Abstract. Thwaites Glacier is rapidly evolving and could make large sea-level contributions in the coming centuries, making it essential to understand the drivers of the ongoing ice loss. Sediment-core analysis suggests that Thwaites Glacier was in a relatively steady state for millennia before its western pinning point ungrounded in the 1940s. Here, we include a first analysis of 1947 aerial imagery of Thwaites Ice Shelf, which shows that it was relatively undamaged, contrasting with the highly-damaged present-day. Additionally, the main outflow and shear margin were displaced ~15 km westwards compared to the present day. We use the MITgcm-WAVI coupled ocean-ice sheet model to create example quasi-steady pre-1940s configurations for Thwaites Glacier, including a most plausible pre-1940s state, finding that healing the damaged ice shelf is necessary to achieve this. Next, we trigger ice retreat and highlight key processes as the model evolves into the present-day configuration, including ice damage, pinning-point ungrounding driven by ocean melting, and ice piracy between eastern and western parts of Thwaites Glacier. By conducting reversibility experiments during the retreat, we find that multiple quasi-steady ice-sheet states are possible under the same ocean forcing, demonstrating the potential for tipping points in the Thwaites system. Either ice damage or increased ocean forcing can eliminate these quasi-steady states, prompting retreat resembling that observed today. Taken together, these results demonstrate that the sea-level contribution from Thwaites Glacier is not simply controlled by ocean warming in the Amundsen Sea, and is highly sensitive to ice-damage feedbacks, which must be incorporated into sea-level projections.

The response of the Antarctic ice sheet to climate change and its contribution to sea level under different emission scenarios are subject to large uncertainties. A key uncertainty is the slipperiness at the ice sheet base and how it is parameterized in glaciological projections. Alternative formulations of the sliding law exist, but very limited access to the ice base makes it difficult to validate them. Here, the Viscous Grain-Shearing (VGS) theory of acoustic propagation in granular material, together with independent estimates of grain diameter and porosity from sediment cores, is used to relate the effective pressure, which is a key control of basal sliding, to seismic observations recovered from Pine Island Glacier, Antarctica. With basal drag and sliding speed derived through satellite observations of ice flow and inverse methods, the new Bayesian sliding law inference - VGS (BASLI-VGS) approach enables a comparison of basal sliding laws within a Bayesian model selection framework. The presented direct link between seismic observations and sliding law parameters can be readily applied to any acoustic impedance data collected in glacial environments underlain by granular material. For rapidly sliding tributaries of Pine Island Glacier, these calculations provide support for a Coulomb-type sliding law and widespread low effective pressures.

Abstract Pine Island and Thwaites glaciers dominate Antarctica's sea‐level contribution. However, it remains unclear whether their ice loss is driven by anomalously warm present‐day ocean conditions, perhaps linked to anthropogenic climate change, or are an ongoing response to historical natural climate anomalies. Here, we probe these drivers by subjecting three state‐of‐the‐art ice‐sheet models to an extreme hypothetical scenario of zero ocean melting. Pine Island responds by thickening and re‐advancing to the prominent seabed ridge on which it was grounded prior to 1940. In contrast, Thwaites continues to lose ice (at a decreasing rate) for 150 years, despite the absence of ocean melting. Since all present‐day ocean forcing via melt is removed, this demonstrates that the ongoing Thwaites ice loss is at least partly supported by the response to a historical perturbation. It also demonstrates that Thwaites' present‐day mass loss trend cannot be reversed by ocean cooling alone, over policy‐relevant centennial timescales.

