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Over the past 60 years, scientists have strived to understand the past, present and future of the Antarctic Ice Sheet. One of the key components of this research has been the mapping of Antarctic bed topography and ice thickness parameters that are crucial for modelling ice flow and hence for predicting future ice loss and ensuing sea level rise. Supported by the Scientific Committee on Antarctic Research (SCAR), the Bedmap3 Action Group aims not only to produce new gridded maps of ice thickness and bed topography for the international scientific community, but also to standardize and make available all the geophysical survey data points used in producing the Bedmap gridded products. Here, we document the survey data used in the latest iteration, Bedmap3, incorporating and adding to all of the datasets previously used for Bedmap1 and Bedmap2, including ice-bed, surface and thickness point data from all Antarctic geophysical campaigns since the 1950s. More specifically, we describe the processes used to standardize and make these and future survey and gridded datasets accessible under the ‘Findable, Accessible, Interoperable and Reusable’ (FAIR) data principles. With the goals to make the gridding process reproducible and to allow scientists to re-use the data freely for their own analysis, we introduce the new SCAR Bedmap Data Portal (bedmap.scar.org, last access: 18 October 2022) created to provide unprecedented open access to these important datasets, through a user-friendly webmap interface. We believe that this data release will be a valuable asset to Antarctic research and will greatly extend the life cycle of the data held within it. Data are available from the UK Polar Data Centre: https://data.bas.ac.uk.

Ice shelf calving is responsible for roughly half the mass lost by Antarctic Ice Shelves and is of vital importance for determining ice-shelf stability. Despite this, it is currently poorly represented in numerical models and as such has seen limited inclusion in numerical simulations of the future Antarctic Ice Sheet. A first step towards improving this situation is assessing the capabilities and limitations of current numerical models regarding calving. The Calving Model Intercomparison Project (CaMIP) is, therefore, being undertaken to address this issue as part of the EU funded Horizon 2020 project PROTECT.We present here an overview of the project, as well as preliminary results from the first round of experiments by a range of participating modelling groups from across the cryospheric community.

As stated in the latest IPCC report, sea level will continue to rise at the end of this century and most likely well beyond, depending on future emission pathways. The Antarctic ice sheet plays an important role, as it is the largest ice sheet and thus the largest source of water storage on Earth. However, projections for Antarctica from ice-sheet models yield very mixed results due to ice-sheet-related processes that are difficult to assess. One of the main sources of uncertainty is the stability of floating ice shelves. Although ice shelves do not directly contribute to sea-level rise, they have been shown to play an important role, as they modulate the grounded ice flow via their buttressing effect. Therefore, it is necessary to assess the stability of ice shelves in a warmer climate to make more accurate predictions and define safe trajectory scenarios. Satellite images show the formation of crevasse in regions with a high deformation rate. These crevasses weaken the stability of the ice shelf, as damage enhances inland ice acceleration and promotes further shearing and retreat. However, most continental-scale ice-sheet models do not account for ice shelf damage and its consequent potential feedback mechanisms. Part of this statement is due to the fact that ice shelves at coarse resolutions show low stability to damage implementation even in simple domains. Here we force a three-dimensional ice-sheet-shelf model with various damage formulations from the literature. Given the high uncertainty in damage formation and propagation, several parameters affecting the stability of the ice shelf are evaluated. Experiments are performed in different domains to test their influence in simple and symmetric cases, such as MISMIP+, as well as in the Amundsen-Sea Embayment. Our results highlight the importance of further research on ice damage, as it has strong implications for projections but is poorly accounted for in ice-sheet models.

