From short-term uncertainties to long-term certainties in the future evolution of the Antarctic Ice Sheet

Domain: earth_science
Published: 2025-12-05
Editorial outcome: minor_revision

Abstract
Robust projections of future sea-level rise are essential for coastal adaptation, yet they remain hampered by uncertainties in Antarctic ice-sheet projections–the largest potential contributor to sea-level change under global warming. Here, we combine two ice-sheet models, systematically sample parametric and climate uncertainties, and calibrate against historical observations to quantify Antarctic ice-sheet changes to 2300 and beyond. By 2300, the projected Antarctic sea-level contributions range from -0.09 m to +1.74 m under low emissions (SSP1-2.6, outer limits of 5-95% probability intervals), and from +0.73 m to +5.95 m under very high emissions (SSP5-8.5). Irrespective of the wide range of uncertainties explored, large-scale Antarctic ice-sheet retreat is triggered under SSP5-8.5, while reaching net-zero emissions well before 2100 strongly reduces multi-centennial ice loss. Yet, even under such strong mitigation, a significant sea-level contribution could still result from West Antarctica. Our results suggest that current mitigation efforts may not be sufficient to avoid self-sustained Antarctic ice loss, making emission decisions taken in the coming years decisive for future sea-level rise.

Introduction
Observations over the past decades show that the Antarctic Ice Sheet is losing mass1, mainly due to ocean-driven ice-sheet retreat in West Antarctica2 and parts of East Antarctica3,4, thereby contributing to global sea-level rise. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR6) concluded that Antarctic mass loss is likely (66–100% outcome probability, see ref. 5 for definitions of likelihood terms and confidence levels) to continue throughout this century under all emission scenarios. By 2100, this contribution could amount to several tens of centimeters of sea-level rise, with the medium confidence projections ranging from −0.01 m to +0.41 m for Shared Socio-economic Pathway SSP1-2.6 and from 0.00 m to +0.57 m for SSP5-8.5 (5–95% percentile range). In a low confidence assessment that includes ice-sheet processes characterized by deep uncertainty6, the upper bound may be higher7,8.

The bulk of Antarctic mass loss, however, is expected beyond the end of this century6, both because slowly-unfolding self-sustained ice loss may already have been triggered in recent decades9,10 or could be initiated in the coming ones11,12, and because changes in Antarctic climate are expected to intensify after 2100, particularly under higher-emission scenarios6.

Yet, no projections of the Antarctic sea-level contribution over this time are provided beyond 2150 in IPCC-AR6. Only the ice-sheet contribution at 2300 was assessed, ranging from −0.3 m to +3.1 m sea-level equivalent, and assigned low confidence, without any likelihood statement6. Given the substantial biophysical and socio-economic impacts of sea-level rise for low-lying coasts13, this constitutes an important gap in global sea-level projections. The lack of confidence in multi-centennial Antarctic projections is because, at the time of IPCC-AR6, only a few individual studies had assessed Antarctic ice loss beyond 2100 under continued global warming, and these studies projected a wide range of outcomes (e.g., refs. 7, 14, 15). By contrast, most ice-sheet projections focused on shorter timescales, typically on the order of several decades16–18.

Since IPCC-AR6, the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6)–a multi-model ensemble–has provided extended projections of Antarctic sea-level contributions through 2300, with a main emphasis on very high emissions (SSP5-8.5 and the Representative Concentration Pathway RCP8.5)19. The focus on very high emissions6,19 mean that we face an imbalance between emission scenarios in sea-level projections, and lack estimates of the Antarctic sea-level contribution to be expected when constraining emissions.

Irrespective of the projection timescales considered, parametric uncertainties, for example, related to surface runoff, sub-shelf melt, or iceberg calving, remain largely unaccounted for in projections of Antarctic mass changes across multiple continental-scale ice-sheet models20. While some individual ice-sheet models have explored uncertainty in unconstrained parameters (e.g., refs. 7, 10, 21), existing multi-model intercomparisons typically include a wide range of ice-sheet models without consistently accounting for parametric uncertainty (e.g., refs. 16, 19).

Furthermore, simulated ice-sheet changes over the past decades have rarely preceded Antarctic projections16,19, even though historical trajectories have been suggested to influence the projected ice loss22. Covering the historical period would allow for conditioning projections of Antarctic mass changes on observations, addressing potential biases in the projected distribution of the ice-sheet contribution to sea-level rise20. So far, only a few studies have calibrated projections on the continental scale with transient satellite observations7,23,24, and even fewer in a Bayesian framework10,25.

To fill the gap in multi-centennial projections of Antarctic mass changes26–28 and provide robust estimates of future sea-level change, we need to systematically account for as many sources of uncertainty as possible (such as future emissions, climate model, ice-sheet model structure and parametric uncertainty) while calibrating with past (observed) mass changes20. Here, we provide a robust assessment of the future evolution of the Antarctic Ice Sheet that, for the first time, jointly accounts for parametric, ice-sheet model structure, and climate model uncertainties in historically-calibrated projections through 2300 under a wider range of emissions, from low to very high. Future Antarctic climate trajectories to 2300 under the extended Shared Socio-Economic Pathways SSP1-2.6 and SSP5-8.529 are derived from four General Circulation Models (GCMs) from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and used to drive two ice-sheet models, with uncertainties in ice–climate interactions explored through a wide range of parameter perturbations. We quantify the contribution of these various sources of uncertainty to the projected range in ice-sheet changes using an analysis of variance, and assess the long-term consequences of the warming projected over the next centuries (that is, the committed ice-sheet response), by maintaining the changes in climate projected for 2300 through the end of the millennium.

Results
Future Antarctic climates
The Antarctic Ice Sheet may face a wide range of future climates over the coming centuries, depending on the emission pathway and strongly modulated by the climate model used (Fig. 1). Here, we base our analysis on a subset of CMIP6 GCMs – the UK Earth System Model (UKESM1-0-LL; ref. 30), the Institut Pierre-Simon Laplace global climate model (IPSL-CM6A-LR; ref. 31), the Community Earth System Model (CESM2-WACCM; ref. 32) and the Meteorological Research Institute Earth System Model (MRI-ESM2-0; ref. 33), which provide some of the few projections up to 2300 available in CMIP6 at the time of this analysis34. These GCMs span a broad range of climate sensitivities, representative of the CMIP6 ensemble35. Despite significant improvements over previous generations, CMIP6 GCMs still show biases in Antarctic climate (e.g., refs. 36–38). For example, the upper Southern Ocean is generally too warm and fresh36, and the Amundsen Sea Low remains poorly captured37.

During the first half of this century, projected Antarctic atmospheric and oceanic warming is similar across the two emission scenarios. Divergence between the higher-emission (SSP5-8.5) and the lower-emission (SSP1-2.6) pathway emerges after 2050, eventually leading to significant changes in Antarctic climate on multi-centennial timescales (Fig. 1). Under SSP1-2.6, the four GCMs considered project a limited Antarctic-averaged atmospheric and oceanic temperature change of +1.0°C–+3.6°C and +0.2°C–+0.7°C, respectively, by 2300 compared to the 1995–2014 period. In contrast, the SSP5-8.5 scenario leads to substantial changes after the end of this century, with Antarctic-averaged atmospheric warming ranging from +12.0°C to +17.0°C and ocean warming from +1.7°C to +3.1°C by 2300 (Fig. 1).

Across the emission pathways, the projected changes in Antarctic climate over the next three centuries show similar characteristics for each CMIP6 GCM considered. Compared to the other three CMIP6 GCMs, MRI-ESM2-0 projects relatively slow and limited changes, especially under very high emissions (with Antarctic oceanic and atmospheric warmings of +1.7°C and 12.2°C, respectively, by 2300). Within each emission pathway, CESM2-WACCM predicts the strongest atmospheric warming of the four GCMs, along with a steady increase in circum-Antarctic ocean temperatures, reaching up to +0.7°C under SSP1-2.6 and +2.0°C under SSP5-8.5 by 2300. In contrast, projections by IPSL-CM6A-LR and UKESM1-0-LL show early and abrupt ocean warming in the first half of this century under both emission pathways. Under SSP5-8.5, these two GCMs yield the strongest multi-centennial Antarctic ocean warming, reaching +2.7°C and +3.1°C, respectively, by 2300.

Bayesian calibration of the multi-model ensemble
To quantify the Antarctic contribution to sea-level rise and its associated uncertainties under these various future climates, we perform a 1400-member ensemble of simulations using two state-of-the-art ice-sheet models: Kori-ULB10,39 and the Parallel Ice Sheet Model40,41 (PISM; see Methods). Future Antarctic climates are derived from the four CMIP6 GCMs described above under the emission pathways SSP1-2.6 and SSP5-8.5 (Fig. 1). Key model parameters controlling the interaction of the ice sheet with the atmosphere and the ocean (Table 2) are varied using Latin Hypercube designs, which sample efficiently across the parameter space42 (as used previously for ice-sheet projections by, for example, refs. 10, 43). Our simulations begin in 1950 to allow, for each ice-sheet model, a Bayesian calibration with observations from the satellite era. Each ensemble member is attributed a weight by comparing the simulated ice-sheet evolution over the past decades with a series of mass balance estimates from the latest Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE1; see Methods and ref. 10).

Through this calibration, the ice-sheet model ensembles are brought closer to the observed ice-sheet mass changes over the past decades (Supplementary Fig. 1a, b). This improved agreement between modeled and observed mass changes is quantitatively confirmed by a reduction in the Continuous Ranked Probability Score (CRPS) following the calibration (Supplementary Fig. 1e–l; see Methods for details). In addition, conditioning on the IMBIE record is effective in reducing the uncertainty in the hindcasts, especially for the Kori-ULB ensemble (Supplementary Fig. 1a) given the wider range of ice–ocean uncertainties explored (Table 2; see Methods).

Some regional biases in Antarctic mass changes, however, persist in the calibrated ensembles. For example, PISM simulations tend to underestimate the observed West Antarctic ice-sheet thinning (Supplementary Fig. 1d, k), whereas Kori-ULB trajectories capture this trend more accurately (Supplementary Fig. 1c, g). Conversely, East Antarctic mass gain is slightly overestimated in the calibrated Kori-ULB ensemble compared to the IMBIE estimates (Supplementary Fig. 1f). These discrepancies between simulated and observed historical ice-sheet changes likely contribute to the overestimation of the ice-sheet net mass balance for both ice-sheet models, especially after 2000 (Supplementary Fig. 1a, b; see e.g., median of posterior distribution). Between 1992 and 2020, the calibrated median net mass balance amounts to −77 Gt yr−1 (−189 Gt yr−1–+1 Gt yr−1, 5–95% percentile range) for the Kori-ULB ensemble and to −59 Gt yr−1 (−157 Gt yr−1–+26 Gt yr−1) for the PISM ensemble, compared to the observed value of −92 ± 18 Gt yr−11 (see Supplementary Fig. 1e–l).

These regional discrepancies may lead to compensatory effects at the continental scale. For example, the calibration tends to favor PISM ensemble members that overestimate mass loss in the Antarctic Peninsula to offset the underestimation of West Antarctic mass loss. In contrast, for Kori-ULB, ensemble members that slightly overestimate West Antarctic mass loss are favored to counteract the model’s bias towards mass gain in East Antarctica. Such compensations may reduce the ability of the calibration to fully capture regional dynamics, and can lead to biases in specific regions despite overall agreement at the continental scale (see Supplementary Fig. 1e–l and Methods).

Projected Antarctic mass changes to 2300
Our multi-model ensemble supports emerging evidence suggesting little scenario-dependence of the Antarctic contribution to sea-level rise by 210010,17,24 (Fig. 2a, zoom-in). In particular, in the coming decades, ice-sheet trajectories under both emission pathways substantially overlap, with projected sea-level contributions ranging between −0.03 m and +0.33 m for SSP1-2.6 and between −0.05 m and +0.37 m for SSP5-8.5 by 2100 (Table 1). On multi-decadal timescales, mass loss remains confined to West Antarctica as well as the Antarctic Peninsula and is offset to some extent by East Antarctic mass gain (Table 1, Supplementary Fig. 2).

After 2100, the projected Antarctic sea-level contribution under both emission pathways starts to diverge, potentially leading to multi-meter ice loss under very high emissions. By 2300, mass loss under SSP5-8.5 ranges from +0.73 m to +5.95 m sea-level equivalent (Fig. 2a, Table 1), primarily driven by a significant grounding-line retreat in the Amundsen Sea Embayment, the Siple Coast, and Weddell Sea regions (Fig. 3c, d). In addition, marine parts of the East Antarctic Ice Sheet, such as the Wilkes and Recovery catchments, potentially lose mass by 2300 (Fig. 3c, d). Multi-centennial ice loss from these East Antarctic marine catchments is only partially offset by accumulation in the interior of East Antarctica, with the lower bound of the 5–95% probability interval reaching less than −0.20 m sea-level equivalent by 2300 under SSP5-8.5 for both ice-sheet models (Table 1, Supplementary Fig. 2c, d).

When strongly constraining emissions according to SSP1-2.6, the projected Antarctic contribution to sea-level rise remains below +1.75 m by 2300 with 95% probability across both ice-sheet models (Fig. 2a, Table 1). Compared to SSP5-8.5, mass loss is substantially reduced due to a lower likelihood of extensive grounding-line retreat in West Antarctica. Nonetheless, the resulting sea-level rise may still require major coastal adaptation efforts. In East Antarctica, mass loss may be avoided by 2300 when following SSP1-2.6, with increased snow accumulation dominating the regional signal and leading to a negative median contribution to sea level for both ice-sheet models (Table 1, Supplementary Fig. 2c, d).

By 2100, our range of the projected Antarctic sea-level contribution is consistent with the IPCC-AR6 assessment6 (based on their medium confidence projections; Fig. 2a), although our upper bound under SSP5-8.5 is slightly lower. The higher IPCC-AR6 upper bound reflects the LARMIP-2 estimates18, which focus on the response to sub-shelf melting under high melt sensitivities representative of the Amundsen Sea Embayment44,45. By 2300, however, our historically calibrated ensembles of simulations yield a higher upper bound than IPCC-AR66 (+4.10 – +4.40 m versus +3.10 m in IPCC-AR6, 83rd percentiles; Fig. 2a) and than ISMIP6 under very high emissions19 (+5.10 – +5.95 m, 95th percentiles, versus +4.40 m, the upper end of ISMIP6 model range). The higher upper bounds presented here and in ISMIP6 likely reflect the application of a range of extended CMIP6 climate trajectories beyond 2100, which were not considered in IPCC-AR6. The sampling of both ice-sheet model structure and wide parametric uncertainties explains our ensembles’ higher upper bounds compared to ISMIP6, despite being constrained by observed mass changes. Yet, even with the strong SSP5-8.5 warming trajectories projected by some of the GCMs considered here, our estimates remain well below the range projected by a single ice-sheet model that considers the Marine Ice Cliff Instability (MICI; +6.87 m – +13.55 m sea-level equivalent under RCP8.57). These projections sampled only a limited subset of MICI-related uncertainty and did not account for broader parametric or climate model uncertainties, which would likely alter these ranges46.

Role of uncertainties in modulating future Antarctic mass loss
Projected Antarctic mass loss is shaped by a shifting interplay between ice-sheet model structures, parameters governing the interactions with the ocean and the atmosphere, and diverging GCM climate trajectories. Structural differences between the ice-sheet models and parameters modulating sub-shelf melting dominate multi-decadal uncertainty. As changes in Antarctic climate intensify and diverge across emission scenarios, parametric uncertainty related to ice–atmosphere interactions and climate model uncertainty increasingly contribute to the spread in the projected Antarctic ice-sheet trajectories. Under very strong warming (SSP5-8.5), this shift in the dominant sources of uncertainty leads to a robust agreement across the ice-sheet models on a multi-meter sea-level contribution by 2300.

To quantify the relative contribution of these different sources of uncertainty in our ensembles, we perform a variance decomposition using an analysis of variance (ANOVA) framework (see Methods; Figs. 4 and 5). This approach attributes the spread in the projected Antarctic sea-level contribution to parameters governing the interaction of the ice sheet with the atmosphere and ocean, the choice of climate model, the ice-sheet model structure, and their two-way interactions (the combined effect of two sources of uncertainty acting together).

In line with previous findings19,47, the choice of ice-sheet model is a major source of uncertainty in Antarctic sea-level projections (Fig. 4), highlighting the influence of differences in model physics (e.g., basal friction laws, calving schemes; see Supplementary Table 1), numerical methods, and initialization approaches. Over the next decades, the influence of this structural uncertainty on the projected ice-sheet trajectories increases under both emission scenarios, with its main effect accounting for up to 40% of the total variance at the continental scale (Fig. 4a, b). This fraction is smaller than in previous assessments19,47, likely because of differences in experimental design: our setup explores a wider range of parametric uncertainty but includes only two ice-sheet models, whereas refs. 19, 47 sampled fewer parameters but a larger and more diverse set of models. In those earlier studies, parametric uncertainty was implicitly embedded in the structural uncertainty, whereas here both sources of uncertainty are explicitly quantified, along with their interactions. Our results suggest that the interactions between parametric and structural uncertainties significantly contribute to the total variance during the next few decades, adding to the main effect of the ice-sheet model structures. The uncertainty in the ice-sheet model structure is illustrated by the systematically higher Antarctic sea-level contribution projected by Kori-ULB compared to PISM throughout this century (Fig. 2a, zoom-in; Table 1). Under SSP1-2.6, PISM projects a zero median Antarctic sea-level contribution by 2100, with a 5-95% range of −0.03 m to +0.04 m. Under SSP5-8.5, the median increases slightly to +0.02 m (-0.05 m to +0.12 m). In contrast, Kori-ULB projects substantially higher estimates: +0.09 m (+0.01 m to +0.33 m) under SSP1-2.6 and +0.08 m (-0.01 m to +0.37 m) under SSP5-8.5. This ice-sheet model divergence mainly stems from a stronger dynamic ice loss in West Antarctica in the projections by Kori-ULB than PISM (Supplementary Fig. 2a, b), as a continuation of the trends simulated by each ice-sheet model over the historical period (Supplementary Fig. 1). While the ice-sheet model structure is the dominant source of uncertainty in West Antarctica and the Antarctic Peninsula throughout this century (Fig. 4c, e, f, h), in East Antarctica, it becomes negligible by the end of this century under both emission scenarios (Fig. 4d, g). In this region, both ice-sheet models consistently project a slight mass gain by 2100 (Table 1, Supplementary Fig. 2c, d).

