Subglacial hydrology modulates basal sliding response of the Antarctic ice sheet to climate forcing

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).</p

A new, fast and unified subglacial hydrological model applied to Thwaites Glacier, Antarctica

Subglacial hydrology is a crucial element for understanding the dynamics of marine ice sheets. Indeed, the presence of subglacial water modulates the ice basal motion, resulting in a modified ice flow across the entire ice sheet. Nonetheless, the subglacial environment is difficult to reach, which makes it necessary to develop models. Many efforts have recently been made in the glaciological and hydrological communities to improve their accuracy and efficiency. Even so, the models currently being developed are typically fairly costly in terms of computing time. As a consequence, conducting numerical simulations over long time scales or running ensemble simulations remains particularly challenging. Here, we propose a simplified approach for coupling subglacial hydrology with the motion of ice. First, we introduce a computationally efficient subglacial hydrology model that is suited for hard and soft bed types as well as efficient and inefficient drainage systems. Then, we show some numerical results based on our implementation of this model within the Kori-ULB ice-sheet code. We first study the impact of subglacial hydrology in the idealized MISMIP configuration. Subsequently, we show results of simulations conducted over Thwaites Glacier which suggest that the coupling of subglacial hydrology with ice flow could significantly increase the contribution of marine ice sheets to future sea-level rise

On the challenges of producing (robust) Antarctic sea level projections

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The long-term future of the Antarctic ice sheet: Uncertainties in ice sheet-Earth system interactions