The Greenland ice-sheet surface melt has increased substantially in intensity and spatial extent over the recent decades. The rapid migration of melt towards upstream areas of Greenland ice sheet is expected to incur major changes in hydrological behaviour of the ice-sheet and outlet glaciers along with changes in export fluxes of carbon, methane, and other nutrient fluxes, which, in turn, will further affect the downstream ecosystem of rivers, fjords and oceans. Subglacial environments are emerging as ecological hotspots, urging detailed understanding of interaction between subglacial-hydrology and biogeochemistry. However, due to their inaccessibility, the hydrology and biogeochemistry of subglacial environment thus far lacks a detailed understanding. Numerical models are, in combination with observational data, ideal tools to advance our understanding.Here, we developed a novel process-based model to investigate the interplay between subglacial-hydrology and (passive and active) tracer dynamics underneath the rapidly changing Greenland ice sheet on seasonal, inter-annual and climate warming relevant timescales. We set up the subglacial-hydrology model GlaDS (Glacier Drainage System model) to simulate seasonal and interannual evolution of distributed and channelized subglacial water flow for Leverett glacier (Southwest Greenland) to explore the geometry, connectivity, and flow dynamics in the seasonally evolving drainage system.We then use the GlaDS results to inform a reaction-transport model (RTM) of Leverett’s subglacial system following the GlaDS set-up. The RTM is run to conduct a series of idealized tracer experiments with the aim of disentangling the transport and reaction controls on subglacial tracer distribution and outflow. Tracers are injected to the system through moulins with the surface meltwater and are either passively transported (passive) or also consumed/produced (active) during their transit through the system. Model results are validated with long-term measurements in this area. Results show that the tracer transport is primarily controlled by subglacial drainage system efficiency, which is regulated by discharge magnitude, topography and moulin locations. The spatial and temporal variation in tracer concentration is further dependent on hydrological interaction between different subglacial components (cavities and channels), location and type of branching of channels, and bed properties.In the future, we will extend the model to wider area of Greenland ice sheet and couple it to multi-component biogeochemical reaction networks with the. aim to understand the evolution of biogeochemical process along with the evolution of hydrology in warming climate.

Recent observations show that the Antarctic ice sheet is currently losing mass at an accelerating rate in areas subject to high sub-shelf melt rates. The resulting thinning of the floating ice shelves reduces their ability to restrain the ice flowing from the grounded ice sheet towards the ocean, hence raising sea level by increased ice discharge. Despite a relatively good understanding of the drivers of current Antarctic mass changes, projections of the Antarctic ice sheet are associated with large uncertainties, especially under high‐emission scenarios. This uncertainty may notably be explained by unknowns in the long-term impacts of basal melting and changes in surface mass balance. Here, we use an observationally-calibrated ice-sheet model to investigate the future trajectory of the Antarctic ice sheet until the end of the millennium related to uncertainties in the future balance between sub-shelf melting and ice discharge on the one hand, and the changing surface mass balance on the other. Our large ensemble of simulations, forced by a panel of CMIP6 climate models, suggests that the ocean will be the main driver of short-term Antarctic mass loss, triggering ice loss in the West Antarctic ice sheet (WAIS) already during this century. Under high-emission pathways, ice-ocean interactions will result in a complete WAIS collapse, likely completed before the year 2500 CE, as well as significant grounding-line retreat in the East Antarctic ice sheet (EAIS). Under a more sustainable socio-economic scenario, both the EAIS and WAIS may be preserved, though the retreat of Thwaites glacier appears to be already committed under present-day conditions. We show that with a regional near-surface warming higher than +7.5°C, which may occur by the end of this century under unabated emission scenarios, major ice loss is expected as the increase in surface runoff outweighs the increase in snow accumulation, leading to a decrease in the mitigating role of the ice sheet surface mass balance.

Promontory ice rises are locally grounded features adjacent to ice shelves that are still connected to the ice sheet. Ice rises are an archive for the atmospheric and ice dynamic history of the respective outflow regions where the presence, absence, or migration of Raymond arches in radar stratigraphy represents a memory of the ice-rise evolution. However, ice rises and their inferred dynamic history are not yet used to constrain large-scale ice flow model spin-ups because matching the arch amplitudes includes many unknown parameters, e.g., those pertaining to ice rheology. In particular, anisotropic ice flow models predict gradients in ice fabric anisotropy on either side of an ice divide. However, this has thus far not been validated with observations. The ground-based phase-sensitive Radio Echo Sounder (pRES) has previously been used to infer ice fabric types for various flow regimes using the co-polarized polarimetric coherence phase as a metric to extract information from the birefringent radar backscatter. Here, we apply this technique using quad-polarimetric radar data along a 5 km transect across a ridge near the triple junction of Hammarryggen Ice Rise at the Princess Ragnhild Coast. A comparison with ice core data collected at the dome shows that the magnitude of ice fabric anisotropy can reliably be reconstructed from the quad-polarimetric data. We use the combined dataset also to infer the spatial variation of ice fabric orientations in the vicinity of the triple junction. The observations are integrated with airborne radar profiles and strain rates based on the shallow ice approximation. We then discuss whether estimated anisotropy from radar polarimetry on ice rises, in general, can be another observational constraint to better ice rises as an archive of ice dynamics.