Beyond 2100, the evolution of the structural uncertainty diverges between the two emission scenarios. Under the SSP1-2.6 warming trajectory, it continues to increase, reaching 50% of the total variance at the continental scale by 2300 (Fig. 4a). In contrast, under SSP5-8.5, the strong atmospheric and oceanic changes projected over Antarctica progressively override the trends inherited from the historical period. Consequently, structural uncertainty decreases and eventually becomes negligible on the continental scale by 2300. At the same time, the contribution of parameters related to ice–atmosphere interactions increases, especially in East Antarctica. By 2300, these parameters become the main source of uncertainty in the projected ice-sheet response under SSP5-8.5, accounting for 31% of the total variance (Fig. 4b). This reflects the growing influence of surface melt and runoff in a warming climate10. The shift in the dominant sources of uncertainty for this very strong warming scenario leads to robust agreement between the two ice-sheet models on a substantial Antarctic sea-level contribution by 2300 (Fig. 2a). Not only do we find a similar median ice loss of +2.67 m and +2.73 m sea-level equivalent in Kori-ULB and PISM, respectively, but also the 5–95% probability intervals are similar (Table 1). The increasing dominance of the climate pathway over the ice-sheet model structure is also particularly evident when explicitly including the emission pathways in the ANOVA (Supplementary Fig. 3).

Across both emission scenarios, the parameters controlling ice–ocean interactions play a key role in driving divergent mass loss trajectories, accounting for up to about 35% of the variance at the continental scale over the next three centuries (Fig. 4). This signal is largely driven by the Kori-ULB ensemble, where parametric uncertainty in ice–ocean interactions dominates the variance in the projected ice-sheet changes, except under very strong warming (SSP5-8.5 beyond 2250; Fig. 5a, b). In contrast, parameters associated with ice–atmosphere interactions consistently dominate the variance in the PISM ensemble (Fig. 5c, d). This difference in the dominant source of uncertainty between both ice-sheet models likely arises from differences in their ensemble designs: the Kori-ULB ensemble, by sampling a variety of sub-shelf melt parameterizations and associated parametric uncertainty in the effective ice–ocean heat flux, is more sensitive to ocean warming (in particular, to the early increase in circum-Antarctic ocean temperatures projected by UKESM1-0-LL and IPSL-CM6A-LR even under SSP1-2.6; Fig. 1). By comparison, the PISM ensemble relies on a single sub-shelf melt parameterization (the Potsdam Ice-Shelf Cavity mOdel, PICO48; see Methods), making it less sensitive to ocean warming. This difference in the sensitivity of the two ice-sheet models ensembles to changes in the ocean is also evident in the contribution of the two-way interaction between ice-sheet model and ocean-related parameters, which quantifies the divergent model responses to ocean forcing. This interaction term specifically captures the influence of the choice of sub-shelf melt parameterization on projected ice-sheet mass changes as well as the ice-sheet model sensitivity resulting from the initialization49, which were hidden within the structural uncertainty in previous assessments19,47. These findings highlight the importance of sampling a range of sub-shelf melt parameterizations and associated parametric uncertainties in systematic ensemble designs, next to continued developments in the representation of ocean-induced melting in ice-sheet models. Similarly, the difference in the sensitivity of the two ice-sheet model ensembles to atmospheric forcing is captured by the two-way interaction between the ice-sheet model and atmosphere-related parameters, which emerges as a significant contributor to the total variance at the continental scale under very strong warming.

Beyond ice-sheet model structure and parametric uncertainties, the trajectory of future Antarctic ice loss is also strongly modulated by the choice of GCM, which explains up to 17% of the total variance in the Antarctic sea-level contribution by 2300, consistent with previous estimates19. Especially in East Antarctica, the GCM divergence beyond 2100 (Fig. 1) represents an important source of uncertainty (Fig. 4d, g). This influence is clearly reflected in the distributions of the Antarctic sea-level contribution by 2300 for the different GCMs, which in some cases do not even overlap in their 17–83% probability intervals (Fig. 2b; comparing, for example, distributions under MRI-ESM2-0 and UKESM1-0-LL). For example, the limited changes in Antarctic climate projected by MRI-ESM2-0 result in consistently lower ice-sheet sea-level contributions by 2300 in both the Kori-ULB and PISM ensemble (Fig. 2b). In contrast, the strong Antarctic atmospheric warming projected by CESM2-WACCM (Fig. 1a) amplifies Antarctic mass loss10, leading to a multi-meter sea-level contribution under SSP5-8.5 emissions for both ice-sheet models (Fig. 2b). The early and abrupt ocean warming characteristic of UKESM1-0-LL and IPSL-CM6A-LR (Fig. 1b) triggers high sea-level contributions compared to the other GCMs in the ocean-sensitive Kori-ULB ensemble, even under SSP1-2.6. In the PISM ensemble, however, these same GCMs lead to lower sea-level contributions than for CESM2-WACCM due to the ensemble’s atmospheric rather than oceanic sensitivity (Fig. 5).

Certain long-term future of the Antarctic Ice Sheet
Irrespective of the wide range of future Antarctic climates as well as the uncertainties in the ice-sheet model structure and parameters sampled in the ensembles, our projections reveal certainties for the long-term evolution of the Antarctic Ice Sheet to the year 3000: a collapse of the West Antarctic Ice Sheet under very high emissions, and substantially reduced sea-level contribution under low emissions.

Under the higher-emission pathway SSP5-8.5, a collapse of the West Antarctic Ice Sheet is triggered across our entire ensemble of simulations. This collapse could already unfold by 2300, is likely (66–100% outcome probability, see ref. 5 for definitions of likelihood terms and confidence levels followed here) by 2500 and becomes virtually certain by 3000, as a committed response to the projected SSP5-8.5 changes in Antarctic climate (Fig. 3c, d). Such ice loss results in a West Antarctic contribution to sea-level rise ranging from +2.76 m to +6.91 m by 3000 (Table 1).

Self-sustained ice loss is also triggered in East Antarctica under the strong SSP5-8.5 warming trajectory, leading to very likely long-term ice-sheet retreat (Fig. 3c, d). This includes both ocean-driven grounding-line retreat in marine catchments and potential ice loss from regions grounded above sea level, amplified by the surface melt-elevation feedback10. As a result, the East Antarctic contribution to sea-level rise by 3000 shows a wide spread, ranging from −0.79 m to +18.05 m (Table 1). Our historically-calibrated projections, linking observed and projected changes to long-term dynamics, consistently indicate that grounding-line retreat in the East Antarctic Wilkes catchment is more likely than in the Aurora catchment under SSP5-8.5 (Fig. 3c, d). This suggests that the currently observed ocean-driven mass loss of Totten Glacier4 (also captured by Kori-ULB during the historical period; Supplementary Fig. 1) does not necessarily imply a substantial long-term retreat. In contrast, even though the Wilkes catchment currently shows limited mass changes50, grounding lines in this region are very likely to retreat over the long term in response to early-millennium SSP5-8.5 warming, consistent with the distinct glaciological and topographic settings of these catchments51.

Under the lower-emission pathway SSP1-2.6, a long-term collapse of the West Antarctic Ice Sheet by 3000 cannot be ruled out, but is considerably less likely (Fig. 3a, b). Uncertainty arises from the Amundsen Sea Embayment, where the likelihood of retreat ranges from about as likely as not (PISM) to virtually certain (Kori-ULB), reflecting the trends projected by the ice-sheet models through 2300 (see previous section). In East Antarctica, constraining emissions under the SSP1-2.6 pathway largely prevents substantial self-sustained ice loss at least until 3000 (Fig. 3a, c), although modest mass loss could still contribute to a sea-level contribution of up to half a meter in both Kori-ULB and PISM (Table 1).

Overall, while very high SSP5-8.5 warming could potentially lead to substantial Antarctic ice loss of up to +25.85 m sea-level equivalent by 3000, following the SSP1-2.6 scenario limits the long-term Antarctic mass loss to below +5.28 m with a 95% probability (Table 1). Note that the spread in the Antarctic sea-level contribution over multi-centennial timescales (with higher amplitudes projected by PISM compared to Kori-ULB; Table 1) is partly due to differences in the simulated magnitude of the water-expulsion effect52 (see Methods).

Methods
Ice-sheet models
The projections of Antarctic mass changes presented here have been carried out with the standalone state-of-the-art ice-sheet models Kori-ULB10,39 and PISM40,41. All simulations were performed at a 16 km spatial resolution. The main characteristics of both ice-sheet models are summarized in Supplementary Table 1.

Kori-ULB. Kori-ULB is a vertically-integrated, thermomechanical, hybrid ice-sheet/ice-shelf model that combines the shallow ice (SIA) and shallow shelf (SSA) approximations for grounded ice, while only the shallow shelf approximation is applied for floating ice shelves to represent ice flow40,41. All Kori-ULB projections are based on the model setup described in ref. 10. To represent grounding-line migration, a flux condition based on ref. 67 is prescribed at the grounding line, following the approach by refs. 68, 69. This parameterization has been shown to produce responses to the loss of buttressing within the range of other ice-sheet models (using different ice-flow approximations), even at coarser resolutions10,69. Basal shear stress and velocities are related in a Weertman sliding law with a basal sliding exponent m=3. The field of basal friction coefficients is inferred following the inverse method of ref. 70 (see below). Basal melting underneath the floating ice shelves is calculated by different sub-shelf melt parameterizations, such as the Potsdam Ice Shelf Cavity mOdel48 (PICO), the plume model71, and simplified parameterizations72,73. Calving at the ice front is determined by the combined penetration depths of surface and basal crevasses (parameterized as a function of the ice velocity, the accumulated strain rate, and the ice thickness), relative to the total ice thickness14,59. The response of the bedrock to changes in ice and ocean load is included using the Elastic Lithosphere-Relaxed Asthenosphere (ELRA) model, with a flexural rigidity of the lithosphere of 1025 Nm and a relaxation time of 3000 years61,74.

Initial ice-sheet conditions are obtained by a transient inverse simulation following refs. 70, 75 under constant historical climatic boundary conditions associated with the year 1950 (see below). To minimize the difference between the modeled and observed ice thickness51, basal friction coefficients under grounded ice and sub-shelf melt rates under floating ice are adjusted iteratively10. The calving front is fixed at its observed present-day position. To limit the impacts of transitioning from the balance sub-shelf melt rates derived during the transient inverse simulation to those determined by a sub-shelf melt parameterization, the initialization is followed by a short 10-yr relaxation run applying historical atmospheric and oceanic boundary conditions.

PISM. Projections with the thermomechanically-coupled ice-sheet model PISM use the model setup of ref. 12, based on a modified version of PISM release v1.09, 76. Ice velocities resulting from the shallow ice and shallow shelf approximations are superimposed over the entire ice-sheet/ice-shelf domain in this hybrid ice-flow model. The position of the grounding line results from hydrostatic equilibrium, and its movement is simulated on the subgrid scale, with a linear interpolation of the basal shear stress across the grounding line77. The sliding of the ice sheet over the underlying bedrock is represented by a general power law of the form

τb = (cid:1)τc (cid:1) u b u b j1(cid:1)q (1)

relating the basal shear stress τb and shallow shelf approximation basal sliding velocities ub. A threshold velocity uth = 100 m yr−1 and sliding exponents q=0.5 and q=0.75 (depending on the initial state associated with each atmospheric climatology, see below) are chosen. The yield stress τc depends on the effective pressure of the subglacial till and the till friction angle. The till friction angle, a material property of the subglacial till, is parameterized depending on the bed elevation78. Sub-shelf melt rates are computed by PICO48 based on the ocean climatology of ref. 79 with basin-wide temperature corrections to match observed aggregated melt rates close to the present day9,48. The rate of iceberg calving at the margins of the ice shelves follows the eigencalving law80, combined with a minimum thickness criterion of 50 m at the calving front, and the removal of ice beyond present-day extents of the Antarctic Ice Sheet81. A viscoelastic Earth-deformation model82,83 is used to include the response of the bedrock and seafloor to changing ice loads. Here, the upper mantle is characterized by a viscosity of 1×1021 Pa s and a density of 3300 kg m−3, and for the lithosphere, a flexural rigidity of 5×1024 Nm is chosen.

Initial ice-sheet states are the result of a spin-up approach under constant historical climatic boundary conditions representing the year 1950. As in ref. 12, a full-physics spin-up ensemble with varying model parameters related to basal sliding next to the SIA enhancement factor is run, starting from a thermal equilibrium obtained under a constant ice-sheet geometry (from ref. 84). The initial ice-sheet state performing well in a scoring22,81 based on the observed ice-sheet geometry84 and ice velocities85 is chosen12.

Sea-level contribution calculation. The calculation of the ice-sheet sea-level contribution in this study is based on the method proposed by ref. 86. This approach estimates the sea-level contribution from a marine ice sheet simulated by an ice-sheet model by considering changes in ice volume above floatation, along with correction terms. These corrections account for the density difference between fresh meltwater and saline ocean water, as well as for changes in mass above floatation that occur for ice grounded below sea level due to isostatic changes, but without actual changes in the sea-level contribution. Specifically, the method includes the water-expulsion effect52 for this bedrock correction term. Consequently, sea-level contributions depend on the magnitude of bedrock changes across the entire grid.

Since the two ice-sheet models used in this study have distinct bedrock adjustment models, they produce varying isostatic responses to changes in ice loading, resulting in different magnitudes for the bedrock correction. In particular, the response to ice unloading in PISM extends beyond the current ice-sheet extent (Supplementary Fig. 4), including under floating ice shelves and in the open ocean, leading to higher corrections and therefore higher sea-level contribution estimates at multi-centennial timescales19 (Table 1).

Antarctic climate over the historical period and until 2300
Changes in Antarctic climate are applied to the standalone ice-sheet models based on independent climate model projections (P1 in Table 2). As in refs. 10, 12, atmospheric and oceanic anomalies derived from the climate model projections are combined with present-day atmospheric and oceanic climatologies (P2 and P3 in Table 2). Atmospheric conditions for the 1995–2014 reference period are based on the polar-oriented regional climate models Modèle Atmosphérique Régional (MARv3.1187) and the Regional Atmospheric Climate MOdel (RACMO2.3p288). Different initial states are constructed for each of these atmospheric climatologies. Similarly, we use ocean present-day conditions based on either the two-dimensional fields from ref. 79, or the three-dimensional fields from ref. 73 (for Kori-ULB only).

For the atmosphere, monthly-mean air temperature and sublimation anomalies, as well as precipitation ratios (thus avoiding negative absolute precipitation89) with respect to the 1995–2014 mean seasonal variations are used. Ocean changes are represented by yearly-averaged temperature and salinity anomalies relative to the 1995–2014 mean. Missing values on the continental shelf (given the coarse horizontal resolution of GCMs) and in currently ice-covered regions are extrapolated following ref. 90.

The ice-sheet simulations begin in the year 1950, allowing for conditioning on observations over the satellite era. Historical simulations are run to the year 2015 using changes in the atmosphere and the ocean derived from CMIP5 NorESM1-M91, given its performance over the historical period around Antarctica92. Future changes in Antarctic climate from 2015 to 2300 under the extended Shared Socio-Economic Pathways SSP1-2.6 and SSP5-8.529 are based on a subset of CMIP6 GCMs (MRI-ESM2-0, IPSL-CM6A-LR, CESM2-WACCM, and UKESM1-0-LL; P1 in Table 2).

Sub-shelf melt rates are computed by PICO48 (P4 in Table 2), given the local thermal forcing approximated from far-field GCM ocean properties94, and depending on a parameter that modulates the effective ice-ocean heat flux Γeff (P5 in Table 2).

Instead of directly applying the surface melt and runoff from the GCMs, these are determined as a function of monthly air temperatures and precipitation in a positive-degree-day (PDD) approach10,95,96, depending on the degree-day factors for the melting of ice Kice and snow Ksnow (P6 and P7 in Table 2). Natural variability is considered using a standard deviation of 4°C and 5°C for Kori-ULB and PISM, respectively. In Kori-ULB, the refreezing of surface melt is governed by a simple thermodynamic parameterization and, among others, controlled by the thickness of the thermally active layer dice, while a constant fraction of surface melt refreezes based on the refreezing factor ψ in PISM (P8 in Table 2). PDD-based projections of the surface mass balance and its components have shown good agreement with climate model projections and observations. The surface mass balance is corrected for changes in elevation following the atmospheric lapse rate γatm (P9 in Table 2), thereby accounting for the surface melt-elevation feedback.