The West Antarctic Ice Sheet (WAIS) has undergone rapid change over the satellite era, characterized by significant thinning, grounding-line retreat, and mass loss. More than a third of the ice loss from this region is from Pine Island Glacier (PIG). However, robust causal links between anthropogenic climate change and PIG ice loss have yet to be established. Here we attempt to quantify the role of anthropogenic climate change in observed retreat of PIG over the 20th century and how this may evolve up to 2200. To do so, we use an ensemble Kalman inversion data assimilation method embedded into an uncertainty quantification framework, called calibrate-emulate-sample (CES). This procedure, which assimilates observations of grounding-line retreat and ice volume, yields observationally constrained probability distributions of both model and climate forcing parameters. Our analysis suggests that it is unlikely that the extent of 20th century PIG retreat would have taken place without anthropogenically driven trends in ice-sheet forcing and that anthropogenic forcing exacerbated the extent of PIG retreat over the 20th century, by approximately 18 %. These results are, importantly, conditional on our choice of initial state. For our chosen initial state, we find that the parameter combinations compatible with these observational constraints require PIG to lose mass (but not experience grounding-line retreat) over the entire simulated period since 1750, not just after the 1940s when grounding-line retreat was initiated. This preconditioned ice mass loss introduces significant uncertainty into our quantification of 20th century forcing contributions. In simulations with no anthropogenic trend in forcing, we still observe significant retreat; this may result either from a larger-than-actual initial state, or may suggest that the earlier ice state preconditioned the industrial era retreat, possibly implicating longer term changes to WAIS in the present retreat.

The West Antarctic Ice Sheet (WAIS) is losing ice and its annual contribution to sea level is increasing. The biggest changes are found in the Amundsen Sea sector of WAIS, which contains two of the most rapidly thinning ice streams, Pine Island Glacier (PIG) and Thwaites Glacier (TG). The future behaviour of these glaciers will impact societies worldwide, yet deep uncertainty remains in the expected rate of ice loss. One prominent question is whether the retreat in this region has already passed a tipping point. In ice-sheet projections using the WAVI model, Thwaites Glacier continues to retreat even in an unrealistic scenario of zero oceanic melting, implying that a tipping point has already been passed. Here, we investigate the robustness of this conclusion to the choices made during the ice sheet model initialisation. In particular, we explore the effects of internal ice temperature, ice shelf extent and initial ice damage on forward runs of the WAVI model under the zero-melt scenario (with no evolving damage). We repeat these experiments for initialisations at different time points within the last ~30 years to assess whether the ice damage in this region passed a tipping point within this timeframe.

Pine Island Glacier is a fast flowing ice stream in West Antarctica. At present, it is rapidly thinning and retreating, and has been since at least the 1970s, when satellite records began. Sediment records indicate that this retreat was initiated in the 1940s, but the influence of climate change on key forcing components only became significant in the 1960s, i.e. the trigger for retreat occurred naturally. However, current ice loss remains responsive to fluctuations in forcing, indicating that Pine Island Glacier is not undergoing a purely unstable retreat after this trigger. This begs the question: to what extent is climate change responsible for the recent retreat of the Pine Island Glacier? Adopting a recently published framework, we assess this question. One major challenge is the computational expense associated with the large ensemble of simulations required to account for significant uncertainties in ice sheet model parameters; to overcome this, we use a two stage Ensemble Kalman Inversion and Model Emulation approach. Ultimately, this procedure yields posterior distributions of parameters, including the trend in forcing resulting from climate change; essentially, this allows us to address the question: given the observed Pine Island Glacier retreat, how large does the trend in forcing have to have been?

The West Antarctic Ice Sheet (WAIS) is losing ice and its annual contribution to sea level is increasing. The future behaviour of WAIS will impact societies worldwide, yet deep uncertainty remains in the expected rate of ice loss. High-impact low-likelihood scenarios of sea-level rise are needed by risk-averse stakeholders but are particularly difficult to constrain. Here, we combine traditional model simulations of the Amundsen Sea sector of WAIS with Gaussian process emulation to show that ice-sheet models capable of resolving kilometre-scale basal topography will be needed to assess the probability of extreme scenarios of sea-level rise. This resolution exceeds many state-of-the-art continent-scale simulations. Our ice-sheet model simulations show that coarser resolutions tend to project a larger range of sea-level contributions than finer resolutions, inflating the tails of the distribution. We therefore caution against relying purely upon simulations 5 km or coarser when assessing the potential for societally important high-impact sea-level rise. Ice sheet simulations which capture km-scale basal topography project a lower probability of high end sea level rise than coarser-scale models, according to a comparison of traditional models of the West Antarctic Ice Sheet with Gaussian process emulation.