Given the potentially high magnitudes and rates of future warming, the long-term evolution of the Antarctic Ice Sheet is highly uncertain. While recent projections under Representative Concentration Pathway 8.5 estimate the Antarctic sea-level contribution by the end of this century between -7.8 cm and 30.0 cm sea-level equivalent (Seroussi et al., 2020), sea-level might continue to rise for millennia to come due to ice sheet inertia, resulting in a substantially higher long-term committed sea-level change. In addition, potentially irreversible ice loss due to several self-amplifying feedback mechanisms may be triggered within the coming centuries, but evolves thereafter over longer timescales depending on the warming trajectory. It is therefore necessary to account for the timescale difference between forcing and ice sheet response in long-term sea-level projections by (i) determining the resulting gap between transient and committed sea-level contribution with respect to changing boundary conditions, (ii) testing the reversibility of large-scale ice sheet changes, as well as (iii) exploring the potential for safe overshoots of critical thresholds when reversing climate conditions from enhanced warming to present-day.Here, we assess the sea-level contribution from mass balance changes of the Antarctic Ice Sheet on multi-millennial timescales, as well as ice loss reversibility. The Antarctic sea-level commitment is quantified using the Parallel Ice Sheet Model (PISM) and the fast Elementary Thermomechanical Ice Sheet (f.ETISh) model by fixing forcing conditions of warming trajectories from state-of-the-art climate models available from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) at regular intervals in time. The ice sheet then evolves for several millennia under constant climate conditions. Finally, the climate forcing is reversed to present-day starting from different stages of ice sheet decline to test for the reversibility of ice loss.Our results suggest that the Antarctic Ice Sheet may be committed to a strong grounding-line retreat or even a collapse of the West Antarctic Ice Sheet when keeping climate conditions constant at warming levels reached during this century. Fixing climate conditions later in time may additionally trigger a substantial decline of the East Antarctic Ice Sheet. We show that the reversibility of Antarctic ice loss as well as the potential for safe overshoots strongly depend on the timing of the reversal of the forcing.

The Antarctic ice sheet’s future contribution to sea level rise is difficult to predict, mostly because of the uncertainty and variability of the surface mass balance (SMB). Ice cores are used to locally (km scale) reconstruct SMB with a very good temporal resolution (up to sub-annual), especially in coastal areas where accumulation rates are high. The number of ice cores records has been increasing these last years, revealing an important spatial variability and different trends of SMB, highlighting the crucial need for greater spatial and temporal representativeness. We present records of density, water stable isotopes (δ18O, δD and deuterium excess), ions concentrations (Na+, K+, Mg+, Ca+, MSA, Cl-, SO42- and NO3-), and continuous electrical conductivity measurement (ECM), as well as age models and resulting surface mass balance from the top 120 m of two ice cores (FK17 and TIR18) drilled on two adjacent ice rises located in coastal Dronning Maud Land and dating back to the end of the 18th century. Both environmental proxies and derived data show contrasting behaviors, suggesting strong spatial and temporal variability at the regional scale. In terms of precipitation proxies, both ice cores show a long-term decrease of deuterium excess (d-excess) and a long-term increase of δ18O, although less pronounced. In terms of chemical proxies, the non-sea-salt sulfate (nssSO42-) concentrations of FK17 are twice the ones of TIR18 and display an increasing trend on the long-term while there is only a small increase after 1950 in TIR18. The SO42- / Na+ ratios show a similar contrast between FK17 and TIR18 and are consistently higher than the sea water ratio, indicating a dominant impact of the nssSO42- on the SO42- signature. The mean long-term SMB is similar for FK17 and TIR18 (0.57 and 0.56 m i.e. a-1 respectively), but the annual records are very different: since the 1950’s, TIR18 shows a continuous decrease while FK17 has shown an increasing trend until 1995 followed by a recent decrease. The datasets presented here offer numerous development possibilities for the interpretation of the different paleo profiles and for addressing the mechanisms behind the spatial and temporal variability observed at the regional scale (tens of km scale) in East Antarctica. The “Mass2Ant IceCores” datasets are available on Zenodo (https://doi.org/10.5281/zenodo.7848435; Wauthy et al., 2023).