Ensemble design
Perturbed-parameter ensembles were produced with each ice-sheet model. These ensembles include up to nine parameters, depending on the ice-sheet model, that govern interactions at the ice-atmosphere and ice-ocean interface. The parameters and their uncertainty ranges (chosen to be as wide as physically plausible97) follow ref. 10 for Kori-ULB, and are very similar for PISM. They are summarized in Table 2. We use a Latin hypercube sampling to generate a 400-member and 300-member ensemble for Kori-ULB and PISM, respectively, assuming uniform prior distributions for all parameters. Each ensemble is then run under both SSP1-2.6 and SSP5-8.5 scenarios, resulting in a total of 1400 simulations.

The designed ensembles include discrete and continuous parameters. The discrete parameters P1 to P3 account for climate forcing uncertainties by including a subset of CMIP6 GCMs (P1) as well as different present-day atmospheric and oceanic climatologies (P2 and P3). Note that P3 only applies to Kori-ULB’s ensemble, while the PISM ensemble only uses ref. 79. We cover uncertainties in ice–ocean interactions by using a variety of sub-shelf melt parameterizations (P4), and sampling, for each of these parameterizations, uncertainty in the parameter that modulates the effective ice–ocean heat flux, i.e., the sensitivity of sub-shelf melt to ocean thermal forcing (P5). Note that PISM’s ensemble does not include a sampling from P4, as only PICO is used (Table 2). Finally, continuous parameters P6 to P9 capture uncertainties in ice–atmosphere interactions, i.e., the degree-day factors for the melting of ice (P6) and snow (P7), and the thickness of the thermally active layer or the refreezing factor in Kori-ULB and PISM, respectively, both influencing the refreezing of surface melt water (P8), and the atmospheric lapse rate (P9).

Calibration
We perform a calibration of each ice-sheet model’s ensemble of simulations independently in a Bayesian framework, as in ref. 10. In particular, a likelihood score is assigned to each ice-sheet model’s ensemble member j based on the discrepancies between a number N of observed ice-sheet net mass balance estimates obs i, i ∈ {1...N} from the latest Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE1; see Supplementary Table 2), which are averaged over several time-periods during the past decades, and the corresponding modeled mass balance mod j i. Assuming independence and a normal distribution of the discrepancies between the observed and simulated ice-sheet mass balance, the multivariate Gaussian likelihood function is given by

Lj = exp (cid:1) 1 2 XN i=1 (mod i j (cid:1) obs i)2 σ2 i (2)

Here, σ2 i denotes the discrepancy variance, determined from the observational σobs i and structural σmod i (approximating the model uncertainties) as σ2 i = (σobs i)2 + (σmod i)2.

Following refs. 10, 21, 98, we assign the structural error by multiplying the observational error, here by a factor of 3 for the Kori-ULB and PISM ensembles. The magnitudes of the structural errors (and therefore discrepancy variances) were chosen to avoid heavily weighted scores for a small number of ensemble members98. These values reflect a judgment that our ability to model reality is far lower than the ability to measure it21,25,97.

With a normalization, a weight wj is then created for each ensemble member with

wj = Lj P Lj (3)

The influence of the calibration and the calibrated trends over the historical period is shown in Supplementary Fig. 1 for each ice-sheet model ensemble. The posterior distributions of the parameter space are given in Supplementary Figs. 5 and 6.

To evaluate the effect of calibration on the ensembles’ distribution of the projected Antarctic sea-level contribution, we calculate the Continuous Rank Probability Score (CRPS), following the approach of ref. 99. The CRPS measures the accuracy and sharpness of a probabilistic forecast by quantifying the difference between the ensemble’s cumulative distribution function (CDF) and that of the observation (treated as a step function). Specifically, the CRPS for an ensemble with CDF F(x) and an observation xobs is given by:

CRPS = Z 1 (cid:1)1 (F(x) (cid:1) H(x (cid:1) xobs)) 2 dx (4)

where H is the Heaviside step function centered at the observed value xobs. A lower CRPS indicates a distribution that more closely matches the observation. Here, we calculate the CRPS for the 1992-2020 average Antarctic mass balance at the continental scale (i.e., as used for the calibration) as well as for different regions (West Antarctica, East Antarctica, and Antarctic Peninsula).

To assess the influence of calibration, we calculate CRPS values normalized by the CRPS of the prior distribution. A normalized CRPS below 1 indicates improved agreement with observations through the calibration. The probability density functions of the prior and posterior distributions, together with the normalized CRPS values, are shown for each ice-sheet model and region in Supplementary Fig. 1e–l. At the continental scale, the calibration significantly reduces the CRPS for both ice-sheet model ensembles (normalized CRPS of 0.27 for Kori-ULB and 0.44 for PISM). However, this is not necessarily the case at the regional scale. For Kori-ULB, the calibration decreases the CRPS performance for the East Antarctic Ice Sheet. For PISM, the CRPS increases for the Antarctic Peninsula and remains largely unchanged for the West Antarctic Ice Sheet. This can be explained by regional biases in the ice-sheet models ensembles: the Kori-ULB ensemble tends to overestimate mass gain in East Antarctica, while the PISM ensemble underestimates West Antarctic mass loss. These regional biases persist in the calibrated ensembles and may lead to overfitting. For instance, underestimated West Antarctic mass loss in the PISM ensemble is compensated for by favoring ensemble members with mass loss in the Antarctic Peninsula beyond observed values during the calibration. Similarly, in the Kori-ULB ensemble, ensemble members slightly overestimating mass loss in West Antarctica are favored to offset model drift in the East Antarctic Ice Sheet. The influence of the structural error on the posterior distributions of the Antarctic sea-level contribution and the normalized CRPS values is shown in Supplementary Fig. 7 and in Supplementary Table 3. Posterior medians are overall insensitive to the choice of structural error (σmod = 2, 3, 4 × σobs), whereas the 5–95% intervals widen with increasing structural error, particularly at longer timescales. We find that assuming a structural error equal to three times the observational uncertainty improves the continental-scale CRPS values while mitigating overfitting at the regional scale.

Sensitivity analysis
To quantify the contribution of different sources of uncertainty to the projected Antarctic contribution to global mean sea-level rise, we perform a variance-based sensitivity analysis using analysis of variance (ANOVA100), following the approach of refs. 19, 47. This method allows us to partition the total variance in the projected Antarctic ice-sheet changes into contributions from individual sources of uncertainty sampled in our ensemble (main effects, representing the fraction of variance explained independently by each source of uncertainty), as well as interactions between them. Interaction terms represent the fraction of variance arising from the combined influence of two sources of uncertainty. For example, the ice-sheet model/GCM interaction captures how the influence of the ice-sheet model depends on the choice of the GCM. These interactions capture non-additive responses that cannot be attributed to any single source of uncertainty alone, meaning that the combined influence of two sources of uncertainty is not simply the sum of their individual effects. In other words, interactions capture variance that emerges only when parameters are varied simultaneously, and cannot be attributed to any one parameter alone.

The ANOVA is performed on the calibrated ensembles using weighted linear regression models and computing variance contributions via weighted sums of squares. We use Type-I (sequential) sums of squares, which allows variance to be attributed incrementally to each factor. The calibration weights wj are passed directly to the MATLAB fitlm function.

We apply the ANOVA separately to each ice-sheet model ensemble (Fig. 5), using all model-specific sources of uncertainty as input factors. In these individual analyses, we focus only on the main effects and do not explicitly analyze interactions between them.

We also perform an ANOVA on the combined ensemble to estimate the contribution of structural differences between the ice-sheet models to the total uncertainty (Fig. 4). The sources of uncertainty are grouped into four broader categories: atmospheric parameters, oceanic parameters, GCM, and ice-sheet model (Fig. 4). Atmospheric parameters include the atmospheric present-day climatology (P2; Table 2), the atmospheric lapse rate (P9) and parameters controlling surface melt (P6 and P7) and runoff (P8). Oceanic parameters include the oceanic present-day climatology (P3), the sub-shelf melt parameterization (P4) and the effective ice–ocean heat flux (P5). Because of the sequential sum of squares and differences in the parameters spaces of the Kori-ULB and PISM ensembles (some parameters are varied only in Kori-ULB and are fixed in PISM; Table 2), we list the ice-sheet model as the first factor in the ANOVA model. This ensures that structural differences between the ice-sheet models are attributed first, preventing artificial inflation of variance contributions to be assigned to parameters (such as the sub-shelf melt parameterization) that would otherwise absorb structural uncertainty.

