Elmer/Ice News

The stability of present-day Antarctic grounding lines

hill2023Theoretical and numerical work has shown that under certain circumstances grounding lines of marine-type ice sheets can enter phases of irreversible advance and retreat driven by the marine ice sheet instability (MISI). Instances of such irreversible retreat have been found in several simulations of the Antarctic Ice Sheet. However, it has not been assessed whether the Antarctic grounding lines are already undergoing MISI in their current position. Here, we conduct a systematic numerical stability analysis using three state-of-the-art ice sheet models: Úa, Elmer/Ice, and the Parallel Ice Sheet Model (PISM). For the first two models, we construct steady-state initial configurations whereby the simulated grounding lines remain at the observed present-day positions through time. The third model, PISM, uses a spin-up procedure and historical forcing such that its transient state is close to the observed one. To assess the stability of these simulated states, we apply short-term perturbations to submarine melting. Our results show that the grounding lines around Antarctica migrate slightly away from their initial position while the perturbation is applied, and they revert once the perturbation is removed. This indicates that present-day retreat of Antarctic grounding lines is not yet irreversible or self-sustained. However, our accompanying paper (Part 2, Reese et al.2023) shows that if the grounding lines retreated further inland, under present-day climate forcing, it may lead to the eventual irreversible collapse of some marine regions of West Antarctica.

See the TiPACCs video explaining the results of these two papers!

To read more: Hill E. A., B. Urruty, R. Reese, J. Garbe, O. Gagliardini, G. Durand, F. Gillet-Chaulet, G. H. Gudmundsson, R. Winkelmann, M. Chekki, D. Chandler and P. M. Langebroek, 2023. The stability of present-day Antarctic grounding lines - Part 1 : No indication of marine ice sheet instability in the current geometry, The Cryosphere, 17, 3739–3759, doi:10.5194/tc-17-3739-2023

and the companion paper:  Reese, R., J. Garbe, E. A. Hill, E. A., B. Urruty, K. A. Naughten, O. Gagliardini, G. Durand, F. Gillet-Chaulet, G. H. Gudmundsson, D. Chandler, P. M. Langebroek and R. Winkelmann, 2023. The stability of present-day Antarctic grounding lines – Part 2 : Onset of irreversible retreat of Amundsen Sea glaciers under current climate on centennial timescales cannot be excluded, The Cryosphere, 17, 3761–3783, doi:10.5194/tc-17-3761-2023

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Inferring the Basal Friction Law From Long Term Changes of Glacier Length, Thickness and Velocity on an Alpine Glacier

gilbert2023Modeling the evolution of glaciers and ice sheets under climate variability requires accurate estimation of ice flow velocities by numerical models. One of the most challenging components of these models is the representation of basal sliding at the rock-ice interface, generally described by relationships between stress and sliding speed. These relationships are mainly derived from laboratory experiments and theoretical studies, and their ability to represent nature still needs to be evaluated. Because direct observation beneath glaciers and ice sheets is difficult, few studies have attempted to validate sliding models from natural scale observations. In this context, our study provides a rare constraint, based on observations on an alpine glacier, on the law that should be used to model glacier sliding on clean bedrock (so-called hard-bed glaciers). We show that a simple power law performs well in explaining long-term glacier behavior for a power exponent of ∼3.1. We suggest that this exponent should be adopted in non-linear power laws incorporated into ice flow models that perform future projections of hard-bedded glaciers and ice sheet evolution.

Read more: Gilbert A., F. Gimbert, O.  Gagliardini and  C. Vincent, 2023 Inferring the basal friction law from long term changes of glacier length, thickness and velocity on an alpine glacierGeophysical Research Letters50, e2023GL104503. doi:10.1029/2023GL104503

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Inquiring past erosions at overdeepenings

egqsj-72-189-2023-avatar-web.jpg At the Last Glacial Maximum (LGM), the Rhine Glacier complex (Rhine and Linth glaciers) featured numerous overdeepened valleys as a consequence of repeated glaciations. Their location is close to the LGM ice margin, far in the lowlands, away from the Alps. Numerical models of ice flow of the Rhine Glacier indicate a poor fit between the sliding distance, a proxy for glacial erosion, and the location of these overdeepenings. Calculations of the hydraulic potential based on the computed time-dependent ice surface elevations of the Rhine Glacier lobe obtained from a high-resolution thermo-mechanically coupled Stokes setup of Elmer/Ice are used to estimate the location of subglacial water drainage routes. Results reveal an elevated subglacial water discharge focused along glacial valleys and overdeepenings under the assumption that of water pressure being equal to the ice overburden pressure. These conditions are necessary for subglacial water to remove basal sediments, expose fresh bedrock, and favor further erosion by quarrying and abrasion. Knowledge of the location of paleo-subglacial water drainage routes appears to be a necessary component in understanding patterns of subglacial erosion beneath paleo-ice masses that can not be explained purely by sliding - a fact that also seems to apply to the here studied overdeepenings in the Swiss lowlands.

 

Cohen, D., Jouvet, G., Zwinger, T., Landgraf, A., and Fischer, U. H., 2023. Subglacial hydrology from high-resolution ice-flow simulations of the Rhine Glacier during the Last Glacial Maximum: a proxy for glacial erosion,. E&G Quaternary Sci. J., 72, 189–201, DOI:10.5194/egqsj-72-189-2023

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