Ice–ocean interaction is crucial for the integrity of ice shelves and thus ice sheet stability. Warm ocean currents lead to enhanced basal melting of ice shelves, which is the dominant component of mass loss for the Antarctic Ice Sheet. Knowing the current melt rates and predicting those under future climate scenarios is thus of great importance. In the course of the ­MIMO-EIS (Monitoring melt where Ice Meets Ocean) Project, we deployed a continuously measuring ApRES (Autonomous phase-sensitive Radio-Echo Sounding) device in the center of Ekström Ice Shelf, recording an hourly time series since April 2020. The continuous time series reveals a seasonal onset of enhanced melt rates, abruptly increasing from

Subglacial processes in Antarctica are difficult to directly observe and are one of the sources of uncertainty when modelling the response of ice sheets to environmental forcing. Subglacial processes pertain to the type of basal sliding or friction law used, where especially the contrast between viscous (linear) and plastic (Coulomb) sliding makes the latter far more responsive to changes at the marine boundary (Ritz et al., 2015; Brondex et al., 2019; Bulthuis et al., 2019; Sun et al., 2020; Kazmierczak et al., 2022).Besides the type of sliding, physical basal conditions, such as basal temperatures, bed properties (hard or soft), subglacial water flow and drainage, till properties, and mechanics, also directly affect the ice sheet flow (Clarke, 2005, Cuffey and Patterson, 2010, Kazmierczak et al., 2022), by affecting the effective pressure (Bueler and Brown, 2009, Winkelmann et al., 2011, van der Wel et al., 2013).In this study, we investigate how variations in effective pressure determines the evolution of the main marine basins of the West Antarctic Ice Sheet (Pine Island and Thwaites glacier) which are currently exhibiting the largest ice mass loss of the Antarctic ice sheet.For this purpose, we employ different types of subglacial hydrology for soft and hard bed configurations, which we adapt to simulate individual drainage basins of the Antarctic ice sheet at a km-scale resolution, thus allowing for proper migration of the grounding line (e.g., Pattyn et al., 2013). These hydrological representations are included in a generalized basal sliding law (Zoet et al., 2020), implemented in the the f.ETISh/Kori model (Pattyn, 2017; Sun et al., 2020). Results are compared to results from a pan-Antarctic ice sheet model (Kazmierczak et al., 2022) and demonstrate the importance of detailed bed topography influencing the subglacial conditions upstream of the grounding line.

Marine ice sheets are complex systems whose response to external forcing is the subject of much attention in the scientific community. In particular, the West Antarctic ice sheet, which could have a significant impact on future sea-level rise, is a major concern. One method of studying the response of marine ice sheets consists in investigating the relationship between the parameters and the equilibrium states of such systems. However, this is typically done by varying these parameters and letting the ice sheets evolve to new steady states, i.e., through transient simulations, which are computationally expensive.An alternative is to consider continuation methods, where the equilibrium state of a system is studied directly as a function of the parameters. Such an approach has already been used in glaciology to study the mechanical behaviour of 1D marine ice sheets (Mulder et al., 2018), highlighting the hysteresis phenomena that had previously been obtained theoretically (Schoof, 2007). However, this study, because it only considers 1D geometries, does not allow to take into account the effect of lateral drag and of complex bedrock geometries, which are two factors that have the potential to stabilize the grounding line (Gudmundsson, 2013, Sergienko and Wingham, 2021).Here we consider the continuation problem in the context of 2D marine ice sheets. This introduces several mathematical difficulties, notably related to the treatment of the distinction between grounded and floating parts. Mathematically, the problem takes the form of a contact problem between the bedrock and the lower part of the ice sheet. This leads to a system of equations that is not differentiable, which is challenging to solve numerically. We address these challenges in the context of the continuation problem, and propose several solutions, including a norm-based approach that is inspired from earlier studies (Mittelmann, 1987). Finally, we present some preliminary results which show that our numerical method is promising.

Major uncertainties in the response of ice sheets to environmental forcing are due to subglacial processes. These processes pertain to the type of sliding or friction law as well as the spatial and temporal evolution of the effective pressure at the base of ice sheets. We evaluate the classic Weertman–Budd sliding law for different power exponents (viscous to near plastic) and for different representations of effective pressure at the base of the ice sheet, commonly used for hard and soft beds. The sensitivity of the above slip laws is evaluated for the Antarctic ice sheet in two types of experiments: (i) the ABUMIP experiments in which ice shelves are instantaneously removed, leading to rapid grounding-line retreat and ice sheet collapse, and (ii) the ISMIP6 experiments with realistic ocean and atmosphere forcings for different Representative Concentration Pathway (RCP) scenarios. Results confirm earlier work that the power in the sliding law is the most determining factor in the sensitivity of the ice sheet to climatic forcing, where a higher power in the sliding law leads to increased mass loss for a given forcing. Here we show that spatial and temporal changes in water pressure or water flux at the base modulate basal sliding for a given power, especially for high-end scenarios, such as ABUMIP. In particular, subglacial models depending on subglacial water pressure decrease effective pressure significantly near the grounding line, leading to an increased sensitivity to climatic forcing for a given power in the sliding law. This dependency is, however, less clear under realistic forcing scenarios (ISMIP6).