Potential additional two-way interactions (for example, within a given category), higher-order interactions involving three or more parameters, and residual unexplained variance may also contribute to the variance but are not explicitly included. These omitted contributions are represented by the whitespace in Figs. 4 and 5.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General comments:\nThis manuscript reports a multi-model, historically-calibrated, numerical modelling experiment to evaluate how the Antarctic\nIce Sheet will response to future warming scenarios. The manuscript uses end-member Shared Socio-Economic Pathways\n(SSPs) for their projections of how the ice sheet may change up to the years 2300 and 3000.\nThe modelling experiment design and methods appear robust, at least to someone with limited direct modelling knowledge\nand expertise. The paper is well-written with reasonably good figures and diagrams, and an acceptable overall structure.\nHowever, throughout the manuscript, I did struggle to identify a clear take-home message that the paper was conveying. The\noverall findings are very much in accordance with previously published ice sheet modelling experiments of future Antarctic\nIce Sheet change: i.e. glaciological change, and sea level rise, is relatively limited up to 2100 with little variation between\nSSPs, but then after 2100 the future scenarios of changes are higher magnitude, and there is divergence between the model\noutputs dependent on SSP used etc. What the authors appear to be stating, is that they have done the following differently to\nprevious studies: (1) extended model runs up to 2300 and beyond (i.e. to yr 3000); (2) calibrated their model runs against\nhistorical observed ice sheet change (i.e. since 1950); and (3) assessed a range of model parametric uncertainties. I do not\nknow the Antarctic Ice Sheet modelling literature sufficiently well enough to distinguish whether these are new and novel\napproaches to the previous experiments done by others. However, it is clear to me that it felt like I had to dig out this potential\nnovelty from the manuscript. The manuscript didn’t necessarily present the findings and key messages particularly well.\nBelow in my specific comments I have tried to make some suggestions for approaches to restructure some of the\nsections/paragraphs and for the text to be more specific, so that the reader is better signposted to the key messages and\nnovelty of what has been done.\nSpecific comments:\nL12-13: Should read “United Kingdom of Great Britain and Northern Ireland” (England, Wales, Scotland & associated\nislands alone is simply ‘Great Britain’)\nL20: “warming” – of what? Ocean, atmosphere etc.?\nL20-24: I suspect this can be further condensed into 2-3 lines.\nL24-25: “perturbed-parameter ensembles” is not terribly accessible language for a Nature Comms abstract aimed at a\nbroader audience.\nL32: “strong changes” – be specific here and quantify what is meant by strong.\nL39-40: “coming years to decades” – be specific about timescale here (e.g. before 2050).\nAbstract (Lines 20-40): Based on the abstract alone, I struggled to understand what the key novel message of the manuscript\nwas. The modelling results replicate what we know already. That’s not necessarily an issue, as reproducibility is good, but it\nwould be good for the abstract (and wider manuscript) to deliver a new contribution to knowledge in some way. We already\nknow that West and East Antarctica will change dramatically by the year 3000 if we continue with business-as-usual type\nscenarios, and that if we want to do something about sea level rise then we need to implement strong mitigation asap.\nL45: “and parts of East Antarctica”?\nL52-54: Based on my reading of the manuscript (particularly line 80), this is a key sentence as it is key to the experiment\ndesign (i.e. what this experiment does that the previous experiments didn’t). It’s effectively what is new and novel about this\nwork. However, I fear it is a bit isolated here and doesn’t really follow from the paragraph it sits at the end of. I wonder if a bit of\nre-structuring of the intro would help, either by: (option 1) open the paragraph with “Parametric uncertainties used in\ncontinental-scale ice-sheet models have not been systematically explored”, then follow with details of the problem and\nexplain what can be done to address this. Follow this approach for paragraphs 2&3 too (i.e. few current modelling\nexperiments extend to 2300/historical calibration); or (option 2): state in opening paragraph what your model experiment will\ndo (i.e. explore parametric uncertainties, model to 2300, calibrate against historical observations etc.) then state why this is a\nstep change in approach/knowledge. Either of these two options would substantively help to emphasise the different\napproach being taken here. At present, the interested reader has to seek the info out and solve the puzzle of where it is and\nhow it connects across the introduction, and the wider manuscript.\nL59: Delete “what is more”\nL62-64: Again, this is a key message for the manuscript. Don’t hide it at the end of the paragraph. Move it to start of\nparagraph (option 1 above) or move to start of introduction section.\nL66: “robust projections” – the implication is that this modelling experiment is more robust than previous ones. Is this the\ncase?\nL65-73: The key message of this paragraph is totally lost. It seems to me that the “uncertainty” angle should be merged with\nissue raised in L52-54 (i.e. parametric uncertainty), whilst this paragraph should be all about the issues of model experiment\ntimescales being too short (i.e. not extending beyond 2200). Longer model experiments (e.g. to 2300) allow coastal planning\nto be implemented effectively. Also, why all the italics in this paragraph?\nL73: unclear to me what is “poorly quantified”. The “interests of coastal planning stakeholders”?\nL72: “coastal planning stakeholders” – they are briefly mentioned here, and then again later in the manuscript, but without\nwho they are being specified, nor any effort being attempted at the model experiment results being distilled in a way that is\nuseful for these so-called “stakeholders”. Perhaps just drop reference to them or be more explicit about who they are and\nwhy they are important.\nL74-84: This is where the authors are stating what is coming in the manuscript, whilst also making the case for the novelty of\ntheir approach. If the rest of the intro section were rewritten to build a strong argument and case for why previous model\nexperiments were limited, this paragraph could be much more effective and hard hitting.\nL79: what is “CMIP”? Define at first use.\nL80 (and L52-53) what are these “parametric uncertainties”? – give detail and examples.\nL88-89: Acronym soup. Please write out model names in full here for the wider readership of the manuscript.\nL92: detail examples of the biases in Antarctic climate in those papers here – they could be critical for evaluating the quality\nof the work done here.\nFigure 1: I am unclear what purpose figure 1c serves. It doesn’t seem to be discussed in the text in any substantive way. It\ncertainly isn’t referred to specifically, whereas 1a and 1b are. It wasn’t clear to me why the “relation between Antarctic-\naveraged atmospheric and circumantarctic ocean temperature changes” was important to include in the main body of the\ntext.\nL99: provide temperature change values as a range, as done later in paragraph for SSP5-8.5 scenario?\nL104-112: Paragraph structure could be improved (e.g. split into temperature and ocean temperature change) to make it\neasier for the reader to distil the primary messages being conveyed. Currently, the atmosphere and ocean changes are too\nblended to make it clear which emission pathway does what.\nL137-140: “likely contribute” – can the authors add a few words here to assess or quantify the likely overestimation of ice\nsheet net mass balance (i.e. not just link to an extended data figure).\nFigure 2: I find this figure extremely challenging to extract information from. The overlapping colours on 2a are simply an\noverlapping blend of purple/brown, whilst the symbols on 2b cannot be discriminated easily. Suggest the authors explore\nhow to improve the quality of the layout and presentation of this figure.\nL151: “By 2300, the modelled Antarctic…..”?\nL153-154: Annotate PIG and TG on Figure 3 or remove reference to them here. Why are these glaciers mentioned\nspecifically, when there are no glaciers referred to for Weddell Sea/Siple Coast?\nL156: why do Wilkes and Recovery “potentially” lose mass? Geomorphologically, it is perhaps arguable whether Recovery\nis a “subglacial basin”, in the sense of Aurora and Wilkes, too. Perhaps refer to Recovery and Wilkes catchments?\nL163-167: This seems to be a key message of this paragraph, but is lost at the end, almost as a throwaway comment “In\nfact…..”. It deserves a paragraph of its own, and some additional detail. It may be the key message of this manuscript that is\nof interest to a wider audience (e.g. a non-discipline specific audience).\nL168: Suggest “aligns broadly” is changed to “are consistent with”\nL171-172: References are required after “…loss to 2300.”\nL183-185: Previously, the manuscript makes a big argument about there being a lack of modelling up to 2300, but here it\nreports that there the ice sheet model intercomparison project has modelling up to 2300. It would be helpful if the authors\ncould explain this apparent contradiction, given that model runs to 2300 are argued as being something the authors do here\nthat is ‘different’ from previous model experiments.\nL185-190: More information is required here about why the lack of an overlap is important. What does this finding tell us\nabout the likely processes operating on the Antarctic Ice Sheet and how realistic MICI is?\nL236-257: As a non-modeller, I found it difficult to takeaway the key messages from this section. Perhaps it is a failing on my\npart, but the alternative is that it is because the key messages the authors are trying to convey are not being signposted\neffectively enough.\nL258-266: Moving this section earlier in this section (perhaps to current line 240?), might likely help address the issues I\nfound with L236-256. This is a key message to convey. Move it to the start of the section, headline what is important, and\nthen provide the detail.\nL268-270: A section opening sentence that does not really say very much. Authors could delete this entirely and simply start\nwith L271, or they could make the statement more specific and less generic. They could consider moving lines 302-305 to\nthe start of this section too.\nL274: rather than “end of this millenium, be specific and say year 3000.\nL273-277: a long sentence. Authors should consider breaking it into multiple sentences.\nL278: What is meant by “tipping dynamics”? If they are hinting at tipping points here, then the end of the sentence (i.e. “very\nlikely long-term ice sheet retreat”) does not fulfil the definition of a tipping point. As the authors do not assess potential\nirreversibility in their experiments, I recommend removing reference to “tipping” or “tipping points”.\nL284-286 (& follow on L287-289): I don’t find this finding a surprise given the different glaciological and topographic\ngeometries of these two basins. Have previous modelling publications suggested otherwise?\nL294: state year 3000, rather than “end of millenium”?\nL296: Instead of “likely as not”, what about “highly likely”?\nL297: “…reflecting the trend in the ice sheet model experiments up to 2300….”?\nL299: Be specific about what is meant by “over the next centuries”. 2 centuries? 3, 4?\nL310-311: I’d argue that L312-348 are mainly about model uncertainties and limitations rather than “Next decades decisive\nfor Antarctica’s long-term future”. As such, I recommend the authors consider changing the title of this section.\nL312-315: Another rather general opening sentence, that doesn’t really make a clear statement about the findings of the\nmodelling experiments undertaken in this study. For the manuscript to be more effective, it needs to have much more\ntargeted and hard-hitting opening sentences for each section. I recommend that the authors carefully review all parts of the\nmanuscript to ensure that it is fully delivering the key messages they want it to make. At present, there is a bit of a disconnect\nbetween what I suspect they think they are saying, and what they are saying in the manuscript. The key messages need to\nbe at the forefront. For example, if ensemble modelling of the type done here (historically calibrated, running to 2300 (and/or\n3000), with assessment of parametric assumptions etc.) is vital, and not done in previous studies, then make it clear what it\nenables. What is the step change? E.g. “Historically-calibrated ensemble modelling enables us to deliver XYZ……..”.\nL314: Again there is a mention of “coastal adaptation planning”, but no details on specifically how the work done here feeds\ninto such activities.\nL324-348: All about model uncertainty and limitations, rather than ‘on-topic’ (i.e. next decades decisive…..) for this section. If\nthe authors persist with this content here (perhaps with a modified section heading), it would be good if the authors can\nevaluate the implications of a model horizontal resolution of 16 km on their model results. This must have implications for\nmodelling retreat via Wilkes vs Aurora Basin, simply because of the widths of the basin entry ‘channels’.\nL351: “some of which may be irreversible” – the authors may wish to reconsider this given that they ‘only’ model up to year\n3000.\nL349-359: this content is more relevant to the section heading than lines 324-348. However, it doesn’t really present\nanything that we didn’t really know already from other work (e.g. substantial long-term Antarctic changes….will occur unless\nmassive mitigation efforts are made). The authors need to make a much stronger case for why what they have done here\nprovides something new and novel. What is the original contribution to knowledge they are making? We know that the “next\nfew years and decades are decisive for the multicentennial fate of the Antarctic Ice Sheet.” What is it that this study has done\nthat is different from previous work? The message that needs to be provided in bold letters is probably the one that the\nauthors outline in the intro section, the rigour of the modelling and analysis that is done here (i.e. assessment of parametric\nuncertainty, model historical-calibration etc.). However, the way the paper is currently written doesn’t relay that message\neffectively at the present time.\nNeil Ross\nNewcastle University", "reviewer_2": "Coulon et al. use multiple models and statistical sampling to project long term sea level rise commitments under different\nemissions pathways. They find that near term projections are strongly influenced by ice sheet model sensitivities, especially\nunder limited warming scenarios. However, long-term projections are dominated by changes in Antarctic climate, particularly\nunder unmitigated emissions. The study emphasises the importance of strong emissions reductions today to prevent multi-\nmetre contribution to sea level rise from both the East and West Antarctic Ice Sheets.\nGeneral comments\nI found this to be an impressive study and, with a few minor edits and some further clarifications, this paper would be a useful\naddition to Nature Comms. It is a significant and much-needed contribution to sea level rise projections over multi-centennial\ntimescales. As the authors note, there are currently few studies that have looked at ice sheet changes over longer\ntimescales. The use of multiple models, their calibration with historical data, use of statistical sampling, and consideration of\nvarious emission scenarios provide an exceptionally robust and comprehensive analysis of the future evolution of the\nAntarctic Ice Sheet and its potential contribution to sea level rise. Furthermore, the authors account for a wide range of\nuncertainties in climate and ice sheet responses. The methodology is especially comprehensive and both the interpretation\nof the data and conclusions drawn are sound.\nSpecific comments\nL49-52 Add reference to: J.L. Bamber, M. Oppenheimer, R.E. Kopp, W.P. Aspinall, R.M. Cooke, Ice sheet contributions to\nfuture sea-level rise from structured expert judgment, Proc. Natl. Acad. Sci. U.S.A. 116 (23) 11195-11200,\nhttps://doi.org/10.1073/pnas.1817205116 (2019). The Pollard and DeConto model isn't the only approach that suggests high\nSLR contributions.\nL55-59 Clarity of these sentences could be improved. Replace the colon with a full stop and make this section more concise.\nL64 No need for brackets here.\nL67-73 Consider rephrasing to emphasise the importance of near-term emissions reductions, as this has clearer policy\nrelevance than adaptation planning on multi-centennial timescales.\nL75 Please state how far “beyond”\nL78-81 Consider adding some examples of parametric uncertainties.\nL120-140 Links go to the wrong tables and figures.\nL120 Adding a brief explanation for using Latin Hypercube sampling would help readers unfamiliar with this method\nunderstand its advantages. Add references to previous use for some helpful context.\nL171-172 “Only a few individual studies have explored the Antarctic ice loss to 2300.” Add references here.\nL107, L213, L245, L261 and Fig 1 caption: Change to circum-Antarctic\nFigure S1: Consider using a larger scale.", "reviewer_3": "Reviewer #1 (Remarks to the Author)\nGeneral comments\nThis manuscript reports a multi-model, historically-calibrated, numerical modelling\nexperiment to evaluate how the Antarctic Ice Sheet will response to future warming\nscenarios. The manuscript uses end-member Shared Socio-Economic Pathways\n(SSPs) for their projections of how the ice sheet may change up to the years 2300 and\n3000.\nThe modelling experiment design and methods appear robust, at least to someone\nwith limited direct modelling knowledge and expertise. The paper is well-written with\nreasonably good figures and diagrams, and an acceptable overall structure. However,\nthroughout the manuscript, I did struggle to identify a clear take-home message that\nthe paper was conveying. The overall findings are very much in accordance with\npreviously published ice sheet modelling experiments of future Antarctic Ice Sheet\nchange: i.e. glaciological change, and sea level rise, is relatively limited up to 2100\nwith little variation between SSPs, but then after 2100 the future scenarios of changes\nare higher magnitude, and there is divergence between the model outputs dependent\non SSP used etc. What the authors appear to be stating, is that they have done the\nfollowing differently to previous studies: (1) extended model runs up to 2300 and\nbeyond (i.e. to yr 3000); (2) calibrated their model runs against historical observed ice\nsheet change (i.e. since 1950); and (3) assessed a range of model parametric\nuncertainties. I do not know the Antarctic Ice Sheet modelling literature sufficiently\nwell enough to distinguish whether these are new and novel approaches to the\nprevious experiments done by others. However, it is clear to me that it felt like I had to\ndig out this potential novelty from the manuscript. The manuscript didn’t necessarily\npresent the findings and key messages particularly well. Below in my specific\ncomments I have tried to make some suggestions for approaches to restructure some\nof the sections/paragraphs and for the text to be more specific, so that the reader is\nIPCC-AR6better signposted to the key messages and novelty of what has been done.\n\nSpecific comments\nL12-13: Should read “United Kingdom of Great Britain and Northern Ireland”\n(England, Wales, Scotland & associated islands alone is simply ‘Great Britain’)\nL20: “warming” – of what? Ocean, atmosphere etc.?\nL20-24: I suspect this can be further condensed into 2-3 lines.\nL24-25: “perturbed-parameter ensembles” is not terribly accessible language for a\nNature Comms abstract aimed at a broader audience.\nL32: “strong changes” – be specific here and quantify what is meant by strong.\nL39-40: “coming years to decades” – be specific about timescale here (e.g. before\n2050).\nAbstract (Lines 20-40): Based on the abstract alone, I struggled to understand what\nthe key novel message of the manuscript was. The modelling results replicate what\nwe know already. That’s not necessarily an issue, as reproducibility is good, but it\nwould be good for the abstract (and wider manuscript) to deliver a new contribution to\nknowledge in some way. We already know that West and East Antarctica will change\ndramatically by the year 3000 if we continue with business-as-usual type scenarios,\nand that if we want to do something about sea level rise then we need to implement\nstrong mitigation asap.\nL45: “and parts of East Antarctica”?\nL52-54: Based on my reading of the manuscript (particularly line 80), this is a key\nsentence as it is key to the experiment design (i.e. what this experiment does that the\nprevious experiments didn’t). It’s effectively what is new and novel about this work.\nHowever, I fear it is a bit isolated here and doesn’t really follow from the paragraph it\nsits at the end of. I wonder if a bit of re-structuring of the intro would help, either by:\n(option 1) open the paragraph with “Parametric uncertainties used in\ncontinental-scale ice-sheet models have not been systematically explored”,\nthen\nfollow with details of the problem and explain what can be done to address this.\nFollow this approach for paragraphs 2&3 too (i.e. few current modelling experiments\nextend to 2300/historical calibration); or (option 2): state in opening paragraph what\nyour model experiment will do (i.e. explore parametric uncertainties, model to 2300,\ncalibrate against historical observations etc.) then state why this is a step change in\napproach/knowledge. Either of these two options would substantively help to\nemphasise the different approach being taken here. At present, the interested reader\nhas to seek the info out and solve the puzzle of where it is and how it connects across\nthe introduction, and the wider manuscript.