The Antarctic ice sheet (AIS) is the largest and yet the most uncertain potential contributor to future sea-level rise. While recent satellite observations have highlighted that the AIS is currently losing mass at an accelerating rate, projections of the future evolution of the ice sheet in a warming climate remain highly uncertain. As future sea-level rise is probably one of the biggest threats imposed on us by climate change, predicting it with the lowest possible uncertainty is of capital societal importance. Uncertainties in the future evolution of the AIS can be explained, notably, by the fact that the ice sheet is capable of abrupt and self-sustained changes associated with several positive feedback mechanisms, especially in its marine areas, i.e. where the ice lies on bedrock below sea level. This is the case for most of the West Antarctic ice sheet (WAIS) as well as for some basins of the East Antarctic ice sheet (EAIS). The interactions between the ice sheet and its surrounding environment (namely the ocean, the atmosphere, and the solid Earth) have been shown to strongly influence its stability, more particularly by triggering or dampening the instabilities threatening the ice sheet. Despite the uncertainties, recent studies suggest that the WAIS will lose mass in the future and eventually (partially) collapse. The uncertainties pertain to when, and to whether the weak Earth structure beneath that area of the ice sheet may be a stabilising factor, as a rapid bedrock uplift in response to ice mass loss has been shown to delay or even limit mass loss. The fate of the EAIS is less clear. A pending question is: will the EAIS lose or gain mass in the future? More specifically, will the grounding line retreat in its marine basins, and if so, can the associated mass loss be compensated by sufficient mass gain due to increased snow accumulation in the interior of the ice sheet?In this thesis, we contribute to clarifying and providing new insights to these questions, and therefore on the long-term future of the AIS. Using a numerical ice-sheet model, we investigate the influence of uncertainties in ice sheet–Earth system interactions on its future stability. In addition, we produce observationally-calibrated projections and associated quantified uncertainties of the evolution of the AIS over the millennium. Our results show that the ocean will be the main driver of Antarctic short-term mass loss, leading to significant retreat in the WAIS (especially in the Amundsen Sea Embayment), even under limited warming. Under sustained warming, however, this may lead to a complete WAIS collapse over the course of the millennium, despite a stabilising weak solid Earth structure beneath West Antarctica. In addition, our results suggest that a sustained warming will likely turn the EAIS into a positive contributor to sea-level rise over the course of the next century. Indeed, we project that the ocean-driven grounding-line retreat in its marine basins, which cannot be efficiently stabilised by bedrock uplift given the rigid structure of the solid Earth in that area, will progressively outweigh the surface mass balance (SMB, the balance between accumulation, sublimation and runoff at its surface). Finally, we show that the mitigating role of the SMB may strongly be reduced under sustained warming, due to a significant increase in surface runoff with increasing temperatures, hence further increasing the net AIS contribution to sea-level rise.La calotte glaciaire Antarctique est le plus gros contributeur potentiel à l'élévation future du niveau marin global, mais aussi le plus incertain. De plus en plus d’observations satellitaires mettent en évidence une actuelle perte de masse de l'Antarctique, et ce à un rythme accéléré. Malgré cela, les projections de l'évolution future de la calotte Antarcique en réponse aux changements climatiques actuels et à venir restent très incertaines. Pourtant, être en mesure de prédire avec la plus faible incertitude possible l'élévation future du niveau de la mer est d'une importance sociétale capitale. Les incertitudes sur l'évolution future de l'Antarctique s'expliquent notamment par le fait que la calotte glaciaire est capable de changements brusques associés à plusieurs mécanismes de rétroactions positives, et ce particulièrement dans les régions où la glace repose sur un socle rocheux situé sous le niveau de la mer. C'est le cas pour la majorité de l'Antarctique de l’Ouest ainsi que dans certains bassins de l’Antarctique de l’Est. Il a été démontré que les interactions entre la calotte glaciaire et son environnement (à savoir l'océan, l'atmosphère et le lit rocheux sous-jacent) influencent fortement sa stabilité, notamment en déclenchant ou en atténuant les instabilités qui la menacent. Malgré les incertitudes, des études récentes suggèrent que, à l'avenir, l’Antarctique de l’Ouest perdra de la masse et finira par (au moins partiellement) s'effondrer. Les incertitudes concernent dès lors le moment de cet effondrement, mais également une potentielle stabilisation de la calotte par le lit rocheux (qui repose sur une région du manteau terrestre particulièrement peu visqueuse) situé sous cette région occidentale. En effet, il a été démontré qu'un rebond rapide du lit rocheux pouvait ralentir voire limiter la perte de masse de la calotte. Le sort de l'Antarctique de l’Est est moins clair. Une des questions en suspens est la suivante :l'Antarctique de l’Est perdra-t-elle ou gagnera-t-elle de la masse ?Plus précisément, perdra-t-elle de la masse dans ses régions potentiellement instables, et, si oui, cette perte de masse sera-t-elle compensée par un gain de masse associé à une augmentation des précipitations neigeuses à l'intérieur du continent ?Cette thèse contribue à clarifier et à apporter de nouveaux éléments de réponses à ces questions, permettant ainsi d’en savoir plus sur l’évolution future de la calotte Antarctique. Pour ce faire, nous étudions, à l'aide d'un modèle numérique de calotte glaciaire, l'influence des incertitudes concernant les interactions de la calotte avec son environnement sur sa stabilité future. De plus, nous produisons, tout en quantifiant les incertitudes, des projections de l'évolution de l’Antarctique au cours du prochain millénaire. Nos résultats montrent que l'océan sera le principal moteur de la perte de masse à court terme de l'Antarctique, engendrant un recul important en Antarctique de l'Ouest (et plus particulièrement au niveau de la mer d'Amundsen), et ce même en cas de réchauffement climatique limité. Dans le cas d'un réchauffement soutenu, cependant, cela pourrait conduire, malgré le rebond du lit rocheux, à un effondrement complet de l’Antarctique de l’Ouest au cours du millénaire. De plus, nos résultats suggèrent qu'un réchauffement soutenu transformera probablement l'Antarctique de l’Est en un contributeur positif à l'élévation du niveau marin global au cours du siècle prochain. En effet, nous prévoyons que le recul de la calotte dans ses bassins instables, non efficacement stabilisé par le rebond du lit rocheux étant donné le manteau terrestre très visqueux dans cette région, l'emportera progressivement sur son bilan de masse de surface (c’est-à-dire l'équilibre entre accumulation, sublimation et ruissellement à la surface de la calotte). Enfin, nous démontrons que le bilan de masse de surface peut être fortement réduit en cas de réchauffement soutenu, en raison d'une augmentation significative du ruissellement de surface avec l'augmentation des températures, augmentant ainsi la contribution nette de l'Antarctique à l'élévation du niveau marin.Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Investigating the Antarctic origin of Meltwater Pulse 1A

