(4), 1025-1059. https://doi.org/10.5194/esd-17-1025-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1025-2026

Keywords: AMOC, Antarctic Ice Sheet, climate tipping points, bipolar seesaw, Southern Ocean, ice sheet modeling, basal melt, Antarctic Bottom Water, deep convection, sea level rise, PISM, freshwater hosing

Firn models are numerical tools that simulate how snow densifies into glacial ice, how heat moves through the column, and how meltwater percolates and refreezes. They are used to interpret satellite observations, to understand firn physics, and to project future surface mass balance of the ice sheets. Every such model integrates coupled differential equations in time, and the boundary conditions at the surface, such as snowfall, melt, temperature, and wind speed, must be supplied at some discrete interval. In the published literature, that climate forcing time step varies enormously, from one hour to days, months, and even years. The reasons for these choices are usually practical rather than physical: forcing data may not exist at finer resolution, files become unwieldy, or computational budgets run out. Until now, the implications of this choice had barely been evaluated.

The research team tested the sensitivity of the IMAU firn densification model, or IMAU-FDM, by forcing it with output from the regional climate model RACMO2.3p2, itself downscaled from the ERA5 reanalysis. They ran simulations for the Antarctic Peninsula Ice Sheet and the southern Greenland Ice Sheet, two regions spanning a wide range of accumulation and melt conditions, using four different forcing time steps: 3 hours, 6 hours, 1 day, and 1 month. The model itself integrates at a 15-minute step, but the climate inputs were averaged to the coarser intervals and held constant between updates. The team focused on firn air content, the vertically integrated pore space of the firn column, a key quantity that determines how much meltwater the firn can absorb before runoff begins.

The results were striking. Across both regions, larger forcing time steps systematically produced more firn air in the final simulated firn layer, and the size of the discrepancy grew with the time step. On the Antarctic Peninsula, the effect was largest on the low-lying ice shelves, where firn air content is naturally small: switching from 3-hourly to daily forcing inflated firn air content by about 15 percent, and monthly forcing by up to 44 percent. In the southern Greenland Ice Sheet, the largest differences appeared in a band at intermediate elevations, where melt is substantial but the firn is not yet fully depleted. There, daily forcing added on average 2.1 meters of firn air compared to 3-hourly forcing, and monthly forcing added 4.0 meters, differences that are far from trivial for meltwater retention studies.

The root cause, the authors show, is the presence or absence of a diurnal cycle in the input data. With 3-hourly or 6-hourly forcing, surface temperatures swing through the daily cycle, reaching the melting point during melt events and dropping below freezing at night. Melt and refreezing therefore alternate in time. With daily or monthly averages, however, temperature and melt become decoupled: the model can simultaneously apply a melt flux and hold the surface below zero degrees, a physically impossible combination. In that unphysical state, any meltwater that the pore space can retain refreezes immediately near the surface, forming a dense, ice-like crust. Subsequent melt events then strip away this dense refrozen material rather than the porous firn beneath it, so the model loses less firn air than it should. The result is an overestimate of the firn’s water-buffering capacity, an artifact born purely of the forcing resolution.

Deeper in the column, a second mechanism amplifies the differences. With 3-hourly forcing, meltwater percolates farther down before refreezing, releasing its latent heat at depth. Because firn conducts heat poorly, that trapped warmth makes the deeper firn warmer, which accelerates densification. Simulations with coarse forcing, by contrast, refreeze water near the surface and keep the deeper column colder, slowing compaction. The team also uncovered a subtler artifact tied to the fresh snow density parameterization used for Antarctica, which depends on instantaneous surface temperature and 10-meter wind speed. Averaging the wind speed over longer intervals changes the snowfall-weighted wind speed, altering the modeled density of newly fallen snow. In nearly melt-free zones this effect can even reverse the sign of the bias, producing slightly less firn air at fine resolution, on the order of half a meter.

The stakes become clearest in Greenland, where melt rates above roughly 500 kilograms per square meter per year can sustain firn aquifers, perennial reservoirs of liquid water buried in the firn. In the team’s simulations, aquifers formed during the spin-up period under 3-hourly forcing but never developed under daily forcing, because the deeper refreezing associated with fine resolution leaves the firn column warmer and less able to freeze all incoming water. Where aquifers did form under both resolutions, the timing and evolution diverged: under fine forcing the aquifer existed from the start of the historical simulation, while under daily forcing it emerged only around 2005. Because liquid water pins the firn temperature to the freezing point, the densification equations respond differently in each case, and the simulated firn air content trends can even acquire opposite signs. The modeled runoff limit, the elevation above which meltwater no longer escapes to the sea, shifted more than 10 kilometers inland between 3-hourly and daily forcing, and up to 35 kilometers compared to monthly forcing.

These discrepancies ripple directly into satellite altimetry, one of the primary tools for tracking ice sheet mass balance. Altimeters measure changes in surface height, but part of that signal reflects firn compaction rather than genuine ice loss, so firn models supply the correction. The study found that surface height changes differed by 0.2 to 0.4 meters on the Antarctic Peninsula depending on forcing resolution, and by 8 to 30 percent of the firn air content at a Greenland aquifer site. Corrections based on coarsely forced simulations would overestimate surface height gains, which means that during periods of thinning, ice loss would be underestimated, and during thickening, ice gain would be underestimated. In a warming Arctic where Greenland’s contribution to sea level rise is closely watched, a bias of this kind matters for projections and for the satellite records that policymakers rely on.

The authors distill two lessons for the modeling community. First, firn models that are prescribed melt fluxes and surface temperatures, as IMAU-FDM is, need a forcing time step fine enough to resolve at least the diurnal cycle; otherwise melt and subfreezing temperatures coexist and the physics breaks down. Models that compute melt internally from a surface energy balance may be less vulnerable to this specific artifact, though they face their own accuracy challenges. Second, parameterizations should be applied only under the conditions and resolutions for which they were developed. The Antarctic fresh snow density formula was tuned for 3-hourly forcing, and the densification equation assumes dry, steady-state snow, yet both are routinely stretched beyond those limits. As climate change accelerates melt across the polar regions, the study argues, the humble choice of a time step deserves the same scrutiny as the equations themselves, because it quietly shapes how much water the firn can hold, where runoff begins, and how vulnerable ice shelves are to collapse.

Subject of Research: Effect of climate forcing time step resolution on firn air content simulated by an ice-sheet firn densification model

Article Title: Impact of climate forcing time step in an ice-sheet firn model

Article References: van den Aker, T. E. A., Kuipers Munneke, P., van de Berg, W. J., Immerzeel, W. W., & van den Broeke, M. R. (2026). Impact of climate forcing time step in an ice-sheet firn model. The Cryosphere, 20(9), 5475-5490. https://doi.org/10.5194/tc-20-5475-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-5475-2026

Keywords: firn, ice sheet, Greenland, Antarctic Peninsula, firn air content, firn model, climate forcing, time step, meltwater retention, firn aquifer, satellite altimetry, The Cryosphere

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