\nL59: Delete “what is more”\nL62-64: Again, this is a key message for the manuscript. Don’t hide it at the end of the\nparagraph. Move it to start of paragraph (option 1 above) or move to start of\nintroduction section.\nL66: “robust projections” – the implication is that this modelling experiment is more\nrobust than previous ones. Is this the case?\nL65-73: The key message of this paragraph is totally lost. It seems to me that the\n“uncertainty” angle should be merged with issue raised in L52-54 (i.e. parametric\nuncertainty), whilst this paragraph should be all about the issues of model experiment\ntimescales being too short (i.e. not extending beyond 2200). Longer model\nexperiments (e.g. to 2300) allow coastal planning to be implemented effectively. Also,\nwhy all the italics in this paragraph?\nL73: unclear to me what is “poorly quantified”. The “interests of coastal planning\nstakeholders”?\nL72: “coastal planning stakeholders” – they are briefly mentioned here, and then\nagain later in the manuscript, but without who they are being specified, nor any effort\nbeing attempted at the model experiment results being distilled in a way that is useful\nfor these so-called “stakeholders”. Perhaps just drop reference to them or be more\nexplicit about who they are and why they are important.\nL74-84: This is where the authors are stating what is coming in the manuscript, whilst\nalso making the case for the novelty of their approach. If the rest of the intro section\nwere rewritten to build a strong argument and case for why previous model\nexperiments were limited, this paragraph could be much more effective and hard\nhitting.\nL79: what is “CMIP”? Define at first use.\nL80 (and L52-53) what are these “parametric uncertainties”? – give detail and\nexamples.\nL88-89: Acronym soup. Please write out model names in full here for the wider\nreadership of the manuscript.\nL92: detail examples of the biases in Antarctic climate in those papers here – they\ncould be critical for evaluating the quality of the work done here.\nFigure 1: I am unclear what purpose figure 1c serves. It doesn’t seem to be discussed\nin the text in any substantive way. It certainly isn’t referred to specifically, whereas 1a\nand 1b are. It wasn’t clear to me why the “relation between Antarctic-averaged\natmospheric and circumantarctic ocean temperature changes” was important to\ninclude in the main body of the text.\nL99: provide temperature change values as a range, as done later in paragraph for\nSSP5-8.5 scenario?\nL104-112: Paragraph structure could be improved (e.g. split into temperature and\nocean temperature change) to make it easier for the reader to distil the primary\nmessages being conveyed. Currently, the atmosphere and ocean changes are too\nblended to make it clear which emission pathway does what.\nL137-140: “likely contribute” – can the authors add a few words here to assess or\nquantify the likely overestimation of ice sheet net mass balance (i.e. not just link to an\nextended data figure).\nFigure 2: I find this figure extremely challenging to extract information from. The\noverlapping colours on 2a are simply an overlapping blend of purple/brown, whilst the\nsymbols on 2b cannot be discriminated easily. Suggest the authors explore how to\nimprove the quality of the layout and presentation of this figure.\nL151: “By 2300, the modelled Antarctic…..”?\nL153-154: Annotate PIG and TG on Figure 3 or remove reference to them here. Why\nare these glaciers mentioned specifically, when there are no glaciers referred to for\nWeddell Sea/Siple Coast?\nL156: why do Wilkes and Recovery “potentially” lose mass? Geomorphologically, it is\nperhaps arguable whether Recovery is a “subglacial basin”, in the sense of Aurora\nand Wilkes, too. Perhaps refer to Recovery and Wilkes catchments?\nL163-167: This seems to be a key message of this paragraph, but is lost at the end,\nalmost as a throwaway comment “In fact…..”. It deserves a paragraph of its own, and\nsome additional detail. It may be the key message of this manuscript that is of interest\nto a wider audience (e.g. a non-discipline specific audience).\nL168: Suggest “aligns broadly” is changed to “are consistent with”\nL171-172: References are required after “…loss to 2300.”\nL183-185: Previously, the manuscript makes a big argument about there being a lack\nof modelling up to 2300, but here it reports that there the ice sheet model\nintercomparison project has modelling up to 2300. It would be helpful if the authors\ncould explain this apparent contradiction, given that model runs to 2300 are argued as\nbeing something the authors do here that is ‘different’ from previous model\nexperiments.\nL185-190: More information is required here about why the lack of an overlap is\nimportant. What does this finding tell us about the likely processes operating on the\nAntarctic Ice Sheet and how realistic MICI is?\nL236-257: As a non-modeller, I found it difficult to takeaway the key messages from\nthis section. Perhaps it is a failing on my part, but the alternative is that it is because\nthe key messages the authors are trying to convey are not being signposted\neffectively enough.\nL258-266: Moving this section earlier in this section (perhaps to current line 240?),\nmight likely help address the issues I found with L236-256. This is a key message to\nconvey. Move it to the start of the section, headline what is important, and then\nprovide the detail.\nL268-270: A section opening sentence that does not really say very much. Authors\ncould delete this entirely and simply start with L271, or they could make the statement\nmore specific and less generic. They could consider moving lines 302-305 to the start\nof this section too.\nL274: rather than “end of this millenium, be specific and say year 3000.\nL273-277: a long sentence. Authors should consider breaking it into multiple\nsentences.\nL278: What is meant by “tipping dynamics”? If they are hinting at tipping points here,\nthen the end of the sentence (i.e. “very likely long-term ice sheet retreat”)\ndoes not\nfulfil the definition of a tipping point. As the authors do not assess potential\nirreversibility in their experiments, I recommend removing reference to “tipping” or\n“tipping points”.\nL284-286 (& follow on L287-289): I don’t find this finding a surprise given the different\nglaciological and topographic geometries of these two basins. Have previous\nmodelling publications suggested otherwise?\nL294: state year 3000, rather than “end of millenium”?\nL296: Instead of “likely as not”, what about “highly likely”?\nL297: “…reflecting the trend in the ice sheet model experiments up to 2300….”?\nL299: Be specific about what is meant by “over the next centuries”. 2 centuries? 3, 4?\nL310-311: I’d argue that L312-348 are mainly about model uncertainties and limitations\nrather than “Next decades decisive for Antarctica’s long-term future”. As such, I\nrecommend the authors consider changing the title of this section.\nL312-315: Another rather general opening sentence, that doesn’t really make a clear\nstatement about the findings of the modelling experiments undertaken in this study.\nFor the manuscript to be more effective, it needs to have much more targeted and\nhard-hitting opening sentences for each section. I recommend that the authors\ncarefully review all parts of the manuscript to ensure that it is fully delivering the key\nmessages they want it to make. At present, there is a bit of a disconnect between\nwhat I suspect they think they are saying, and what they are saying in the manuscript.\nThe key messages need to be at the forefront. For example, if ensemble modelling of\nthe type done here (historically calibrated, running to 2300 (and/or 3000), with\nassessment of parametric assumptions etc.) is vital, and not done in previous studies,\nthen make it clear what it enables. What is the step change? E.g.\n“Historically-calibrated ensemble modelling enables us to deliver XYZ……..”.\nL314: Again there is a mention of “coastal adaptation planning”, but no details on\nspecifically how the work done here feeds into such activities.\nL324-348: All about model uncertainty and limitations, rather than ‘on-topic’ (i.e.\nnext\ndecades decisive…..) for this section. If the authors persist with this content here\n(perhaps with a modified section heading), it would be good if the authors can\nevaluate the implications of a model horizontal resolution of 16 km on their model\nresults. This must have implications for modelling retreat via Wilkes vs Aurora Basin,\nsimply because of the widths of the basin entry ‘channels’.\nL351: “some of which may be irreversible” – the authors may wish to reconsider this\ngiven that they ‘only’ model up to year 3000.\nL349-359: this content is more relevant to the section heading than lines 324-348.\nHowever, it doesn’t really present anything that we didn’t really know already from\nother work (e.g. substantial long-term Antarctic changes….will occur unless massive\nmitigation efforts are made). The authors need to make a much stronger case for why\nwhat they have done here provides something new and novel. What is the original\ncontribution to knowledge they are making? We know that the “next few years and\ndecades are decisive for the multicentennial fate of the Antarctic Ice Sheet.” What is it\nthat this study has done that is different from previous work? The message that\nneeds to be provided in bold letters is probably the one that the authors outline in the\nintro section, the rigour of the modelling and analysis that is done here (i.e.\nassessment of parametric uncertainty, model historical-calibration etc.). However, the\nway the paper is currently written doesn’t relay that message effectively at the present\ntime.\n\nNeil Ross\nNewcastle University"}, "author_response": "Response to the comments of the reviewers for the manuscript\n‘From short-term uncertainties to long-term certainties in the future evolution of the\nAntarctic Ice Sheet’\nby V. Coulon, A.K. Klose, T. Edwards, F. Turner, F. Pattyn, and R. Winkelmann\nWe would like to thank the reviewers for carefully reading our manuscript and for their efforts\nin creating their review comments. We considered their suggestions thoroughly and adapted\nthe manuscript accordingly.\nWe provide a point-by-point response to all comments below. The reviewers’ comments are\ngiven in bold and black font, the authors’ reply in normal and blue font. A revised manuscript\nand a manuscript highlighting the changes are attached.\nIn the revised manuscript, we have particularly addressed the following points:\n● We have restructured the Introduction to highlight the novelty of our work.\n● We have updated the calibration of our ice-sheet models ensembles and\ndemonstrate the benefits of this calibration approach based on the Continuous\nRanked Probability Score (CRPS).\n● We have quantified the roles of the various sources of uncertainty sampled in\nour ensemble in the range of projected Antarctic ice-sheet changes based on an\nanalysis of variance (ANOVA). This analysis quantitatively underlines the key\nmessages with respect to the roles of uncertainties in modulating future Antarctic\nmass loss. The respective section has also been restructured and reformulated\naccordingly.\n● We have restructured and reformulated major parts of the Results to clarify the\nfindings and key messages of our work.\nWe are grateful for the opportunity to further improve our manuscript and are looking forward\nto your feedback.\nYours Sincerely,\nViolaine Coulon, Ann Kristin Klose, Tamsin Edwards, Fiona Turner, Frank Pattyn, and\nRicarda Winkelmann\nReviewer #1 (Remarks to the Author)\nGeneral comments\nThis manuscript reports a multi-model, historically-calibrated, numerical modelling\nexperiment to evaluate how the Antarctic Ice Sheet will response to future warming\nscenarios. The manuscript uses end-member Shared Socio-Economic Pathways\n(SSPs) for their projections of how the ice sheet may change up to the years 2300 and\n3000.\nThe modelling experiment design and methods appear robust, at least to someone\nwith limited direct modelling knowledge and expertise. The paper is well-written with\nreasonably good figures and diagrams, and an acceptable overall structure.\nHowever, throughout the manuscript, I did struggle to identify a clear take-home message that\nthe paper was conveying. The overall findings are very much in accordance with\npreviously published ice sheet modelling experiments of future Antarctic Ice Sheet\nchange: i.e. glaciological change, and sea level rise, is relatively limited up to 2100\nwith little variation between SSPs, but then after 2100 the future scenarios of changes\nare higher magnitude, and there is divergence between the model outputs dependent\non SSP used etc. What the authors appear to be stating, is that they have done the\nfollowing differently to previous studies: (1) extended model runs up to 2300 and\nbeyond (i.e. to yr 3000); (2) calibrated their model runs against historical observed ice\nsheet change (i.e. since 1950); and (3) assessed a range of model parametric\nuncertainties. I do not know the Antarctic Ice Sheet modelling literature sufficiently\nwell enough to distinguish whether these are new and novel approaches to the\nprevious experiments done by others. However, it is clear to me that it felt like I had to\ndig out this potential novelty from the manuscript. The manuscript didn’t necessarily\npresent the findings and key messages particularly well. Below in my specific\ncomments I have tried to make some suggestions for approaches to restructure some\nof the sections/paragraphs and for the text to be more specific, so that the reader is\nIPCC-AR6better signposted to the key messages and novelty of what has been done.\nWe thank the reviewer for this constructive and thoughtful review, which has been extremely\nhelpful in improving the readability of the manuscript as well as the clarity of its novelty and\nkey messages. We agree that these aspects were not sufficiently emphasized in the original\nversion.\nSignificant Antarctic mass changes are expected after 2100, both because slowly-unfolding\nself-sustained ice loss could be triggered in the coming decades, and because climate\nchanges are expected to intensify beyond the end of the century, especially under\nhigher-emission scenarios. However, in IPCC-AR6 (Fox-Kemper et al., 2021),\nmulti-centennial Antarctic sea-level projections were assigned low confidence and not\nassociated with a likelihood statement. This is due to the small number of available studies\nthat had assessed Antarctic ice loss beyond 2100 under continued global warming and the\nlarge spread among them (e.g. DeConto and Pollard, 2016; Bulthuis et al., 2019; DeConto et\nal., 2021).\nSince IPCC-AR6, ISMIP6-2300 (Seroussi et al., 2024) has made an important step forward\nby providing a large multi-model ensemble (16 ice-sheet models) projecting the Antarctic\nice-sheet evolution to 2300. This assessment mainly focused on very high emissions\n(SSP5-8.5 and RCP8.5), while the lower-emission pathway was represented by a single\nGeneral Circulation Model (GCM). In addition, ISMIP6-2300 did not include any systematic\nparametric uncertainty sampling and did not calibrate projections against historical\nobservations. These two aspects are not specific to ISMIP6 but remain broader challenges\nin Antarctic modelling for providing robust projections: parametric uncertainty has rarely\nbeen covered consistently across ice-sheet models, and only a few studies have calibrated\nprojections on the continental scale with satellite observations (Golledge et al., 2019;\nDeConto et al., 2021; Lowry et al., 2021), and even fewer in a Bayesian framework (Ritz et\nal., 2015; Coulon et al., 2024).\nOur study builds on and extends previous work in several key ways:\n1. Emission pathway uncertainty – balanced assessment of both low (SSP1-2.6) and\nvery high (SSP5-8.5) emissions, each with four GCMs.\n2. Parametric uncertainty – systematic sampling of parameters controlling ice–climate\ninteractions across two continental-scale ice-sheet models.\n3. Historical calibration – Bayesian calibration against observed ice-sheet mass\nchange over the past decades, which increases the robustness of our projections.\n4. Extended timescales – projections to 2300, and extended until 3000, which allows\nus to explore the long-term consequences of early-millennium warming.\nThese elements together provide, to our knowledge, the first comprehensive multi-centennial\nassessment of the Antarctic ice-sheet evolution that consistently integrates historical\ncalibration, a wide range of emission pathways and parametric uncertainty across two\nice-sheet models.\nBased on this framework, we find:\n● Under very high emissions (SSP5-8.5), Antarctic ice loss could reach several meters\nby 2300, while reaching net-zero emissions well before 2100 (SSP1-2.6) limits mass\nloss to below 1.75 m. Even then, the associated sea-level rise would still require\nmajor coastal adaptation.\n● The dominant sources of uncertainty shift over time: ice-sheet changes are initially\nmost sensitive to ice-sheet model structures and parameters controlling sub-shelf\nmelting, but with stronger warming (projected after 2150 under SSP5-8.5), the role of\nGCM trajectories and surface melt/runoff parameters becomes increasingly\nimportant.\n● By 3000, we can assign high confidence to substantial, self-sustained Antarctic mass\nloss under strong early-millennium warming, including a virtually certain collapse of\nthe West Antarctic Ice Sheet within our probabilistic projections. While substantial\nEast Antarctic ice loss may be avoided under low emissions, widespread West\nAntarctic retreat is less likely but cannot be excluded.\nWhile some of these aspects in the future Antarctic ice-sheet evolution are consistent with\nprevious work, our framework highlights the (committed) ice-sheet response in a more\nrobust way than before by providing calibrated probabilistic time series of Antarctic ice-sheet\nchanges to 2300 under different SSPs. Given the balanced assessment under SSP1-2.6 and\nSSP5-8.5, this allows us, for example, to give robust estimates of Antarctic mass changes\nunder constrained emissions.\nIn the revised manuscript, we have restructured the Introduction to highlight these points of\nnovelty more clearly, and revised the Results and Discussion to emphasize the key\ntake-home messages. We have also added an explicit ANOVA-based quantification of the\nrelative roles of emissions, GCMs, ice-sheet model structures, and parametric uncertainties,\nwhich quantitatively underlines the key messages in the Section on “Role of uncertainties in\nmodulating future Antarctic mass loss” and extends previous uncertainty quantifications (e.g.\nSeroussi et al., 2023; Seroussi et al., 2024).\nSpecific comments\nL12-13: Should read “United Kingdom of Great Britain and Northern Ireland”\n(England, Wales, Scotland & associated islands alone is simply ‘Great Britain’)\nThanks, this has been updated as “UK”.\nL20: “warming” – of what? Ocean, atmosphere etc.?\n“warming” has been replaced by “global warming”.\nL20-24: I suspect this can be further condensed into 2-3 lines.\nAgreed, we have reformulated this part of the Abstract.\nL24-25: “perturbed-parameter ensembles” is not terribly accessible language for a\nNature Comms abstract aimed at a broader audience.\nAgreed, we have replaced “perturbed parameter ensembles” by “systematically sampling\nparametric and climate model uncertainties”. Note that examples for parameters governing\nice–climate interactions are given in the Introduction, as suggested in another comment.\nL32: “strong changes” – be specific here and quantify what is meant by strong.\nWe find that changes in the Antarctic climate start to dominate the ice-sheet evolution after\n2150 under the higher-emission pathway. This can be inferred from the ANOVA analysis that\nhas been added in the revised manuscript. We have reformulated this sentence, stressing\nthe conditions (“after 2150 under very high emissions”) for this shift in the dominant source\nof uncertainty.\nL39-40: “coming years to decades” – be specific about timescale here (e.g. before\n2050).\nWe agree that specifying a timescale would be helpful, but in this case it is difficult to be\nmore precise. Given that even the lower-emission scenario may lead to significant West\nAntarctic ice loss, our intent is to emphasize that any emission reduction in the near-term will\nbe helpful in reducing the likelihood of substantial mass loss later in this century and beyond.\nWe tried to reformulate this sentence to clarify the context.\nAbstract (Lines 20-40): Based on the abstract alone, I struggled to understand what\nthe key novel message of the manuscript was. The modelling results replicate what\nwe know already. That’s not necessarily an issue, as reproducibility is good, but it\nwould be good for the abstract (and wider manuscript) to deliver a new contribution to\nknowledge in some way. We already know that West and East Antarctica will change\ndramatically by the year 3000 if we continue with business-as-usual type scenarios,\nand that if we want to do something about sea level rise then we need to implement\nstrong mitigation asap.\nWe have reformulated the Abstract and major parts of the manuscript to clarify the novelty of\nour work. Please also see our response to the general reviewer comment for a more detailed\ndiscussion of the novelty of our work and respective adjustments in the revised manuscript.\nL45: “and parts of East Antarctica”?\nAgreed, we have adjusted the formulation as suggested.