The stability of present-day Antarctic grounding lines – Part B: Possible commitment of regional collapse under current climate
Abstract. Observations of ocean-driven grounding line retreat in the Amundsen Sea Embayment in Antarctica give rise to the question of a collapse of the West Antarctic Ice Sheet. Here we analyse the committed evolution of Antarctic grounding lines under present-day climate conditions to locate the underlying steady states that they are attracted to and understand the reversibility of large-scale changes. To this aim, we first calibrate the sub-shelf melt module PICO with observed and modelled melt sensitivities to ocean temperature changes. Using the new calibration, we run an ensemble of historical simulations from 1850 to 2015 with the Parallel Ice Sheet Model to create model instances of possible present-day ice sheet configurations. Then, we extend a subset of simulations best representing the present-day ice sheet for another 10,000 years to investigate their evolution under constant present-day climate forcing. We test for reversibility of grounding line movement if large-scale retreat occurs. While we find parameter combinations for which no retreat happens in the Amundsen Sea Embayment sector, we also find admissible model parameters for which an irreversible retreat takes place. Hence, it cannot be ruled out that the grounding lines – which are not engaged in an irreversible retreat at the moment as shown in our companion paper (Part A, Urruty et al., subm.) – will evolve towards such a retreat under current climate conditions. Importantly, an irreversible collapse in the Amundsen Sea Embayment sector evolves on millennial timescales and is not inevitable yet, but could become so if forcing on the climate system is not reduced in the future. In contrast, we find that allowing ice shelves to regrow to their present geometry means that large-scale grounding line retreat into marine basins upstream of Filchner-Ronne and Ross ice shelves is reversible. Other grounding lines remain close to their current positions in all configurations under present-day climate.
Ronja Reese et al.
Status: final response (author comments only)
RC1: 'Comment on tc-2022-105', Michele Petrini, 01 Jul 2022
- AC1: 'Reply on RC1', Ronja Reese, 31 Oct 2022
RC2: 'Comment on tc-2022-105', Anonymous Referee #2, 06 Jul 2022
- AC2: 'Reply on RC2', Ronja Reese, 31 Oct 2022
Ronja Reese et al.
Ronja Reese et al.
Viewed (geographical distribution)
In this paper, Reese et al. use the ‘Parallel Ice Sheet Model’ (PISM) and the sub-shelf melt module ‘Potsdam Ice-shelf Cavity mOdel’ (PICO) to analyse the multi-millennial evolution of the Antarctic grounding-lines under a constant, present-day climate forcing, and the reversibility of associated large-scale changes. The authors first calibrate the sub-shelf melt module PICO against observed (Dotson ice-shelf) and modelled (Filchner-Ronne ice-shelf) melt sensitivity to ocean temperature changes. Optimised PICO parameters are then used in an ensemble of continuous spin-up (pre-industrial forcing) - historical (1850-2015 forcing) PISM simulations, which are evaluated against present-day observations. Simulations showing best agreement are then extended for 10,000 years beyond the historical period under constant present-day climate forcing and bathymetry. The evolution of the Antarctic grounding-lines is then analysed, and reversibility is tested for simulations showing large-scale retreat by reverting climate forcing to pre-industrial conditions.
In my opinion, this is a great paper, addressing an extremely relevant scientific topic (future states and reversibility of Antarctic grounding-lines) with the use of advanced modelling tools (e.g., PICO instead of simpler sub-shelf melt parameterizations) and innovative techniques to calibrate numerical modelling results against observations (e.g., PICO parameters optimisation, PISM ensemble scoring methods). The study presents some limitations (e.g., no isostasy, no full equilibrium reached at the end of the simulations), but these are clearly discussed throughout the manuscript, and are in my opinion acceptable considering the technical challenges (and, likely, computational costs) associated with this type of study.
In view of this, I consider this work definitely worthy of publication, and I commend the authors for the great deal of technical work they have undertaken.
I have only two major comments, mainly related to the quality and number of figures included in the manuscript. In fact, I think some important figures are missing, and some of the included figures do not allow the reader to easily verify what is stated in the main text.
P1, L8: I would add to the abstract the fact that isostatic rebound is not accounted for in the simulations, e.g., “…under constant present-day climate forcing and bathymetry”.
P2, L54-56: I suggest rephrasing, and including a citation for CMIP5.
P3, L62: ‘... let the ice sheet states evolve’.
P3, L65: ‘... to occur eventually under…’.
P3, L65-67: I suggest rephrasing, e.g., ‘To test … retreat, the simulations showing large-scale retreat are extended for 20,000 years under reverted pre-industrial forcing’.
P5, L125-126: I suggest rephrasing.
P6, L134: ‘The present paper can be understood to investigates…’.
P6, L149: ‘...which differ in between across…’.
P6, L.160-165: I found these sentences not very clear, I suggest rephrasing.
P7, L165: ‘...we hope aim to represent’.
P11, Section 4.1: I suggest including how the SMB is calculated in PISM, either in this section or in Section 4.2.
P12, L291: I suggest specifying how many thousand years, rather than stating ‘several’. Also, I’d use ‘at 8 km horizontal resolution’.
P12, L301: I suggest including the notation Hmax as in Table 2.
P13, L315: I would remove ‘the best run is used … in Urruty et al.’, as this is not relevant for this paper.
P15, L326: I would expand here on what quasi-equilibrium means, and what are the implications. I think it would be enough to move the text at P16, L365-369 at the beginning of the section.
P15, L335: I would add some text linking the discrepancy from observation to simulated and observed sub-shelf melt rates pattern (either here, or in the discussion section). Also, the same could be done with simulated and observed SMB.
P15, L343-345: I would split this sentence in two.
P15, L348-350: I suggest rephrasing.
P16, L374-376: I suggest rephrasing.
P21, L468: I would use ‘Moreover’ instead of ‘in particular’.
P21, L468-470: I would also add something like ‘...as well as glacial isostatic rebound, which can induce changes in the local bathymetry and ice shelf cavity geometry’. Also, I think the paper by Whitehouse et al. 2019, (‘Solid Earth change and the evolution of the Antarctic Ice Sheet’, Nature Comms.) should be cited here.
P24, L541: Rather than ‘...change in sub-shelf melt rates is thought to be a major trigger…’ I would use something stronger, e.g., ‘... recent observations and modelling suggest that…’.