Sub-shelf melting is one of the dominant drivers of Antarctica's ice sheet mass loss. Various sub-shelf melt rate parameterizations exist for standalone ice models, but they lack the capability to capture complex ocean circulation within ice shelf cavities. To overcome drawbacks of standalone models and to improve melt parameterizations, high resolution coupling of ice sheet and ocean models are capable of hindcasting past decennia and be compared to observations.Here, we present results of a hindcast (1985-2018) of the new circumpolar coupled Southern Ocean – Antarctic ice sheet configuration, developed within the framework of the PARAMOUR project. The configuration is based on the ocean and sea ice model NEMO3.6-LIM3 and the ice sheet model f.ETISh v1.7. The coupling routine facilitates exchange of monthly sub-shelf melt rates (from ocean to ice model) and evolving ice shelf cavity geometry (from ice to ocean model).For a selection of warm and cold cavities, we investigate the impact of the coupling frequency (more precisely, the frequency of updating the ice shelf cavity geometry within the ocean model) and the coupling itself on the sub-shelf melt rates and its feedback on the ice dynamics. We further compare the sub-shelf melt rates and the response of the ice sheet with observations and other studies.

A large uncertainty in the response of ice sheet models to environmental forcing is stems from are fragmented knowledge on basal sliding and friction operating at the base of the ice sheet. Recent studies (Ritz et al., 2015; Brondex et al., 2019; Bulthuis et al., 2019) have demonstrated that plastic sliding laws lead to a higher ice sheet sensitivity compared to viscous ones. Of interest is therefore a regularized Coulomb friction law, which leads to viscous or power-law behaviour at low ice velocities and Coulomb plastic behaviour at high ice velocities (Schoof, 2005; Gagliardini et al., 2007; Joughin et al., 2019; Zoet and Iverson, 2020; Helanow et al., 2021). Here, we analyze different descriptions of the regularized Coulomb friction law, with and without a dependence on effective pressure in an ice sheet model of intermediate complexity (f.ETISh; Pattyn, 2017) for idealized (MISMIP+) and real case (Thwaites Glacier, Antarctica) ice sheet geometries. We furthermore derived grounding line flux conditions based on boundary layer theory and compared to a numerical flowline model. It is shown that the grounding line flux for the regularized Coulomb friction law independent on effective pressure has a dependence on ice thickness to the power n+3 for the pure plastic case.

Major uncertainties in the response of ice sheets to environmental forcing are due to subglacial processes. These processes pertain to the type of basal sliding or friction law especially the contrast between viscous (linear) and plastic (Coulomb) sliding makes the latter far more responsive to changes at the marine boundary (Ritz et al., 2015, Brondex et al., 2019, Bulthuis et al., 2019, Sun et al., 2020).Besides the sliding type, physical basal conditions, such as basal temperature conditions, bed properties (hard or soft), subglacial water flow and drainage, or till properties and mechanics, add to the sensitivity of ice sheet flow (Clarke, 2005, Cuffey and Patterson, 2010). These parameters can influence the effective pressure (Bueler and Brown, 2009, Winkelmann et al., 2011, van der Wel et al., 2013, Bueler and van Pelt, 2015).In this study, we investigate the sensitivity due to the effective pressure variation in the main marine basins of the West Antarctic ice sheet (Pine Island and Thwaites glacier) where most of the current ice mass loss is situatedFor this purpose, we use different subglacial hydrologies in a thermomecanical coupled model (f.ETISh; Pattyn, 2017), adapted to simulate individual drainage basins of the Antarctic ice sheet at a km spatial resolution. This prevents us to apply flux conditions at the grounding line and simulate grounding line migration (Pattyn et al., 2013). Results show a complex and wide variety of mass changes in forcing scenarios following CMIP6 climatologies.