During the last glacial termination, a phase of abrupt global sea-level rise called Meltwater Pulse 1A took place around 14 500 BP. Although the timing and the magnitude of this event have become better constrained, a causal link between the MWP-1A and an accelerated ice loss from the Antarctic ice sheet has still not been proven. Understanding the origin of this meltwater pulse is of the highest importance when considering the current uncertainty surrounding a potential collapse of the Antarctic ice sheet in response to present-day climate change.We simulated the evolution of the Antarctic ice sheet over the last 40 kyr using the ice-sheet model f.ETISh (Pattyn, 2017). A large ensemble of 54 runs is used to calibrate the model to modern and geologic data and determine a best-fit simulation. Moreover, a sensitivity analysis is carried out to assess the conditions for the occurrence of significant melting of the Antarctic ice sheet. With the best-fit simulation, the influence of an accentuated oceanic melting coincident with Meltwater Pulse 1A is tested to evaluate the likely contribution of Antarctica to this event. For the Antarctic ice sheet to have made a meaningful contribution to the Meltwater Pulse 1A, a sufficient sea-level equivalent ice volume must have existed at the Last Glacial Maximum. Then, this ice volume must have been discharged at the correct time and at a rate fast enough to contribute to this rapid sea-level rise (Golledge et al. 2014).Results reveal that a sufficient ice volume has existed at the Last Glacial Maximum in Antarctica. However, abrupt rises in seal-level and air temperatures, characteristic of the last glaciation termination, do not, on their own, trigger rapid retreat of the modelled ice sheet. For the Antarctic ice sheet to have made a meaningful contribution to the Meltwater Pulse 1A, a specific interaction between the ice sheet and its surrounding ocean – independent of the atmospheric forcing - seems to be necessary. More specifically, rates of freshwater flux from the Antarctic ice sheet are highest when, by a still undetermined mechanism, an episode of accentuated sub-shelf oceanic melting occurs. This latter instigates a positive feedback that further accelerates retreat of marine-based sectors of the Antarctic ice sheet. Under these conditions, a contribution of nearly 1.2 meters of sea-level equivalent from the Antarctic ice sheet is predicted at the time of MWP-1A, representing a meaningful but still minor (between 5 and 10%) contribution to this event. Even forced by such a substantial accentuation of sub-shelf melting, the Antarctic ice sheet can therefore not be considered as the major contributor of MWP-1A.info:eu-repo/semantics/publishe

Modelling the Antarctic ice sheet over the historical period: challenges, improvements, and weather vs climate

Invited talkinfo:eu-repo/semantics/nonPublishe

Sensitivity of a spatially-varying Elastic Lithosphere-Relaxed Asthenosphere (ELRA) isostatic model of the Antarctic ice sheet

On glacial-interglacial time scales, isostatic effects of ice sheet volume changes may have a large impact on timingand magnitude of retreating and advancing ice through grounding-line migration. A common isostatic model usedin ice-flow modelling is the Elastic Lithosphere-Relaxed Asthenosphere (ELRA) model. It considers an elasticlithosphere, defined by a given effective lithosphere thickness and a relaxation equation for asthenospheric responsewith a characteristic time scale as a function of asthenosphere viscosity. However, effective lithosphere thicknessin Antarctica ranges from tens (West-Antarctica) to hundreds of meters (East Antarctica), leading to a flexuralrigidity that varies spatially across several orders of magnitude. Furthermore, recent studies also point out to alarge spatial variability in asthenosphere viscosity. Here, we explore in a sensitivity analysis both spatially uniformand spatially varying values of flexural rigidity and asthenosphere viscosity applied to a model of the Antarcticice sheet, forced by background temperature and sea-level changes over the last 40,000 years, thus covering thelast glacial-interglacial transition. Results demonstrate a higher sensitivity for the West Antarctic ice sheet, whereasthenosphere viscosity essentially influences timing and magnitude of grounding-line retreat during the glacialinterglacialtransition.info:eu-repo/semantics/publishe

