the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The stability of present-day Antarctic grounding lines – Part A: No indication of marine ice sheet instability in the current geometry
Benoît Urruty
Emily A. Hill
Ronja Reese
Julius Garbe
Olivier Gagliardini
Gael Durand
Fabien Gillet-Chaulet
G. Hilmar Gudmundsson
Ricarda Winkelmann
Mondher Chekki
David Chandler
Petra M. Langebroek
Abstract. Theoretical and numerical work has firmly established that 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 past and future evolution of the Antarctic Ice Sheet. However, hitherto the stability regime of Antarctic Ice Sheet grounding lines in their current position has not been assessed. Here we conduct a systematic numerical stability analysis of all the grounding lines of the Antarctic Ice Sheet to determine if they are currently undergoing irreversible retreat through MISI. To do this, we initialise three state-of-the-art ice-flow models, Úa, Elmer/Ice, and PISM, to replicate the current geometry of the Antarctic Ice Sheet, and then apply small, but numerically significant, perturbations in ocean-induced ice-shelf melt. We find that the grounding lines around Antarctica migrate slightly away from their initial position while the perturbation is applied, and then revert to the initial state once the perturbation is removed. There is no indication of irreversible or self-sustaining retreat. This suggests that present-day grounding-line retreat is driven by external climate forcing alone. Hence, if the currently observed mass imbalance were to be removed, the grounding-line retreat would likely stop. However, under present-day climate forcing, further grounding-line retreat is expected, and our accompanying paper (Part B, Reese et al., 2022) shows that this could eventually lead to a collapse of some marine regions of West Antarctica.
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Benoît Urruty et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2022-104', Anonymous Referee #1, 16 Aug 2022
- AC1: 'Author's response to anonymous referee', Benoit Urruty, 12 Oct 2022
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RC2: 'Review of Urruty et al.', Alexander Robinson, 28 Aug 2022
This study presents a comprehensive evalulation of the possibility for large-scale, internally driven retreat of present-day Antarctic grounding lines. Two different ice sheet models were spun-up to approximate a steady-state with the present-day geometry of Antarctica, while a third model was spun-up to approximate the ice sheet after transient historical forcing since 1850. Perturbation analysis was then used to determine whether a temporal increase in basal melting over 20 years could cause the ice sheet to undergo strong grounding-line retreat that would continue after the forcing was removed. In all experiments, the present-day geometry was found to be a stable configuration, in the sense that all major grounding lines essentially returned to their original position.
This study is very interesting, timely and well done. The experiments are designed to test a specific hypothesis, and the results are convincing. Furthermore, overall the authors do a good job of discussing the various caveats to their methods and using the complementary strengths of the different models and experimental setups to confirm their findings. Particularly, I think the value of the study comes across quite well in the discussion section.
In contrast to the first reviewer, I find no major impediments to publication. I do agree that some of the framing in the Introduction and Methods could be more precise, with a few comments noted below. But I would recommend publication after only minor revisions.
Specific comments
L46: Delete "In the future," as it doesn't seem to fit. Maybe instead add an "also" to become "have also shown potential"
L51: larger event => larger one
L52: marine basins => marine basins,
L57: "The aim of this paper is to determine if stable grounding-line positions exist in the current geometry of the ice sheet." <= Rephrase here. The current geometry has been stable for several thousand years now.
L63: "control parameter that satisfies the steady state condition" <= It should be a control variable that satisfies the steady-state condition, right? The perturbation is applied to a parameter and the control variable (e.g., grounding-line position) is allowed to evolve. Please revise.
L71-73: "The existence of such stable steady states is also strong indication that the currently observed retreat of Antarctic grounding lines is purely driven by changes in the external drivers such as oceanic forcing." <= It could also be an indication that the ice-sheet continues to evolve due to past climatic forcing, since as mentioned, in reality it is not in steady state. Consider adding some nuance here, which would flow better into the next paragraph which treats this point.
L74: ice sheet state => ice-sheet state
L100: set-ups => setups
L205: What is the motivation for this formula for error in grounding-line position? Add a sentence or two, as there could be many ways to define this error.
L287: 500 years => 480 years [right?]
L415: "The spin-up procedure..." <= Fragment, please revise.
L440: firstly => first
L536: committing => producing
Citation: https://doi.org/10.5194/tc-2022-104-RC2 - AC2: 'Author's response to Alexander Robinson', Benoit Urruty, 12 Oct 2022
Benoît Urruty et al.
Benoît Urruty et al.
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