In September 2021, two jökulhlaups were released into the Skaftá river from western Vatnajökull icecap (Iceland). Such floods have been known since 1955. These jökulhlaups originate from two subglacial lakes under 1–3 km wide and 50–150 m deep depressions in the glacier surface, commonly referred to as ice cauldrons, formed by geothermal melting. The average time interval between jökulhlaups from each cauldron is ~2 a. The jökulhlaups travel ~40 km under a glacier that reaches maximum thickness of ~750 m and emerge in the Skaftá river at the terminus of Skaftárjökull outlet glacier. In addition to increased subglacial water flow and river discharge, these floods are responsible for seismicity and tremor onset, inside the cauldrons, along the subglacial channels, at the outlet where the subglacial channels meet the river, as well as on the river path. We used seismic interferometry or cross-correlation of seismic noise to analyse data from 2015 to the end of 2021. We computed cross-correlation functions for 27 seismic stations and for frequencies between 0.5 and 8 Hz. To characterize these floods, we calculated the propagation velocities based on the cross-correlation functions and for each frequency band. We located seismic signatures both during the floods period and previous times by using a grid-search method based on the approach of Ballmer et al. 2013, which calculates theoretical differential times and provides probabilities of locations as the summed stack amplitudes of correlograms. These daily location grids allowed us to analyse the spatial evolution of probability of location during these floods as well as compare them with previous “non-flood” periods. Furthermore, based on these location grids, we were able to compute temporal series for isolated locations (pixels) such as ice cauldrons, a hydrological station located on the river path, or any other target, allowing us to analyse the temporal evolution of location probability during the floods as well as compare it with the last six years of data. The resolution of this temporal evolution ranges from monthly to hourly. By using six years of seismic data, we were also able to compare the 2021 floods with floods due to the same ice cauldrons, for example in August 2018 and September 2019.This study provides an insight on how relevant seismic interferometry can be in the monitoring of such processes, with the purpose of being fully automatic in the near future.

Ice mass loss on Greenland and Antarctica is a major contributor to sea level change and will thereby profoundly impact the world's infrastructure (e.g. transport, roads, ground water, housing) over the next decades. In order to react and adjust now accordingly, precise estimates of sea level change are needed. Though, future changes in sea level are provided by Earth system models, which rarely include ice sheet models, or by standalone ice sheet models. Hence, feedbacks between ice and atmosphere-ocean are overseen. Local scale coupled models can help bridging this gap by estimating how feedbacks between the different Earth systems affect global sea level estimates.Here, we present results from a coupled simulation of the ocean-sea ice model NEMO3.6-LIM3 (1/24° grid ~ less than 2 km grid spacing) and the ice sheet model BISICLES (on 0.5-4km spatial resolution). The coupling routine is done via python code including variable exchange, pre- and postprocessing, done offline every 3 months, following the setup described in Pelletier et al., 2021.Simulated ice mass changes, grounding line position and ice velocity changes of this high-resolution coupling scheme (between 1993-2014) are compared to observations and results of uncoupled simulations. We further discuss which processes might be neglectable and which are the main drivers of ice velocity acceleration and changes in sub-shelf ocean circulation. Pelletier, C., Fichefet, T., Goosse, H., Haubner, K., Helsen, S., Huot, P.-V., Kittel, C., Klein, F., Le clec'h, S., van Lipzig, N. P. M., Marchi, S., Massonnet, F., Mathiot, P., Moravveji, E., Moreno-Chamarro, E., Ortega, P., Pattyn, F., Souverijns, N., Van Achter, G., Vanden Broucke, S., Vanhulle, A., Verfaillie, D., and Zipf, L.: PARASO, a circum-Antarctic fully-coupled ice-sheet - ocean - sea-ice - atmosphere - land model involving f.ETISh1.7, NEMO3.6, LIM3.6, COSMO5.0 and CLM4.5, Geosci. Model Dev. Discuss. [preprint], https://doi.org/10.5194/gmd-2021-315, in review, 2021.

Marine ice sheets are complex systems with a highly non-linear behavior. There remains a large uncertainty about how various physical processes such as the basal friction and the subglacial hydrology affect the dynamics of the grounding line (GL). One possibility to better understand their mechanical behavior consists in adopting a boundary-layer analysis close to the GL. Specifically, one can derive a so-called flux condition, which is an analytical expression for the amount of ice that flows through this GL per unit time. In turn, this flux condition can provide useful information about the grounding-line dynamics, including the presence of hysteresis (Schoof, 2007b).Several studies have introduced hybrid friction laws to model friction between the grounded part of the ice sheet and the bedrock (Schoof, 2005, Gagliardini et al., 2007). These friction laws behave as power-law friction laws far from the GL and plastically closer to it. Recent experiments have shown that these models are more realistic than the usual power-law friction (Zoet and Iverson, 2020). In parallel, sophisticated models for the subglacial hydrology have been developed (Bueler and van Pelt, 2015).In this presentation, we show that the flux conditions previously derived for the Weertman friction law (Schoof, 2007a) and the Coulomb friction law (Tsai et al., 2015) can be extended to a flux condition for the general Budd friction law, with two different simple effective-pressure models for the subglacial hydrology. Using asymptotic developments, we provide a justification for the existence and uniqueness of a solution to the boundary-layer problem. Finally, we generalize our results to hybrid friction laws, based on a parametrization of the flux condition.