The relative contributions of anthropogenic climate change and internal variability in sea level rise from the West Antarctic Ice Sheet are yet to be determined. Even the way to address this question is not yet clear, since these two are linked through ice-ocean feedbacks and probed using ice sheet models with substantial uncertainty. Here we demonstrate how their relative contributions can be assessed by simulating the retreat of a synthetic ice sheet setup using an ice sheet model. Using a Bayesian approach, we construct distributions of sea level rise associated with this retreat. We demonstrate that it is necessary to account for both uncertainties arising from both a poorly-constrained model parameter and stochastic variations in climatic forcing, and our distributions of sea level rise include these two. These sources of uncertainty have only previously been considered in isolation. We identify characteristic effects of climate change on sea level rise distributions in this setup, most notably that climate change increases both the median and the weight in tails of distributions. From these findings, we construct metrics quantifying the role of climate change on both past and future sea level rise, suggesting that its attribution is possible even for unstable marine ice sheets.

The Amundsen Sea sector has some of the fastest-thinning ice shelves in Antarctica, caused by high, ocean-driven basal melt rates, which can lead to increased ice streamflow, causing increased sea level rise (SLR) contributions. In this study, we present the results of a new synchronously coupled ice-sheet–ocean model of the Amundsen Sea sector. We use the Wavelet-based, Adaptive-grid, Vertically Integrated ice sheet model (WAVI) to solve for ice velocities and the Massachusetts Institute of Technology general circulation model (MITgcm) to solve for ice thickness and three-dimensional ocean properties, allowing for full mass conservation in the coupled ice–ocean system. The coupled model is initialised in the present day and run forward under idealised warm and cold ocean conditions with a fixed ice front. We find that Thwaites Glacier dominates the future SLR from the Amundsen Sea sector, with a SLR that evolves approximately quadratically over time. The future evolution of Thwaites Glacier depends on the lifespan of small pinning points that form during the retreat. The rate of melting around these pinning points provides the link between future ocean conditions and the SLR from this sector and will be difficult to capture without a coupled ice–ocean model. Grounding-line retreat leads to a progressively larger Thwaites Ice Shelf cavity, leading to a positive trend in total melting, resulting from the increased ice basal surface area. Despite these important sensitivities, Thwaites Glacier retreats even in a scenario with zero ocean-driven melting. This demonstrates that a tipping point may have been passed in these simulations and some SLR from this sector is now committed.

Ice sheet models are used to improve our understanding of the past, present, and future evolution of ice sheets.To do so, they solve the equations describing the flow of ice when forced by other climate elements, particularly the atmosphere and oceans.We present WAVI.jl, an ice sheet model written in Julia.WAVI.jl is designed to make ice sheet modelling more accessible to beginners and low-level users, whilst including sufficient detail to be used for addressing cutting-edge research questions.

The West Antarctic Ice Sheet (WAIS) is losing ice and its annual contribution to sea level is increasing. The future behaviour of WAIS will impact societies worldwide, yet deep uncertainty remains in the expected rate of ice loss. High impact low likelihood scenarios of sea level rise are needed by risk-averse stakeholders but are particularly difficult to constrain. Here we combine traditional model simulations of the Amundsen Sea sector of WAIS with Gaussian process emulation to show that ice-sheet models capable of resolving kilometre-scale basal topography will be needed to assess the probability of extreme scenarios of sea level rise. This resolution exceeds many state-of-the-art continent-scale simulations. Our model simulations show that lower resolutions tend to overestimate future sea level contribution and inflate the tails of the distribution. We therefore caution against relying purely upon low resolution simulations when assessing the potential for societally important high impact sea level rise.

Computer simulations are becoming an essential tool in many scientific fields from molecular dynamics to aeronautics. In glaciology, future predictions of sea level change require input from ice sheet models. Due to uncertainties in the forcings and the parameter choices for such models, many different realisations of the model are needed in order to produce probabilistic forecasts of sea level change. For these reasons, producing robust probabilistic forecasts from an ensemble of model simulations over regions of interest can be extremely expensive for many ice sheet models. Multi-fidelity experimental design (MFED) is a strategy that models the high-fidelity output of the simulator by combining information from various resolutions in an attempt to minimize the computational costs of the process and maximize the accuracy of the posterior. In this paper, we present an application of MFED to an ice-sheet simulatorand demonstrate potential computational savings by modelling the relationship between spatial resolutions. We also analyze the behavior of MFED strategies using theoretical results from sub-modular maximization.