\nL52-54: Based on my reading of the manuscript (particularly line 80), this is a key\nsentence as it is key to the experiment design (i.e. what this experiment does that the\nprevious experiments didn’t). It’s effectively what is new and novel about this work.\nHowever, I fear it is a bit isolated here and doesn’t really follow from the paragraph it\nsits at the end of. I wonder if a bit of re-structuring of the intro would help, either by:\n(option 1) open the paragraph with “Parametric uncertainties used in\ncontinental-scale ice-sheet models have not been systematically explored”, then\nfollow with details of the problem and explain what can be done to address this.\nFollow this approach for paragraphs 2&3 too (i.e. few current modelling experiments\nextend to 2300/historical calibration); or (option 2): state in opening paragraph what\nyour model experiment will do (i.e. explore parametric uncertainties, model to 2300,\ncalibrate against historical observations etc.) then state why this is a step change in\napproach/knowledge. Either of these two options would substantively help to\nemphasise the different approach being taken here. At present, the interested reader\nhas to seek the info out and solve the puzzle of where it is and how it connects across\nthe introduction, and the wider manuscript.\nWe fully agree and thank you for these very relevant suggestions. As developed above, we\nhave completely restructured the Introduction to better highlight the novelty of our work,\nfollowing your advice as closely as possible.\nL59: Delete “what is more”\nOk.\nL62-64: Again, this is a key message for the manuscript. Don’t hide it at the end of the\nparagraph. Move it to start of paragraph (option 1 above) or move to start of\nintroduction section.\nAgreed, thank you for the suggestion. As developed above, we have completely restructured\nthe Introduction, and hope that this clarifies the key messages.\nL66: “robust projections” – the implication is that this modelling experiment is more\nrobust than previous ones. Is this the case?\nYes, for the reasons outlined above (joint sampling of uncertainties in the ice-sheet model\nstructure, climate models, emissions and parameters as well as calibrating against\nobservations), we believe that our work provides a significant step forward with respect to a\nrobust and comprehensive assessment of the future evolution of the Antarctic Ice Sheet\ncompared to the few previous studies that have assessed the Antarctic contribution to\nsea-level rise beyond 2100.\nL65-73: The key message of this paragraph is totally lost. It seems to me that the\n“uncertainty” angle should be merged with issue raised in L52-54 (i.e. parametric\nuncertainty), whilst this paragraph should be all about the issues of model experiment\ntimescales being too short (i.e. not extending beyond 2200). Longer model\nexperiments (e.g. to 2300) allow coastal planning to be implemented effectively. Also,\nwhy all the italics in this paragraph?\nAgreed. We have included two distinct paragraphs addressing the uncertainty and\ntimescales in the restructured Introduction to clarify the key messages and novelty of our\nwork. The italics have been removed.\nL73: unclear to me what is “poorly quantified”. The “interests of coastal planning\nstakeholders”?\nWe were referring to “high-end projections”. This sentence has been removed in the revised\nmanuscript.\nL72: “coastal planning stakeholders” – they are briefly mentioned here, and then\nagain later in the manuscript, but without who they are being specified, nor any effort\nbeing attempted at the model experiment results being distilled in a way that is\nuseful for these so-called “stakeholders”. Perhaps just drop reference to them or be\nmore explicit about who they are and why they are important.\nWe agree. As suggested, we have removed the reference to coastal planning stakeholders\nin the revised manuscript.\nL74-84: This is where the authors are stating what is coming in the manuscript, whilst\nalso making the case for the novelty of their approach. If the rest of the intro section\nwere rewritten to build a strong argument and case for why previous model\nexperiments were limited, this paragraph could be much more effective and hard\nhitting.\nThank you for this relevant and constructive comment. We have substantially restructured\nthe Introduction following your advice, and hope that these revisions clarify the novelty of\nour work.\nL79: what is “CMIP”? Define at first use.\nWe have included a definition of “CMIP” in the revised manuscript.\nL80 (and L52-53) what are these “parametric uncertainties”? – give detail and\nexamples.\nWe have added examples for parametric uncertainties in the revised manuscript.\nL88-89: Acronym soup. Please write out model names in full here for the wider\nreadership of the manuscript.\nAgreed. We have added the full names of the GCMs in the revised manuscript.\nL92: detail examples of the biases in Antarctic climate in those papers here – they\ncould be critical for evaluating the quality of the work done here.\nThank you for this suggestion. We have included the examples that the upper Southern\nOcean is generally too warm and fresh (Beadling et al., 2020), and that the Amundsen Sea\nLow remains poorly captured (Bracegirdle et al., 2020) in GCMs of the Coupled Model\nIntercomparison Project Phase 6 (CMIP6).\nFigure 1: I am unclear what purpose figure 1c serves. It doesn’t seem to be discussed\nin the text in any substantive way. It certainly isn’t referred to specifically, whereas 1a\nand 1b are. It wasn’t clear to me why the “relation between Antarctic-averaged\natmospheric and circumantarctic ocean temperature changes” was important to\ninclude in the main body of the text.\nWe agree that panel c in Figure 1 was not included in the main text in a detailed way, so we\nhave removed it in the revised manuscript.\nL99: provide temperature change values as a range, as done later in paragraph for\nSSP5-8.5 scenario?\nAgreed, thank you for the suggestion. We have added the range of the projected\nAntarctic-averaged atmospheric and oceanic temperature change under SSP1-2.6.\nL104-112: Paragraph structure could be improved (e.g. split into temperature and\nocean temperature change) to make it easier for the reader to distil the primary\nmessages being conveyed. Currently, the atmosphere and ocean changes are too\nblended to make it clear which emission pathway does what.\nThank you for this comment. However, the goal of this paragraph is to focus on characteristic\natmospheric and oceanic changes in Antarctica for each GCM considered in this work that\nare independent of the emission pathway. For example, projections by CESM2-WACCM\nindicate the strongest atmospheric warming in Antarctica of the four GCMs considered within\neach emission pathway. IPSL-CM6A-LR and UKESM1-0-LL project an early and abrupt\nAntarctic ocean warming in the first half of this century under both emission pathways.\nIn the revised manuscript, we have added an introductory sentence to this paragraph,\nclarifying its main aim, and have reformulated parts of the paragraph. We hope that these\nadjustments clarify the main aim and key message of this paragraph.\nL137-140: “likely contribute” – can the authors add a few words here to assess or\nquantify the likely overestimation of ice sheet net mass balance (i.e. not just link to an\nextended data figure).\nWe have added a comparison of the calibrated median ice-sheet net mass balance and the\nobserved mass balance between 1992 and 2020 in the revised manuscript to quantify the\noverestimation of the ice-sheet net mass balance.\nFigure 2: I find this figure extremely challenging to extract information from. The\noverlapping colours on 2a are simply an overlapping blend of purple/brown, whilst the\nsymbols on 2b cannot be discriminated easily. Suggest the authors explore how to\nimprove the quality of the layout and presentation of this figure.\nThank you for this important comment. In the revised manuscript, we have adjusted Figure 2\nas follows:\n● The colouring and linestyle, representing the two ice-sheet models and emission\nscenarios, have been changed.\n● Box-and-whiskers showing the Antarctic sea-level contribution projected by the\nice-sheet models in 2100 and 2300 have been added in Figure 2a.\n● The markers in Figure 2b, indicating the different GCMs, have been removed to\nimprove clarity.\nL151: “By 2300, the modelled Antarctic…..”?\nWe have added “projected” (instead of “modelled”) in the revised manuscript.\nL153-154: Annotate PIG and TG on Figure 3 or remove reference to them here. Why\nare these glaciers mentioned specifically, when there are no glaciers referred to for\nWeddell Sea/Siple Coast?\nThis is a good point. We have removed Pine Island and Thwaites glaciers from the text, and\nnow refer to the Amundsen Sea Embayment.\nL156: why do Wilkes and Recovery “potentially” lose mass? Geomorphologically, it is\nperhaps arguable whether Recovery is a “subglacial basin”, in the sense of Aurora\nand Wilkes, too. Perhaps refer to Recovery and Wilkes catchments?\nWe find that grounding-line retreat in the Wilkes and Recovery catchments may occur by\n2300 under the SSP5-8.5 emission pathway for some ensemble members, but is overall less\nlikely than retreat in West Antarctica. This is reflected by a probability of being ungrounded\nof < 0.5 in Figure 3c and d.\nIn the revised manuscript, “subglacial basin” has been changed to “catchment”.\nL163-167: This seems to be a key message of this paragraph, but is lost at the end,\nalmost as a throwaway comment “In fact…..”. It deserves a paragraph of its own, and\nsome additional detail. It may be the key message of this manuscript that is of interest\nto a wider audience (e.g. a non-discipline specific audience).\nThanks for pointing this out. In the revised manuscript, a separate paragraph is focused on\nthe Antarctic sea-level contribution under the SSP1-2.6 emission pathway, starting with this\nkey message.\nL168: Suggest “aligns broadly” is changed to “are consistent with”\nOk.\nL171-172: References are required after “…loss to 2300.”\nGiven the restructuring of this section, this sentence has been removed in the revised\nmanuscript.\nL183-185: Previously, the manuscript makes a big argument about there being a lack\nof modelling up to 2300, but here it reports that there the ice sheet model\nintercomparison project has modelling up to 2300. It would be helpful if the authors\ncould explain this apparent contradiction, given that model runs to 2300 are argued as\nbeing something the authors do here that is ‘different’ from previous model\nexperiments.\nWe understand the confusion. As developed above, projections of the Antarctic ice-sheet\nevolution through 2300 have indeed been carried out in ISMIP6 (Seroussi et al., 2024),\ncontributing to filling the gap in global sea-level projections beyond 2100 in IPCC-AR6.\nHowever, our study goes beyond the ISMIP6-2300 ensemble in several key aspects. In\naddition to considering a wide range of emissions, from low to very high (balancing\nSSP1-2.6 and SSP5-8.5), we extend the ISMIP6 framework by (i) systematically sampling\nparametric uncertainty in ice–climate interactions in a coordinated ensemble design across\ntwo continental-scale ice-sheet models, and (ii) calibrating the projections against historical\nobservations. This has been clarified in the revised Introduction.\nL185-190: More information is required here about why the lack of an overlap is\nimportant. What does this finding tell us about the likely processes operating on the\nAntarctic Ice Sheet and how realistic MICI is?\nOur ensemble shows a higher upper bound than ISMIP6-2300 (Seroussi et al., 2024), but is\nstill below the uncertainty range provided by DeConto et al. (2021) using an ice cliff collapse\nparameterization. However, it should be noted that DeConto et al. (2021) sample a very\nlimited range of uncertainty associated with the Marine Ice Cliff Instability and not any other\nparameter or climate model uncertainty, which would probably alter these ranges towards an\noverlap (Li et al. 2023). This sentence has been rephrased as “Yet, even with the strong\nSSP5-8.5 warming trajectories projected by some of the GCMs considered here, our\nestimates remain well below the range projected by a single ice-sheet model that considers\nthe Marine Ice Cliff Instability (MICI; +6.87 m-+13.55 m of sea-level equivalent under\nRCP8.5). These projections sampled only a limited subset of MICI-related uncertainty and\ndid not account for broader parametric or climate model uncertainties, which would likely\nalter these ranges (Li et al. 2023).”\nL236-257: As a non-modeller, I found it difficult to takeaway the key messages from\nthis section. Perhaps it is a failing on my part, but the alternative is that it is because\nthe key messages the authors are trying to convey are not being signposted\neffectively enough.\nIn the revised manuscript, we have completely restructured and reformulated this section on\nthe “Role of uncertainties in modulating future Antarctic mass loss”. In addition, we have\nincluded an ANOVA-based quantification of the relative contribution of the different sources\nof uncertainty to the spread in Antarctic sea-level projections, as suggested by Reviewer 3.\nWe hope that these revisions clarify the key messages of this section.\nL258-266: Moving this section earlier in this section (perhaps to current line 240?),\nmight likely help address the issues I found with L236-256. This is a key message to\nconvey. Move it to the start of the section, headline what is important, and then\nprovide the detail.\nGiven the restructuring of the section on the “Role of uncertainties in modulating future\nAntarctic mass loss”, this paragraph on high-end sea-level contribution – which was\ndistracting from the key messages – has been removed in the revised manuscript.\nL268-270: A section opening sentence that does not really say very much. Authors\ncould delete this entirely and simply start with L271, or they could make the statement\nmore specific and less generic. They could consider moving lines 302-305 to the start\nof this section too.\nWe have reformulated this statement to be more specific, by briefly stating the key\n‘certainties’ that our analysis reveals with respect to the committed response of the Antarctic\nIce Sheet by year 3000.\nL274: rather than “end of this millenium, be specific and say year 3000.\nWe have changed “end of this millennium” to “3000” in the revised manuscript.\nL273-277: a long sentence. Authors should consider breaking it into multiple\nsentences.\nThanks. We have split up this sentence in the revised manuscript.\nL278: What is meant by “tipping dynamics”? If they are hinting at tipping points here,\nthen the end of the sentence (i.e. “very likely long-term ice sheet retreat”) does not\nfulfil the definition of a tipping point. As the authors do not assess potential\nirreversibility in their experiments, I recommend removing reference to “tipping” or\n“tipping points”.\nWe have changed “Tipping dynamics” to “Self-sustained ice loss”.\nL284-286 (& follow on L287-289): I don’t find this finding a surprise given the different\nglaciological and topographic geometries of these two basins. Have previous\nmodelling publications suggested otherwise?\nThank you for this point. Indeed, the higher likelihood of grounding-line retreat in the Wilkes\ncatchment compared to the Aurora catchment under higher-emission scenarios is consistent\nwith their distinct glaciological and topographic settings (Morlighem et al., 2020). Previous\nmodelling studies have highlighted the potential vulnerability of both catchments (e.g.,\nGolledge et al., 2015; DeConto & Pollard, 2016; Garbe et al., 2020; Ritz et al., 2015), but\nhave not provided historically-calibrated projections to 2300 and beyond that would allow a\ndirect, quantitative comparison of the multi-centennial evolution of Wilkes versus Aurora. Our\nframework addresses this gap, linking observed and projected changes to long-term\ndynamics and providing more robust estimates of future retreat across timescales. In the\nrevised manuscript, these aspects have been added.\nL294: state year 3000, rather than “end of millenium”?\nWe have changed “end of this millennium” to “3000” in the revised manuscript.\nL296: Instead of “likely as not”, what about “highly likely”?\nThroughout the whole manuscript, we follow IPCC language (see Box 1.1 in Chen et al.,\n2021) when it comes to likelihood statements as the one in L296. In the revised manuscript,\nwe would like to continue following this language, and thus use “about as likely as not”. We\nhave added a reference to this IPCC language at its first use, and have put all likelihood\nterms in italics for clarity.\nL297: “…reflecting the trend in the ice sheet model experiments up to 2300….”?\nWe have changed “reflecting the trend in the ice sheet model experiments up to 2300” to\n“reflecting the trends projected by the ice-sheet models up to 2300”.\nL299: Be specific about what is meant by “over the next centuries”. 2 centuries? 3, 4?\nWe have changed “over the next centuries” to “at least until 3000”.\nL310-311: I’d argue that L312-348 are mainly about model uncertainties and limitations\nrather than “Next decades decisive for Antarctica’s long-term future”. As such, I\nrecommend the authors consider changing the title of this section.\nWe agree. We have changed the title of this section to “Remaining uncertainties in the timing\nof ice loss”, and kept the title “Next decades decisive for Antarctica’s long-term future” for the\nconcluding section.\nL312-315: Another rather general opening sentence, that doesn’t really make a clear\nstatement about the findings of the modelling experiments undertaken in this study.\nFor the manuscript to be more effective, it needs to have much more targeted and\nhard-hitting opening sentences for each section. I recommend that the authors\ncarefully review all parts of the manuscript to ensure that it is fully delivering the key\nmessages they want it to make. At present, there is a bit of a disconnect between\nwhat I suspect they think they are saying, and what they are saying in the manuscript.\nThe key messages need to be at the forefront. For example, if ensemble modelling of\nthe type done here (historically calibrated, running to 2300 (and/or 3000), with\nassessment of parametric assumptions etc.) is vital, and not done in previous studies,\nthen make it clear what it enables. What is the step change? E.g.\n“Historically-calibrated ensemble modelling enables us to deliver XYZ……..”.\nThank you for this very helpful comment. Following this advice, we have reformulated major\nparts of the manuscript to better highlight the key messages. In particular, in this section, we\nclearly emphasize the step change provided by our study: historically-calibrated ensemble\nmodelling in a Bayesian framework to 2300 under SSPs (balancing low- and very-high\nemissions), combining two ice-sheet models, and systematically sampling parametric\nuncertainties. This allows us to provide robust, probabilistic projections of the Antarctic\ncontribution to sea-level rise, addressing a gap in IPCC-AR6. We hope these revisions\nclarify the main findings of our study and their significance.\nL314: Again there is a mention of “coastal adaptation planning”, but no details on\nspecifically how the work done here feeds into such activities.\nWe have changed the focus of this statement to reflect the need of robust multi-centennial\nprojections for “long-term policy and decision-making” instead of “coastal adaptation\nplanning”.\nL324-348: All about model uncertainty and limitations, rather than ‘on-topic’ (i.e.\nnext decades decisive…..) for this section. If the authors persist with this content here\n(perhaps with a modified section heading), it would be good if the authors can\nevaluate the implications of a model horizontal resolution of 16 km on their model\nresults. This must have implications for modelling retreat via Wilkes vs Aurora Basin,\nsimply because of the widths of the basin entry ‘channels’.\nThank you for this suggestion. We have added a note at the end of this section to\nacknowledge that our ensemble uses a horizontal resolution of 16 km, which may affect the\nrepresentation of narrow basin entry channels (e.g., in Wilkes and Aurora catchments) and\nthus influence the timing of grounding-line retreat.\nL351: “some of which may be irreversible” – the authors may wish to reconsider this\ngiven that they ‘only’ model up to year 3000.\nWe have changed “some of which may be irreversible” to “some of which may be irreversible\non human timescales”. We agree that we do not specifically assess the reversibility of the\nprojected ice-sheet changes here. However, based on previous modelling studies (e.g.\nGarbe et al., 2020) and our process-understanding with respect to relevant feedback\nmechanisms (e.g. Weertman, 1974; Schoof, 2007), together with response timescales of the\nAntarctic Ice Sheet (reaching up to multiple centuries to millennia; e.g. Golledge et al., 2015;\nKlose et al., 2024), we can expect these changes to be irreversible on human timescales\n(Lee et al., 2021).\nL349-359: this content is more relevant to the section heading than lines 324-348.\nHowever, it doesn’t really present anything that we didn’t really know already from\nother work (e.g. substantial long-term Antarctic changes….will occur unless\nmassive mitigation efforts are made). The authors need to make a much stronger case\nfor why what they have done here provides something new and novel. What is the\noriginal contribution to knowledge they are making? We know that the “next few years\nand decades are decisive for the multicentennial fate of the Antarctic Ice Sheet.” What\nis it that this study has done that is different from previous work? The message that\nneeds to be provided in bold letters is probably the one that the authors outline in the\nintro section, the rigour of the modelling and analysis that is done here (i.e.