With a sea-level rise potential of 58 m sea-level equivalent, the future evolution of the Antarctic Ice Sheet under progressing warming is of importance for coastal communities, ecosystems and the global economy. Short-term projections of the sea-level contribution from Antarctica in the recent ice sheet model intercomparison ISMIP6 range from a slight mass gain (-7.8 cm) to a mass loss of up to 30.0 cm sea-level equivalent at the end of the century under Representative Concentration Pathway 8.5 (Seroussi et al., 2020, Edwards et al., 2021). However, due to high inertia of the system, the ice sheet response to perturbations in its climatic boundary conditions are rather slow. Consequences of potentially triggered unstable ice loss due to positive feedback mechanisms may therefore play out over long timescales (on the order of millennia). Projections of the committed sea-level change at a given point in time, that is the sea-level change which arises by fixing the climatic boundary conditions and letting the ice sheet evolve over several millennia, might differ substantially from the sea-level change expected at that point in time (Winkelmann et al., 2022).Previous assessments of the long-term contribution to sea-level rise from the Antarctic Ice Sheet have been primarily restricted to a single model and have rarely explored the full range of intra- and inter-model parameter uncertainties. Here, we determine the long-term, multi-millennial sea level contribution from mass balance changes of the Antarctic Ice Sheet by means of two ice sheet models, the Parallel Ice Sheet Model (PISM) and the fast Elementary Thermomechanical Ice Sheet (f.ETISh) model. More specifically, we assess the response of the Antarctic Ice Sheet to atmospheric and oceanic forcing conditions derived from state-of-the-art climate model projections available from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) under the Shared Socioeconomic Pathways SSP5-8.5 and SSP1-2.6 available until the year 2300. The sea-level commitment from the Antarctic Ice Sheet is quantified by branching off at regular intervals in time and running the ice sheet models for several millennia under fixed climate conditions. Key uncertainties related to ice dynamics as well as to interactions with the bed, atmosphere and ocean are taken into account in an ensemble approach.

The vast majority of the Antarctic ice sheet is covered with snow that compacts under its own weight and transforms into ice below the surface. However, in some areas, this typically blue-colored ice is directly exposed at the surface. These so-called "blue ice areas" represent islands of negative surface mass balance through sublimation and/or melt. Moreover, blue ice areas expose old ice that is easily accessible in large quantities at the surface, and some areas contain ice that extends beyond the time scales of classic deep-drilling ice cores.Observation and modeling efforts suggest that the location of blue ice areas is related to a specific combination of topographic and meteorological factors. In the literature, these factors are described as (i) enhanced katabatic winds that erode snow, due to an increase of the surface slope or a tunneling effect of topography, (ii) the increased albedo of blue ice (with respect to snow), which enhances ablative processes, and (iii) the presence of nunataks (mountains protruding the ice) that act as barriers to the ice flow upstream, and prevent deposition of blowing snow on the lee side of the mountain. However, it remains largely unknown which role the physical processes play in creating and/or maintaining blue ice at the surface of the ice sheet.Here, we study how a combination of environmental and topographic factors lead to the observation of blue ice. We also quantify the relevance of the single processes and build an interpretable model aiming at not only predicting blue ice presence, but also explaining why it is there. To do so, data is fed into a convolutional neural network, a machine learning algorithm which uses the spatial context of the data to generate a prediction on the presence of blue ice areas. More specifically, we use a U-Net architecture that through convolutions and linked up-convolutions allows to obtain a semantic segmentation (i.e., a pixel-level map) of the input data. Ground reference data is obtained from existing products of blue ice area outlines that are based on multispectral observations. These products contain considerable uncertainties, as (i) the horizontal change from snow to ice is gradual and a single threshold in this transition is not applicable uniformly over the continent, and (ii) the blue ice area extent is known to vary seasonally. Therefore, we train our deep learning model with a loss function with increasing weight towards the center of blue ice areas.Our first results indicate that the neural network predicts the location of blue ice relatively well, and that surface elevation data plays an important role in determining the location of blue ice. In our ongoing work, we analyze both the predictions and the neural network itself to quantify which factors posses predictive capacity to explain the location of blue ice. Eventually this information may allow us to answer the simple yet important question of why blue ice areas are located where they are, with potentially important implications for their role as paleoclimate archives and for their evolution under changing climatic conditions.