The West Antarctic Ice Sheet is thinning and losing mass at an accelerating rate. However these changes have yet to be formally attributed to anthropogenic climate change, primarily because of the potential for positive feedbacks on ice sheet mass loss which may have been triggered even within the limits of natural internal climate variability. This begs the question: has the thinning, mass loss, and ultimately sea level rise from Antarctica resulted from anthropogenic changes? Or, is the ongoing mass loss simply the result of a positive feedback playing out on the long timescales on which ice sheets evolve? We have developed a framework to address this question, in which forcing is applied via variable ice-shelf basal melt rates with large internal variability. This framework is suitable, in particular, for use in systems with strong feedback potential. An idealised example shows that this framework permits statistically robust attribution statements to be made, even in systems that are highly susceptible to feedbacks, demonstrating the feasibility of such attribution studies for the West Antarctic Ice Sheet.

<p>The Amundsen Sea Sector has some of the highest thinning rates of ice shelves in Antarctica, thought to be driven by high, but interannually variable, ocean driven melt rates. This thinning can lead to increased ice flow speeds, eventually leading to sea level rise. To fully represent these processes and other feedbacks, a fully coupled ice/ocean model must be used. Therefore, a fully synchronous mass conservative coupled ice-sheet/ocean model of the Amundsen Sea Sector has been developed. This new coupled model builds upon previous coupling developments and involves coupling of the WAVI ice-sheet model to the 3D ocean model MITgcm, via the Streamice ice-sheet model. Coupled model projections are presented, examining ice grounding line retreat rates and ice mass loss, along with ocean driven melt rate evolution. The sensitivity of these results to ocean forcings is shown, specifically the thickness of the relatively warm Circumpolar Deep Water layer and its variability. In addition, we discuss the impact of the present-day initialisation and tuning of the coupled model.</p>

Accurate predictions of basal melt rates on ice shelves are necessary for precise projections of the future behaviour of ice sheets. The computational expense associated with completely resolving the cavity circulation using an ocean model makes this approach unfeasible for multi-century simulations, and parametrizations of melt rates are required. At present, some of the most advanced melt rate parametrizations are based on a one-dimensional approximation to the melt rate that emerges from the theory of subglacial plumes applied to ice shelves with constant basal slopes and uniform ambient ocean conditions; in this work, we present an asymptotic analysis of the corresponding equations in which non-constant basal slopes and typical ambient conditions are imposed. This analysis exploits the small aspect ratio of ice shelf bases, the relatively weak thermal driving and the relative slenderness of the region separating warm, salty water at depth and cold, fresh water at the surface in the ambient ocean. We construct an approximation to the melt rate that is based on this analysis, which shows good agreement with numerical solutions in a wide variety of cases, suggesting a path towards improved predictions of basal melt rates in ice-sheet models.

Abstract. We present the result of the third Marine Ice Sheet Model Intercomparison Project, MISMIP+. MISMIP+ is intended to be a benchmark for ice-flow models which include fast sliding marine ice streams and floating ice shelves and in particular a treatment of viscous stress that is sufficient to model buttressing, where upstream ice flow is restrained by a downstream ice shelf. A set of idealized experiments first tests that models are able to maintain a steady state with the grounding line located on a retrograde slope due to buttressing and then explore scenarios where a reduction in that buttressing causes ice stream acceleration, thinning, and grounding line retreat. The majority of participating models passed the first test and then produced similar responses to the loss of buttressing. We find that the most important distinction between models in this particular type of simulation is in the treatment of sliding at the bed, with other distinctions – notably the difference between the simpler and more complete treatments of englacial stress but also the differences between numerical methods – taking a secondary role.

We use an ice sheet model with realistic initial conditions to forecast how the Amundsen Sea sector of West Antarctica responds to recently observed rates of submarine melting. In these simulations, we isolate the effects of a positive feedback, driven by submarine melt in new ocean cavities flooded during retreat, by allowing the present climate, calving front and melting beneath existing ice shelves to persist over the 21st century. Even without additional forcing from changes in climate, ice shelf collapse, or ice cliff collapse, the model predicts slow, sustained retreat of West Antarctica, driven by the marine ice sheet instability and current levels of ocean-driven melting. When observed rates of melting are included in new subglacial ocean cavities, the simulated sea level contribution increases, and for sufficiently intense melting it accelerates over time. Conditional Bayesian probabilities for sea level contributions can be derived but will require improved predictions of ocean heat delivery.