\nassessment of parametric uncertainty, model historical-calibration etc.). However, the\nway the paper is currently written doesn’t relay that message effectively at the present\ntime.\nAs mentioned in our response to previous related comments, we have significantly\nrestructured and reformulated the manuscript and hope that those key messages are now\nefficiently conveyed.\nNeil Ross\nNewcastle University\nReviewer #2 (Remarks to the Author)\nCoulon et al. use multiple models and statistical sampling to project long term sea\nlevel rise commitments under different emissions pathways. They find that near term\nprojections are strongly influenced by ice sheet model sensitivities, especially\nunder limited warming scenarios. However, long-term projections are dominated by changes\nin Antarctic climate, particularly under unmitigated emissions. The study emphasises\nthe importance of strong emissions reductions today to prevent multi-metre\ncontribution to sea level rise from both the East and West Antarctic Ice Sheets.\nGeneral comments\nI found this to be an impressive study and, with a few minor edits and some further\nclarifications, this paper would be a useful addition to Nature Comms. It is a\nsignificant and much-needed contribution to sea level rise projections over\nmulti-centennial timescales. As the authors note, there are currently few studies that\nhave looked at ice sheet changes over longer timescales. The use of multiple models,\ntheir calibration with historical data, use of statistical sampling, and consideration of\nvarious emission scenarios provide an exceptionally robust and comprehensive\nanalysis of the future evolution of the Antarctic Ice Sheet and its potential\ncontribution to sea level rise. Furthermore, the authors account for a wide range of\nuncertainties in climate and ice sheet responses. The methodology is especially\ncomprehensive and both the interpretation of the data and conclusions drawn are\nsound.\nWe thank the reviewer for this positive evaluation of our manuscript, and have included the\nspecific comments given below in the revised version.\nSpecific comments\nL49-52 Add reference to: J.L. Bamber, M. Oppenheimer, R.E. Kopp, W.P. Aspinall, R.M.\nCooke, Ice sheet contributions to future sea-level rise from structured expert\njudgment, Proc. Natl. Acad. Sci. U.S.A. 116 (23) 11195-1200,\nhttps://doi.org/10.1073/pnas.1817205116&#x00A0;(2019). The Pollard and DeConto\nmodel isn't the only approach that suggests high SLR contributions.\nThank you for suggesting this reference. In the revised manuscript, the Introduction has\nbeen restructured and reformulated. In particular, we have included the IPCC-AR6\nassessment assigned medium confidence, and have added a note on potentially higher\nupper bounds in the Antarctic sea-level contribution, as estimated when including MICI\n(DeConto et al., 2021) or in the structured expert judgement by Bamber et al. (2019).\nL55-59 Clarity of these sentences could be improved. Replace the colon with a full\nstop and make this section more concise.\nAgreed. In the revised manuscript, we have restructured and reformulated the Introduction.\nL64 No need for brackets here.\nOk.\nL67-73 Consider rephrasing to emphasise the importance of near-term emissions\nreductions, as this has clearer policy relevance than adaptation planning on\nmulti-centennial timescales.\nThank you for the suggestion. In the revised manuscript, we have restructured and\nreformulated the Introduction. We feel that the emphasis on near-term emission reduction is\nno longer relevant here. However, we address this aspect in the final section of the revised\nmanuscript\nL75 Please state how far “beyond”\nWe have added “to the end of this millennium” in the revised manuscript.\nL78-81 Consider adding some examples of parametric uncertainties.\nWe have added examples for parametric uncertainties in the revised manuscript.\nL120-140 Links go to the wrong tables and figures.\nThanks for pointing this out. We have made sure that the links go to the correct tables and\nfigures in the revised manuscript.\nL120 Adding a brief explanation for using Latin Hypercube sampling would help\nreaders unfamiliar with this method understand its advantages. Add references to\nprevious use for some helpful context.\nWe agree. We have added a brief explanation of the Latin Hybercube sampling and\nrespective references in the revised manuscript.\nL171-172 “Only a few individual studies have explored the Antarctic ice loss to 2300.”\nAdd references here.\nIn the revised manuscript, a modified version of this sentence has been moved to the\nIntroduction, and relevant references have been added.\nL107, L213, L245, L261 and Fig 1 caption: Change to circum-Antarctic\nWe have changed “circumantarctic” to “circum-Antarctic” in the revised manuscript.\nFigure S1: Consider using a larger scale.\nDone.\nReviewer #3 (Remarks to the Author)\nMy comments can be found in my review attachment.\nSummary\nThis article examines the future of the Antarctic Ice Sheet (AIS) and its impact on sea\nlevel rise using two ensemble models calibrated within a Bayesian framework for two\nextreme emission scenarios: low emission (SSP1-2.6) and very-high emission\n(SSP5-8.5). Each ensemble addresses a broad spectrum of uncertainties associated\nwith ice-climate interactions (ocean and atmosphere). The findings indicate that\nuncertainties in historical and short to long-term projections (up to 2100)\npredominantly stem from the selection of the ice sheet model (Kori or PISM),\nice-climate interactions, and the climate model (AOGCM). In contrast, over\nmulti-centennial to millennium timescale (from 2100 to 3000), the chosen emission\nscenario emerges as the most significant uncertainty factor. Notably, under the\nextremely high emission scenario (SSP5-8.5), there is a high likelihood of substantial\nmass loss from the West Antarctic Ice Sheet (WAIS) and a significant probability of\nloss from certain subglacial basins of the East Antarctic Ice Sheet (EAIS), such as\nWilkes. Conversely, adhering to the Paris Agreement and limiting warming to 2\ndegrees (SSP1-2.6) considerably reduces the likelihood of WAIS loss and prevents\nself- sustained ice loss from the EAIS.\nI enjoyed reading this manuscript, which is well-written and succinct! The figures are\nclear, as are the introduction, methods, results, and discussion sections. I believe this\nis an important paper and serves as a valuable complement to the ISMIP work on the\nAIS. It perfectly complements Seroussi et al. (2024) by more thoroughly exploring\nice-climate interactions and SSP uncertainties. The goal of reaching a broader\naudience has been very successfully achieved by using terms characterizing\nuncertainties similar to those used by the IPCC (very likely, virtually certain, etc.).\nThank you for the positive feedback, and the very constructive and relevant suggestions\nprovided in the following comments.\nMajor comments\nI really appreciate the work of this team, and this article seems like an excellent\nculmination of their previous research. However, I am somewhat uncertain about the\nadded value of this paper, as it primarily appears to summarize the findings for a\nbroader audience from Klose et al. (2024). The main distinction seems to be the\ninclusion of the Bayesian calibration proposed in Coulon et al. (2024), which\nsignificantly affects the results for Kori due to differences in the prior ensemble\ncompared to Klose et al. (2024), but has minimal impact on the PISM results.\nAdditionally, there seems to be insufficient evidence demonstrating the superior\nperformance of the posterior ensemble over the prior ensemble during the\nobservation period. Figure A1 indicates a significant reduction in uncertainty for the\nKori posterior ensemble, yet this is not observed for PISM, and the posterior median\noccasionally falls outside the observational range for both models.\nThank you for this constructive comment.\nWe would like to clarify that the work presented here is not a summary of previous findings\nfrom Klose et al. (2024), but is based on new ensembles of projections specifically designed\nfor this study. In these ensembles, various sources of uncertainty are considered to provide\na robust assessment of the future evolution of the Antarctic Ice Sheet through 2300 and\nextended to the end of this millennium. Compared to Klose et al. (2024), parametric\nuncertainty in ice–climate interactions are consistently included here for both Kori-ULB and\nPISM, next to uncertainty in future emissions and climate model uncertainty, which\nrepresents a substantial advance: To the best of our knowledge, such a comprehensive\nassessment of the Antarctic ice-sheet changes beyond the end of this century, especially for\nthe SSPs and calibrated with observations, does not exist so far. It directly addresses the\n‘low confidence’-statement on the estimated sea-level contribution from the Antarctic Ice\nSheet through 2300 in IPCC-AR6 (Fox-Kemper et al., 2021) and substantially adds to\nSeroussi et al. (2020) and Seroussi et al. (2024). To clarify the novelty of our work, the\nIntroduction has been significantly restructured, following comments from Reviewer 1. For a\nbroader discussion of the novelty of our study, we also refer to our response to the related\ngeneral comment of Reviewer 1.\nThat said, we agree that the added value of the Bayesian calibration was not sufficiently\nclear in the original manuscript. The calibration has been revised and its benefits are\ndemonstrated using Continuous Rank Probability Scores, as suggested below. In addition,\nexplanations regarding the stronger reduction in uncertainty for the Kori-ULB ensemble than\nfor the PISM one, and the median occasionally falling outside of the observational range are\nprovided in our responses to specific comments below.\nTo enhance the paper and further distinguish it from Klose et al. (2024), I suggest two\nmodifications:\n1. Rewrite the section “Role of uncertainties in modulating future Antarctic mass\nloss”: Your ensemble adds substantial value to the findings of Seroussi et al. 2023\nand 2024 by delving deeper into ice-climate interactions and SSP uncertainty. It would\nbe beneficial to include a figure showing the time evolution of Sobol\nindices/percentage of variance (similar to Fig. 9 in Seroussi et al. 2023, Fig. 15 in\nSeroussi et al. 2024, and Fig. 6 in Jager et al. 2024). This would provide strong visual\nsupport for this section and clarify which sources of uncertainty are most significant\nup to the year 2300. Additionally, a new paragraph comparing this with Fig. 15 from\nSeroussi et al. 2024 could emphasize the importance of considering uncertainty from\nice-climate interactions and SSP.\nWe would like to thank you for this very relevant suggestion. In the revised manuscript, we\nhave quantified the contribution of the different sources of uncertainty included in our\nensembles by calculating the percentage of variance using ANOVA, both by combining the\nPISM and Kori-ULB ensembles to quantify the structural uncertainty, and separately for each\nice-sheet model ensemble. These results are shown in Figures 4 and 5 of the revised\nmanuscript, as well as in Extended Data Figure 3. We believe this addition significantly\nimproves the manuscript and provides strong visual support for the discussion on the role of\nthe different sources of uncertainty.\nThe section has been rewritten accordingly, with a clearer statement on the added value of\nour approach and a comparison to the ISMIP6-2300 ensemble.\n2. Demonstrate the improvement of the ensemble through Bayesian calibration:\nIn Jager et al. 2024, I proposed a methodology to determine whether the posterior\nensemble is better than the prior ensemble. Similar to practices in machine learning\nand hydrology, it involves splitting your observation dataset into a training set and a\ntesting set. I believe this method could be easily applied to your study, but if you have\nanother approach to demonstrate the benefits of Bayesian calibration, I would be\ndelighted to learn about it. Here is a suggestion for implementing it:\na. Split the ensemble into a training set and a testing set. Given that you have 18\nobservations across 3 sectors, consider using observations from only 2 sectors (5\nper sector) as the training set, with the remainder as the testing set.\nb. Use the training set for Bayesian calibration to determine your weights.\nc. Evaluate the performance of the posterior ensemble against the prior one using the\ntesting set. Various metrics, such as the continuous rank probability score (CRPS)\nfrom Eq. 3 in Jager et al. 2024, can be used for this comparison. Additional metrics\nare available here: [https://www.cawcr.gov.au/projects/verification/].\nd. Restart with another training and testing set.\nAgain, thank you for the relevant comment and suggestion. You are correct to say that the\neffect of the calibration is more evident for the Kori-ULB ensemble than for the PISM\nensemble. We attribute this to the following two main reasons:\n● The PISM ensemble produces a much smaller spread than the Kori-ULB one by\ndesign (less ice–ocean uncertainties were explored). Obtaining a larger reduction in\nvariance from the PISM prior to posterior would require reducing the model\ndiscrepancy, but this would imply giving more weight or trust to one ice-sheet model\nover the other, which is something we wanted to avoid.\n● The PISM ensemble underestimates the observed mass loss of the West Antarctic\nIce Sheet. In contrast, for Kori-ULB, a model drift leads to an overestimation of mass\ngain in East Antarctica. This influences the efficiency of the calibration using the\nregional mass balance estimates from IMBIE, which is more pronounced for the\nPISM ensemble than for Kori-ULB (given that West Antarctica dominates the\nhistorical mass changes signal).\nFor these reasons, we have revised our calibration and now use continental-scale\nobservational estimates instead of regional ones. This allows for a more efficient calibration\nof the PISM ensemble.\nHowever, this reduces the number of observational constraints (6 values), making it difficult\nto apply the cross-validation methodology you suggest (splitting the data into training and\ntesting sets). That said, we agree that it is helpful to use a quantitative metric to assess the\ninfluence of the calibration. Therefore, we now calculate the Continuous Rank Probability\nScore (CRPS) for Antarctic-wide, West Antarctic, East Antarctic, and Antarctic Peninsula\nmass-balance observations over the 1992-2020 period using IMBIE estimates (Otosaka et\nal., 2023). The normalised CRPS values, as well as the comparison between prior and\nposterior distributions, are now shown in Extended Data Figure 1. We hope this effectively\nillustrates the benefits of the calibration for both ice-sheet models.\nWe also acknowledge the limitation of calibrating against continental rather than regional\nmetrics. To compensate for the model drift towards mass gain in East Antarctica in Kori-ULB,\nensemble members characterized by a slight overestimation of West Antarctic mass loss are\nfavored in the calibration. For PISM, the calibration tends to favor ensemble members that\noverestimate mass loss in the Antarctic Peninsula to offset the underestimation of West\nAntarctic mass loss. A similar statement has been added to the section “Bayesian calibration\nof the multi-model ensemble” of the revised manuscript.\nI’ve noticed that some statements in the paper may not fully align with the widely\naccepted understanding of SSPs, emissions, and mitigation efforts, or at least differ\nfrom my personal understanding. This issue seems to extend beyond your study and\nreflects a broader trend in our community, where SSP5-8.5 is still often treated as a\n'business-as- usual' scenario, which it is not. According to the latest IPCC Summary\nfor Policymakers (2022), the policies currently implemented are projected to lead to a\nwarming of about 3.2°C (ranging between 2.2°C and 3.5°C, with medium confidence).\nThis projection is slightly higher than SSP2-4.5 (around 2.8°C) but lower than\nSSP3-7.0 (around 3.9°C), and significantly lower than SSP5-8.5 (around 4.8°C). Other\nliterature also suggests that we are not currently on a trajectory consistent with\nSSP5-8.5 (Raftery et al., 2017; Hausfather and Peters, 2020; Hausfather and Moore,\n2022; Pielke et al., 2022).\nGiven this context, you have two options:\n1. Perform new simulations with an intermediate SSP such as SSP2-4.5: This\napproach would allow you to explore whether current policies are sufficient to prevent\nsignificant contributions from the AIS, or if more aggressive policies are needed. It\nalso provides valuable information to stakeholders by representing a 'most\nprobable/likely path', serving as a median path between two less likely extremes.\n2. Revise some of your statements downwards: While this option is simpler as it\ndoes not require running new simulations (and I understand that there are limited\nclimate models that have run until 2300 with SSP2-4.5, though some exist from\nCMIP5 with RCP4.5, e.g., Hezel et al. 2014), it would significantly reduce the impact\nof your paper. Specifically, it would necessitate revising the last high-impact\nsentence in the abstract: 'Our results imply that strong mitigation efforts are needed\nin the coming years to decades to reduce the risk of self-sustained sea-level rise\nover the next centuries.'"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have undertaken a very thorough, comprehensive and thoughtful response to all reviewers’ comments, and the\nmanuscript is now in excellent shape. The tracked change document clearly evidences substantive rewriting and\nreorganisation of the manuscript text, to deliver a much improved and highly robust manuscript. The figures and diagrams\nare also much improved.\nI particularly appreciate the very considered and positive way in which the authors approached suggestions for improving\nthe overall messaging of the manuscript. As someone who is not a modeller, I was wary of providing such suggestions,\nfeeling slightly out of my depth(!). However, I am delighted to see how effectively the changes made by the authors have\nimproved the manuscript. The introduction is now focused, well structured, and justifies this study and the knowledge gap it\naddresses very effectively. The introduction, and the closing sections of the manuscript (lines 355-469), now accurately\nreflect and fairly represent, the quality, depth, and novelty of the research done in this study. I can easily identify and absorb\nthe important information provided by this important study now. If I, as a non-modeller with slightly rusty mathematical and\nstatistical skills, can do this then the authors can be confident that they are now able to target and impact a very broad\naudience with their manuscript.\nI also fully understand the author team’s logic where they did not implement recommendations from the reviewers, and agree\nwith their decision making where changes have not been made.\nI consider it a privilege to have been able to contribute (in a minor reviewer way) to the development of this manuscript, and\nit was a pleasure to read the very robust and focused revised manuscript. The authors have approached the reviews in a\nvery positive and constructive way and have every right to be proud of what they have produced. To quote reviewer 3, the\nwork is “truly impressive”, as well as now being very accessible. I look forward to using the published version of the\nmanuscript for both research and teaching purposes.\nI have one minor comment (relating to lines 366-367) – I didn’t think there was 6.91 m of West Antarctic ice to lose? Is this a\ntypo?\nNeil Ross\nNewcastle University", "reviewer_2": "I appreciate the effort that the authors have put into the revisions. I’m happy that they have addressed all of my comments. I\nhave no additional suggestions and believe the manuscript is now in good shape and ready for publication.", "reviewer_3": "This manuscript provides a robust analysis of the future evolution of the Antarctic Ice Sheet (AIS) and its contribution to sea\nlevel rise under two contrasting emission scenarios: low-emission (SSP1-2.6) and high-emission (SSP5-8.5). Using an\nensemble modeling approach within a Bayesian framework, the study addresses uncertainties in ice-climate interactions\n(ocean and atmosphere) and ice sheet model structure (Kori and PISM).\nThe findings highlight that, for projections up to 2100, uncertainties are primarily driven by ice sheet model choice and ice-\nocean interactions. Beyond 2100, however, the emission scenario becomes the dominant factor. Under SSP5-8.5, the study\npredicts a high likelihood of significant mass loss from the West Antarctic Ice Sheet (WAIS) and destabilization of subglacial\nbasins in the East Antarctic Ice Sheet (EAIS), such as Wilkes. In contrast, adherence to SSP1-2.6, aligned with the Paris\nAgreement, substantially reduces the risk of WAIS collapse and prevents irreversible EAIS loss.\nThe manuscript is well-written, concise, and methodologically sound. Figures are clear, and the structure—from introduction\nto discussion—is well-organized. This work is a valuable contribution to the field, complementing prior studies like ISMIP\nand extending the understanding of ice-climate interactions and SSP uncertainties. The use of IPCC-aligned uncertainty\nterms enhances accessibility for a broader audience.\nThe authors have addressed all major concerns from my previous review, particularly regarding methodology and scenario\ninterpretation. I now consider this paper ready for publication as is, with only minor optional refinements to further polish the\ndetails.