How well is the Antarctic climate over the last decades represented in climate models and how predictable is its future evolution? These questions delve into the specificities of the Antarctic climate, a system characterized by large natural fluctuations and complex interactions between the ice sheet, ocean, sea ice and atmosphere. The PARAMOUR project aims at improving our understanding of key processes which control the variability and predictability of the Antarctic climate at the decadal timescale. In this context, we introduce PARASO, a novel fully-coupled regional ocean - sea-ice - ice-sheet - atmosphere climate model over an Antarctic circumpolar domain covering the full Southern Ocean. The state-of-the-art models used are f.ETISh1.7 (ice sheet), NEMO3.6 (ocean), LIM3.6 (sea ice), COSMO5.0 (atmosphere) and CLM4.5 (land), which are run at a horizontal resolution close to 1/4°. One key feature of this tool resides in a novel two-way coupling interface for representing the ocean - ice-sheet interactions, through explicitly resolved ice-shelf cavities. We also consider the impact of atmospheric processes on the Antarctic ice sheet through surface mass exchanges between COSMO-CLM and f.ETISh. Our developments include a new surface tiling approach to combine open-ocean and sea-ice covered cells within COSMO. Using a 30 year-long run, we investigate the model performance and the interannual-to-decadal variability of the simulated Antarctic climate. The focus is on the interactions between the atmosphere, ocean and ice components at the regional scale and the links with larger spatial scales. Specific attention is paid to the mass balance of ice sheets and ice shelves, which influences both the ice sheet dynamics and the changes in the ocean and atmosphere. The system and its performance will be documented in this presentation together with some aspects of decadal variability from a 30-year integration forced with reanalyses (ERA5 and ORAS5). Early results of a 3-member retrospective forecast driven by EC-Earth will also be presented.

Over the last decades, the Antarctic Ice Sheet (AIS) has been losing mass, mainly through ice discharge and sub-shelf melting (Rignot et al., 2019). More specifically, recent observations show that the AIS is currently losing mass at an accelerating rate in areas subject to strong ocean-induced melt. At the same time, no long-term trend in snowfall accumulation changes can be detected in the interior of the ice sheet. Due to these current trends, basal melting has often been considered as the main driver of future Antarctic mass loss. However, even though stronger basal melting of ice shelves is projected to drive future AIS mass loss, recent studies (e.g. Seroussi et al., 2020) have shown that surface mass balance (SMB, the balance of accumulation through snowfall and ablation through erosion, sublimation and runoff) has a strong potential in controlling the future stability and evolution of the Antarctic Ice Sheet. With increasing temperatures, SMB is expected to increase in Antarctica in the future as a result of enhanced snowfall. As long as the warming remains modest, other AIS SMB components (such as runoff) will likely continue to play a minor role in future SMB changes (Lenaerts et al., 2019; Kittel et al., 2021). Under high-emission scenarios, however, future runoff is likely to significantly compensate for mass gain through snowfall (Kittel et al 2021). The balance between these competing processes is still a matter of debate and, as of yet, there is no consensus on estimates of the future mass balance of the Antarctic Ice Sheet (Seroussi et al., 2020).Here, we investigate the relative importance of SMB changes and ocean-induced melt on the long-term (multi-centennial to multi-millennial) AIS response as well as their associated uncertainties. To do so, we force two ice sheet models (fETISh and PISM) with atmospheric and oceanic projections inferred from a subset of models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) under the Shared Socioeconomic Pathways (SSP) 5-8.5 and SSP1-2.6. Changes in precipitation rate and air temperature are corrected for elevation changes and used as inputs to a positive degree-day scheme which estimates changes in snowfall, rainfall and surface runoff. Climate projections are used as forcing until the year 2300 and afterwards no climate trend is applied, allowing to investigate the long-term impacts of early-millennia warming (often called sea-level commitment).Taking into account key uncertainties in both atmospheric and oceanic forcing, our results predict that atmosphere-ice surface interactions will have an important role on the AIS stability under high-end future emission scenarios. We also show the increasingly important role of the melt-elevation feedback for multi-centennial projections of the AIS. Finally, we find that modelling choices regarding the atmosphere forcing have a significant influence on the future sea-level contribution from the AIS under high-end emission scenarios, leading to a spread from a few centimeters to several meters contribution over the coming millennia.