On calculating the sea-level contribution in marine ice-sheet models

Estimating the contribution of marine ice sheets to sea-level rise is complicated by ice grounded below sea level that is replaced by ocean water when melted. The common approach is to only consider the ice volume above floatation, defined as the volume of ice to be removed from an ice column to become afloat. With isostatic adjustment of the bedrock and external sea-level forcing that is not a result of mass changes of the ice sheet under consideration, this approach breaks down, because ice volume above floatation can be modified without actual changes in the sea-level contribution. We discuss a generalised approach for estimating the sea-level contribution from marine ice sheet models that conserves mass and consistently takes the effect of bedrock changes into account.info:eu-repo/semantics/nonPublishe

Sensitivity of a spatially-varying Elastic Lithosphere-Relaxed Asthenosphere (ELRA) isostatic model of the Antarctic ice sheet

On glacial-interglacial time scales, isostatic effects of ice sheet volume changes may have a large impact on timingand magnitude of retreating and advancing ice through grounding-line migration. A common isostatic model usedin ice-flow modelling is the Elastic Lithosphere-Relaxed Asthenosphere (ELRA) model. It considers an elasticlithosphere, defined by a given effective lithosphere thickness and a relaxation equation for asthenospheric responsewith a characteristic time scale as a function of asthenosphere viscosity. However, effective lithosphere thicknessin Antarctica ranges from tens (West-Antarctica) to hundreds of meters (East Antarctica), leading to a flexuralrigidity that varies spatially across several orders of magnitude. Furthermore, recent studies also point out to alarge spatial variability in asthenosphere viscosity. Here, we explore in a sensitivity analysis both spatially uniformand spatially varying values of flexural rigidity and asthenosphere viscosity applied to a model of the Antarcticice sheet, forced by background temperature and sea-level changes over the last 40,000 years, thus covering thelast glacial-interglacial transition. Results demonstrate a higher sensitivity for the West Antarctic ice sheet, whereasthenosphere viscosity essentially influences timing and magnitude of grounding-line retreat during the glacialinterglacialtransition.info:eu-repo/semantics/publishe

Solid Earth and local sea-level change effects on grounding-line stability of the Antarctic ice sheet

Both isostatic effects of ice sheet volume changes and gravitationally-consistent local sea-level variations areknown to have a large impact on timing and magnitude of retreating and advancing ice through grounding-linemigration on glacial-interglacial time scales.Full self-gravitating viscoelastic solid-Earth models (SGVEM) incorporate gravitational, rotational and bedrockdeformational responses to ice-ocean mass redistribution and are thus able to solve the sea-level equation. Onthe other side of the spectrum are ELRA models (Elastic Lithosphere-Relaxed Asthenosphere), often used inconjunction with ice-sheet models. They consider an elastic lithosphere, defined by a given effective lithospherethickness and a relaxation equation for asthenospheric response with a characteristic response time as a function ofasthenosphere viscosity. However, several recent studies suggest strong lateral variations in lithospheric thicknessand asthenosphere viscosity between Eastern and Western Antarctica. More specifically, effective lithospherethickness and mantle viscosity variability in Antarctica induce large spatial variations - across several orders ofmagnitude - of both the flexural rigidity and the asthenospheric response time, with weaker Earth structure thanpreviously thought in Western Antarctica. It has been shown that the combination of bedrock uplift and localsea-level lowering associated with grounding-line retreat reduces Antarctic ice sheet (AIS) mass loss, with greaterstabilization occurring for weaker solid Earth (Gomez et al. 2015, Konrad et al. 2015). Properly approximatingthe interactions of the ice sheet with the solid Earth and local sea-level response is thus key to understand thestability and evolution of the AIS.Here, we develop a simplified Earth model based on the ELRA model that approximates the lateral variationsof the Antarctic Earth structure, leading to spatially varying asthenospheric response time and effectivelithosphere thickness. This is further combined with a gravitationally consistent description of the local sealevelnear the margin of ice sheets as a reaction to local mass changes. We explore the sensitivity of theice sheet-solid Earth-sea level system by performing a series of perturbations in fringing ice shelves leadingto grounding line retreat and analyse the effect of different model configurations on the stability of grounding lines.info:eu-repo/semantics/inPres