\nMinor comments:\nL36-38: I agree with both parts of the sentence, but I am unsure whether the second part is a direct consequence of the first,\nor if that is what you intended to show. Your results indicate that SSP1-2.6, which is more optimistic than current mitigation\nefforts, may not be sufficient to entirely avoid self-sustained Antarctic ice loss, and this loss is significantly less likely under\nSSP1-2.6 compared to SSP5-8.5, which represents a more pessimistic scenario than current mitigation efforts. These are\ntwo distinct findings, yet the current wording implies that the second result is a consequence of the first. I recommend\nrephrasing for clarity.\nL88-89: all ice-sheet models using inverse method are calibrated with satellite observations. Perhaps you mean transient\nsatellite observations?\nFig. 2: I really like the updated style of the figures! The colors are visually appealing and work well, though I hope they won’t\nconfuse readers familiar with the IPCC reports, where light blue is often used for SSP1-1.9 and light red for SSP3-7.0. As it\nstands, the figure is very clear and effective. For panel b, however, the names of the GCMs on the bottom axis appear a bit\ncrowded and stacked—this might just be my perspective, but it could benefit from slight adjustments to improve readability.\nL289-290: It seems that the negligible structural uncertainty by 2300 may result from opposing effects between the Antarctic\nPeninsula (where this uncertainty is quite significant) and the West Antarctic Ice Sheet. Additionally, since there are only two\nice sheet models, when the medians of the two models intersect, their contribution to the overall uncertainty effectively drops\nto zero. However, I suspect that this uncertainty would begin to grow again beyond 2300. This changes the conclusion of\nthis sentence somewhat.\nL295-300: Similar to my previous comment, I think it is likely coincidental that both models produce the same median at this\npoint. The curves appear quite different, and it seems probable that beyond 2300, PISM would project higher mass loss. For\nexample, introducing a third model would likely alter these conclusions.\nFig. 4: Why is the total so far from 1 at the beginning? It looks much better in Fig. 5. From my perspective, this discrepancy\nmight arise from the ISM/atmosphere and ISM/ocean uncertainties, which could be much higher initially—especially\nISM/ocean. By 2100, most of the uncertainty comes from Kori, and its uncertainty is largely driven by ocean processes.\nTherefore, ISM/ocean should be the dominant source of uncertainty in 2100 in Fig. 4.\nAfter reviewing the methodology, I realized this wasn’t immediately clear. In Fig. 4, you are not showing all ocean-related\nuncertainties but only the common source of uncertainty explored in both models: the ice-ocean heat flux. However, given\nthe importance of other ocean-related uncertainties in Kori, it seems likely that these would account for much of the white\nspace in Fig. 4. It would be helpful to include all ocean-related uncertainties in Fig. 4. Even though PISM does not explore\nsub-shelf melt parameterization, it uses Mpico, which falls within Kori’s parameter space. This suggests it should be\npossible to account for this uncertainty.\nHere’s a potential approach: For Kori, you have the same number of members for each parameter (400/5 = 80 members per\nparameter). For Mpico (parameter 0), you have 80 members from Kori and 300 members from PISM. You could shuffle these\n380 members to create five smaller groups of 80 members each. (I realize 5 × 80 ≠ 380, but if you instead use groups of 20\nmembers, this could work: 19 groups of 20 members with Mpico, 4 groups of 20 members with Plume, etc.). Afterward, you\ncould compute the variance between these groups to estimate the variance due to sub-shelf melt parameterization. Dividing\nthis variance by the total variance would give you the contribution of this uncertainty. A similar approach could be applied for\noceanic present-day climatology.\nFor the variance from the interaction between ISM and ice shelf parameterization, I think the value should be exactly the\nsame: though I haven’t thought through all the details and have just made a quick sketch, it seems that by using the previous\nmethod you split the variance due to the melt parameterization half in the main effect and half in the interaction. Another way\nto show this uncertainty is to attribute all the variance to the interaction: you compute the variance due to melt\nparameterization in the Kori ensemble and divide it by the total variance of both ensembles.\nFinally, I didn’t see any explicit mention that these sensitivity indices are computed on the calibrated ensemble (though I\nassume they are). It might be helpful to clarify this in your description of the sensitivity analysis, including how you account\nfor the weights in this process. Also, if this is the case, the method proposed above would need to be modified slightly.\nInstead of forming groups with the same number of members, you would create groups with the same total weight.\nL453: A good citation to add for sliding and subglacial hydrology: Zhao, C., Gladstone, R., Zwinger, T. et al. Subglacial water\namplifies Antarctic contributions to sea-level rise. Nat Commun 16, 3187 (2025). https://doi.org/10.1038/s41467-025-58375-\nof the first part. To me, the second part seems to be a consequence of the lower likelihood of extreme sea level rise under\nSSP1-2.6.\nMethods:\nL518-519: I’m a bit confused about the q value. Does it depend on your spin-up calibration (L536-538)? In the end, do you\nobtain a single q value for all members, or does it vary between members based on the scoring described in L537?\nL592-593: I don’t quite understand how natural variability is accounted for. Do you vary the temperature using a Gaussian\ndistribution with a variance of 4°C and 5°C?\nL666: Fortunately, the CRPS decreases across the entire domain, as expected since the data was used for calibration! It\nwould have been a major red flag if that weren’t the case. That said, while it doesn’t always decrease at the regional scale, it\ndoes in most cases, which is a good sign for robustness.\nHope it helps, and good luck with your revisions!\nEliot Jager, University of Helsinki", "reviewer_author_response": "Response to the comments of the reviewers for the manuscript\n‘From short-term uncertainties to long-term certainties in the future evolution of the\nAntarctic Ice Sheet’\nby V. Coulon, A.K. Klose, T. Edwards, F. Turner, F. Pattyn, and R. Winkelmann\nWe are glad to read that you are happy with the revised manuscript. Thanks a lot again for\nthe constructive comments in the previous round of revisions, which were very helpful in\nimproving our manuscript.\nAs described in the Methods section, the calculation of the ice-sheet sea-level contribution in\nthis study is based on the method proposed by Goelzer et al. (2020), which includes the\nwater-expulsion effect (Pan et al., 2021), next to other correction terms. Consequently,\nsea-level contributions depend on the magnitude of bedrock changes across the entire grid.\nBy year 3000, this water-expulsion effect can become significant due to the substantial\nisostatic response to changes in ice loading in West Antarctica, adding to the changes in ice\nvolume.\nReviewer #2 (Remarks to the Author)\nI appreciate the effort that the authors have put into the revisions. I’m happy that they\nhave addressed all of my comments. I have no additional suggestions and believe the\nmanuscript is now in good shape and ready for publication.\nThank you very much again for the very helpful comments and suggestions in the previous\nround of revisions.\nReviewer #3 (Remarks to the Author)\nThis manuscript provides a robust analysis of the future evolution of the Antarctic Ice\nSheet (AIS) and its contribution to sea level rise under two contrasting emission\nscenarios: low-emission (SSP1-2.6) and high-emission (SSP5-8.5). Using an\nensemble modeling approach within a Bayesian framework, the study addresses\nuncertainties in ice-climate interactions (ocean and atmosphere) and ice sheet model\nstructure (Kori and PISM).\nThe findings highlight that, for projections up to 2100, uncertainties are primarily\ndriven by ice sheet model choice and ice-ocean interactions. Beyond 2100, however,\nthe emission scenario becomes the dominant factor. Under SSP5-8.5, the study\npredicts a high likelihood of significant mass loss from the West Antarctic Ice Sheet\n(WAIS) and destabilization of subglacial basins in the East Antarctic Ice Sheet (EAIS),\nsuch as Wilkes. In contrast, adherence to SSP1-2.6, aligned with the Paris Agreement,\nsubstantially reduces the risk of WAIS collapse and prevents irreversible EAIS loss.\nThe manuscript is well-written, concise, and methodologically sound. Figures are\nclear, and the structure—from introduction to discussion—is well-organized. This\nwork is a valuable contribution to the field, complementing prior studies like ISMIP\nand extending the understanding of ice-climate interactions and SSP uncertainties.\nThe use of IPCC-aligned uncertainty terms enhances accessibility for a broader\naudience.\nThe authors have addressed all major concerns from my previous review, particularly\nregarding methodology and scenario interpretation. I now consider this paper ready\nfor publication as is, with only minor optional refinements to further polish the details.\nThank you very much for the effort in thoroughly reviewing our manuscript and providing\nfurther suggestions for refinement. We have adapted the manuscript accordingly. Please see\nour detailed responses below.\nMinor comments:\nL36-38: I agree with both parts of the sentence, but I am unsure whether the second\npart is a direct consequence of the first, or if that is what you intended to show. Your\nresults indicate that SSP1-2.6, which is more optimistic than current mitigation\nefforts, may not be sufficient to entirely avoid self-sustained Antarctic ice loss, and\nthis loss is significantly less likely under SSP1-2.6 compared to SSP5-8.5, which\nrepresents a more pessimistic scenario than current mitigation efforts. These are\ntwo distinct findings, yet the current wording implies that the second result is a\nconsequence of the first. I recommend rephrasing for clarity.\nThank you for this comment. Our results show that\n(1) Antarctic ice loss can be limited under low emissions (SSP1-2.6) compared to very\nhigh emissions (SSP5-8.5), especially by avoiding substantial ice-sheet retreat in\nEast Antarctica.\n(2) Even though ice loss may be limited under SSP1-2.6 (with stronger mitigation efforts\ncompared to current climate policies), widespread West Antarctic ice-sheet retreat\ncannot be fully excluded.\nTo our understanding, the statement in L36-38 reflects and summarizes these key results (1)\nand (2) of our manuscript in that “current mitigation efforts may not be sufficient to avoid\nself-sustained ice loss”, which means that stronger mitigation efforts compared to current\nclimate policies are needed (i.e. “emission decisions taken in the coming years are\ndecisive”).\nL88-89: all ice-sheet models using inverse method are calibrated with satellite\nobservations. Perhaps you mean transient satellite observations?\nThank you for this comment. We have changed “satellite observations” to “transient satellite\nobservations”. Note that the calibration applied here is conceptually different to data\nassimilation in a snapshot (e.g. Gagliardini et al., 2013; Gillet-Chaulet et al., 2016;\nMorlighem et al., 2013) or transient inversion (Choi et al., 2023; Badgeley et al., 2025).\nBadgeley, J.A., Morlighem, M., and Seroussi", "reviewer_h": "Increased sea-level contribution from\nnorthwestern Greenland for models that reproduce observations, Proceedings of the\nNational Academy of Sciences, 122 (25), e2411904122,\nhttps://doi.org/10.1073/pnas.2411904122, 2025.\nChoi, Y., Seroussi, H., Morlighem, M., Schlegel, N.-J., and Gardner, A.: Impact of\ntime-dependent data assimilation on ice flow model initialization and projections: a case\nstudy of Kjer Glacier, Greenland, The Cryosphere, 17, 5499–5517,\nhttps://doi.org/10.5194/tc-17-5499-2023, 2023.\nGagliardini, O., Zwinger, T., Gillet-Chaulet, F., Durand, G., Favier, L., de Fleurian, B., Greve,\nR., Malinen, M., Martín, C., Råback, P., Ruokolainen, J., Sacchettini, M., Schäfer, M., Seddik,\nH., and Thies, J.: Capabilities and performance of Elmer/Ice, a new-generation ice sheet\nmodel, Geosci. Model Dev., 6, 1299–1318", "reviewer_research": "Earth Surface, 118, 1746-1753", "reviewer_289": "It seems that the negligible structural uncertainty by 2300 may result from\nopposing effects between the Antarctic Peninsula (where this uncertainty is quite\nsignificant) and the West Antarctic Ice Sheet. Additionally, since there are only two\nice sheet models, when the medians of the two models intersect, their contribution to the\noverall uncertainty effectively drops to zero. However, I suspect that this uncertainty\nwould begin to grow again beyond 2300. This changes the conclusion of this\nsentence somewhat.\nWe understand your point and agree that structural uncertainty in the Antarctic sea-level\ncontribution is not strictly zero and may indeed increase beyond 2300, for example, due to\ncontrasting behaviours between the Antarctic Peninsula and the West Antarctic Ice Sheet,\nor when including additional ice-sheet models. However, the main aim of this section is to\nhighlight the shift in the dominant sources of uncertainty (from ice-sheet model structure to\nclimate forcing) for this very high warming scenario. This shift results in relatively robust\nagreement between the ice-sheet models in terms of multi-centennial mass loss, and is\nparticularly evident when including the SSP scenario in the ANOVA as a source of\nuncertainty as shown in Supplementary Figure 3.\nTo further stress this key message, we have added the following sentence at the end of this\nparagraph: “The increasing dominance of the climate pathway over ice-sheet model\nstructure is also particularly evident when explicitly including the emission pathways in the\nANOVA (Supplementary Fig. 3).”\nL295-300: Similar to my previous comment, I think it is likely coincidental that both\nmodels produce the same median at this point. The curves appear quite different, and\nit seems probable that beyond 2300, PISM would project higher mass loss. For\nexample, introducing a third model would likely alter these conclusions.\nPlease see our response to the previous related comment.\nFig. 4: Why is the total so far from 1 at the beginning? It looks much better in Fig. 5.\nFrom my perspective, this discrepancy might arise from the ISM/atmosphere and\nISM/ocean uncertainties, which could be much higher initially—especially\nISM/ocean. By 2100, most of the uncertainty comes from Kori, and its uncertainty is\nlargely driven by ocean processes. Therefore, ISM/ocean should be the dominant\nsource of uncertainty in 2100 in Fig. 4.\nAfter reviewing the methodology, I realized this wasn’t immediately clear. In Fig. 4, you\nare not showing all ocean-related uncertainties but only the common source of\nuncertainty explored in both models: the ice-ocean heat flux. However, given\nthe importance of other ocean-related uncertainties in Kori, it seems likely that these\nwould account for much of the white space in Fig. 4. It would be helpful to include all\nocean-related uncertainties in Fig. 4. Even though PISM does not explore\nsub-shelf melt parameterization, it uses Mpico, which falls within Kori’s parameter\nspace. This suggests it should be possible to account for this uncertainty.\nHere’s a potential approach: For Kori, you have the same number of members for\neach parameter (400/5 = 80 members per parameter). For Mpico (parameter 0), you\nhave 80 members from Kori and 300 members from PISM. You could shuffle these\n380 members to create five smaller groups of 80 members each. (I realize 5 × 80 ≠ 380, but\nif you instead use groups of 20 members, this could work: 19 groups of 20 members\nwith Mpico, 4 groups of 20 members with Plume, etc.). Afterward, you could compute\nthe variance between these groups to estimate the variance due to sub-shelf melt\nparameterization. Dividing this variance by the total variance would give you the\ncontribution of this uncertainty. A similar approach could be applied for\noceanic present-day climatology.\nFor the variance from the interaction between ISM and ice shelf parameterization, I\nthink the value should be exactly the same: though I haven’t thought through all the\ndetails and have just made a quick sketch, it seems that by using the previous\nmethod you split the variance due to the melt parameterization half in the main effect\nand half in the interaction. Another way to show this uncertainty is to attribute all the\nvariance to the interaction: you compute the variance due to melt\nparameterization in the Kori ensemble and divide it by the total variance of both ensembles.\nThank you for this very helpful comment. It is correct that, since Figure 4 only displays the\ncommon parameters sampled in the ensembles of both ice-sheet models, uncertainty related\nto the sub-shelf melt parameterisation and the present-day ocean climatology appear as\nunexplained variance (within the white space).\nWe have followed your suggestion and have included all ensemble parameters in the\nrevised Figure 4. To do this, we use Type-I (sequential) sums of squares, which allows\nvariance to be attributed incrementally to each source of uncertainty and their interactions\nand have listed ‘ice-sheet model’ as the first parameter. Given the constant values of the\n‘sub-shelf melt parameterisation’ and the ‘ocean present-day climatology’ in the PISM\nensemble, this ensures that structural (ice-sheet model) differences between Kori-ULB and\nPISM are accounted for before attributing variance to within-model parameter perturbations,\nand avoids artificial inflation of ‘sub-shelf melt parameterisation’ and the ‘ocean present-day\nclimatology’ effects that would have been confounded with the ice-sheet model structure.\nVariance fractions now sum closer to 1.\nFinally, I didn’t see any explicit mention that these sensitivity indices are computed on\nthe calibrated ensemble (though I assume they are). It might be helpful to clarify this\nin your description of the sensitivity analysis, including how you account for the\nweights in this process. Also, if this is the case, the method proposed above would\nneed to be modified slightly.\nInstead of forming groups with the same number of members, you would create groups\nwith the same total weight.\nWe thank the reviewer for pointing this out. In fact, in the previous manuscript version, the\nANOVA was computed on the uncalibrated ensemble. You are correct that this should have\nbeen clarified, and that it is more consistent to apply the sensitivity analysis on the calibrated\nensemble. We have revised Figures 4 and 5 and compute the variance decomposition on\nthe calibrated ensemble using weighted linear regression (fitlm in MATLAB), which allows\nus to include the calibration weights directly. Variance contributions are obtained from Type-I\n(sequential) weighted sums of squares and expressed as fractions of the total weighted\nvariance. This ensures that the sensitivity analysis is fully consistent with the calibrated\nprojections presented in the manuscript. The Methods section has been clarified accordingly.\nThe scripts for reproducing our ANOVA analysis will be made available on Zenodo.\nWe would like to stress that while including the weighting in the ANOVA shifts absolute\nvariance fractions slightly, as expected, it does not affect our main findings.\nL453: A good citation to add for sliding and subglacial hydrology: Zhao, C.,\nGladstone, R., Zwinger, T. et al. Subglacial water amplifies Antarctic contributions to\nsea-level rise. Nat Commun 16, 3187 (2025).\nhttps://doi.org/10.1038/s41467-025-58375-4\nThank you for pointing out this reference. We have included it in the Discussion.\nL466-469: Like in the abstract, I’m not sure I understand why the second part of the\nsentence is presented as a consequence of the first part. To me, the second part\nseems to be a consequence of the lower likelihood of extreme sea level rise under\nSSP1-2.6.\nPlease see our response to the previous related comment.\nMethods:\nL518-519: I’m a bit confused about the q value. Does it depend on your spin-up\ncalibration (L536-538)? In the end, do you obtain a single q value for all members, or\ndoes it vary between members based on the scoring described in L537?\nFull-physics spin-up ensembles varying key model parameters including the basal sliding\nexponent q are run for the MAR and RACMO-based historical atmospheric climatologies,\nand the ensemble members performing well in the applied scoring are chosen for projecting\nthe Antarctic ice-sheet evolution with PISM. The chosen initial ice-sheet states for the MAR-\nand RACMO-based climatology have distinct basal sliding exponents q. Likewise, for each\nhistorical atmospheric climatology, initial ice-sheet states with Kori-ULB are generated\n(based on transient inverse simulations), and are associated with respective basal friction\nfields. This has been clarified in the revised manuscript.\nL592-593: I don’t quite understand how natural variability is accounted for. Do you\nvary the temperature using a Gaussian distribution with a variance of 4°C and 5°C?\nThe PDD approach assumes a daily temperature variability in the form of “white noise” with\na standard deviation (as specified in the manuscript) around a monthly mean atmospheric\ntemperature.\nL666: Fortunately, the CRPS decreases across the entire domain, as expected since\nthe data was used for calibration! It would have been a major red flag if that weren’t\nthe case. That said, while it doesn’t always decrease at the regional scale, it does in\nmost cases, which is a good sign for robustness.\nThanks a lot again for suggesting the CPRS to assess the effect of the calibration on our\nensemble in your previous comments. This has been very helpful to improve our manuscript.\nHope it helps, and good luck with your revisions!\nEliot Jager, University of Helsinki"}, "author_response": ""}]

Decision letter