Accurate dynamical models of the Antarctic ice sheet with carefully specified initial conditions and well‐calibrated rheological parameters are needed to forecast global sea level. By adapting an inverse method previously used in electric impedance tomography, we infer present‐day flow speeds within the ice sheet. This inversion uses satellite observations of surface velocity, snow accumulation rate, and rate of change of surface elevation to estimate the basal drag coefficient and an ice stiffness parameter that influences viscosity. We represent interior ice motion using a vertically integrated approximation to incompressible Stokes flow. This model represents vertical shearing within the ice and membrane stresses caused by horizontal stretching and shearing. Combining observations and model, we recover marked geographical variations in the basal drag coefficient. Relative changes in basal shear stress are smaller. No simple sliding law adequately represents basal shear stress as a function of sliding speed. Low basal shear stress predominates in central East Antarctica, where thick insulating ice allows liquid water at the base to lubricate sliding. Higher shear stress occurs in coastal East Antarctica, where a frozen bed is more likely. Examining Thwaites glacier in more detail shows that the slowest sliding often coincides with elevated basal topography. Differences between our results and a similar adjoint‐based inversion suggest that inversion or regularization methods can influence recovered parameters for slow sliding and finer scales; on broader scales we recover a similar pattern of low basal drag underneath major ice streams and extensive regions in East Antarctica that move by basal sliding.

The contribution to sea level to 2200 from the grounded, mainland Antarctic Peninsula ice sheet (APIS) was calculated using an ice-sheet model initialized with a new technique computing ice fluxes based on observed surface velocities, altimetry and surface mass balance, and computing volume response using a linearized method. Volume change estimates of the APIS resulting from surface massbalance anomalies calculated by the regional model RACMO2, forced by A1B and E1 scenarios of the global models ECHAM5 and HadCM3, predicted net negative sea-level contributions between –0.5 and –12mm sea-level equivalent (SLE) by 2200. Increased glacier flow due to ice thickening returned 15% of the increased accumulation to the sea by 2100 and 30% by 2200. The likely change in volume of the APIS by 2200 in response to imposed 10 and 20 km retreats of the grounding line at individual large outlet glaciers in Palmer Land, southern Antarctic Peninsula, ranged between 0.5 and 3.5mm SLE per drainage basin. Ensemble calculations of APIS volume change resulting from imposed grounding-line retreat due to ice-shelf break-up scenarios applied to all 20 of the largest drainage basins in Palmer Land (covering 40% of the total area of APIS) resulted in net sea-level contributions of 7–16mm SLE by 2100, and 10–25mm SLE by 2200. Inclusion of basins in the northern peninsula and realistic simulation of grounding-line movement for AP outlet glaciers will improve future projections.

Inverse methods, where surface data are 'inverted' in order to quantify basal properties of ice sheets, play a major role in initializations. The balance-velocity method is a unique linear initialization, in which accumulation, surface elevation and thickness data are used to calculate the velocities and basal conditions required to maintain the observed ice-sheet altimetric signal, resulting in an estimate of the basal sliding viscosity that is guaranteed to be non-negative. We examine the observation that balance velocities based on the shallow-ice approximation (SIA) are extremely dependent on grid size, showing that Antarctic balance velocities on a 1 km grid are excessively over-channelized. Incorporating the membrane-stress approximation into balance-velocity calculations and comparing them with a simplified analytical solution shows that numerical error monotonically decreases with grid resolution and over-channelization is eliminated for Newtonian and non-Newtonian rheology. In contrast, for the SIA reducing grid size below the membrane-stress coupling length fails to improve accuracy. However, since this approach is nonlinear, a unique viscosity solution is not guaranteed, and in practice 'sliding viscosity' estimates are noisy. This raises problems of the sensitivity of these estimates to data and model errors, which may mean using inverse or smoothing techniques in association with balance-velocity methods in many, if not all, practical applications.