Relative importance of the mechanisms triggering the Eurasian ice sheet deglaciation
Abstract. The last deglaciation (21000 to 8000 years BP) of the Eurasian ice sheet (EIS), is thought to have been responsible for a sea level rise of about 20 meters. While many studies have examined the timing and rate of the EIS retreat during this period, many questions remain about the key processes that triggered the EIS deglaciation 21,000 years ago. Due to its large marine-based parts in the Barents-Kara and British Isles sectors, EIS is often considered as a potential analog of the current West Antarctic ice sheet (WAIS). Identifying the mechanisms that drove the EIS evolution might provide a better understanding of the processes at play in the West Antarctic destabilization. To investigate the relative impact of key drivers on the EIS destabilization we used the three-dimensional ice sheet model GRISLI (version 2.0) forced by climatic fields from five PMIP3/PMIP4 LGM simulations. In this study, we performed sensitivity experiments to test the response of the simulated Eurasian ice sheets to surface climate, oceanic temperatures (and thus basal melting under floating ice tongues) and sea level perturbations. Our results highlight that the EIS retreat is primarily triggered by atmospheric warming. Increased atmospheric temperatures further amplify the sensitivity of the ice sheets to sub-shelf melting. These results contradict those of previous modelling studies mentioning the central role of basal melting on the deglaciation of the marine-based Barents-Kara ice sheet. However, we argue that the differences with previous works are mainly related to differences in the methodology followed to generate the initial LGM ice sheet. We conclude that being primarily sensitive to the atmospheric forcing, the Eurasian ice sheet cannot be considered as a direct analogue of the present-day West Antarctic ice sheet. However, because of the expected rise in atmospheric temperatures, risk of hydrofracturing is increasing and could ultimately put the WAIS in a configuration similar to the pas Eurasian ice sheet.
Victor van Aalderen et al.
Status: open (until 12 Apr 2023)
- RC1: 'Comment on egusphere-2023-34', Anonymous Referee #1, 27 Mar 2023 reply
Victor van Aalderen et al.
Victor van Aalderen et al.
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Review of van Aalderen et al., 2023
This is an interesting and generally well-written manuscript, that explores the sensitivity of a simulated Eurasian Ice Sheet (EIS) to different forcings. I agree with the general approach – use a palaeo-ice sheet to explore the impact of different processes within the framework of an ice sheet model. I also think that such approaches are much needed to help elucidate the long-term processes that operate within ice sheets. We can increase the number of ice sheets we study by using the palaeo examples. However, I have a few major concerns, outlined below, which make me question the conclusions of the manuscript. The largest concern is that the main finding that atmospheric processes dominate over ocean processes for the EIS, is model set-up specific, rather than an interpretable generalisation as presented.
Model specific result?
My main concern is that the results presented in the manuscript are specific to the model-setup, and not a general representation of the actual processes that drove the deglaciation of the EIS. Potential reasons why the results may be model specific are highlighted below (basal sliding, MISI, sensitivity to other parameters, spin up). These model specific results are interesting enough to other ice sheet modellers, and perhaps Quaternary Scientists who study the EIS. But, the manuscript is presented as if the model were close enough to reality that the results represent what the ice sheet actually did (for which there is no current verification conducted against say proxy records). Foremost, this requires a title change, perhaps:
Relative importance of the mechanisms triggering the Eurasian ice sheet deglaciation in the GRISLI2.0 ice sheet model
Then throughout the manuscript, it is worth highlighting that this is a model-specific result. For example, in the abstract (L19) “EIS retreat is primarily triggered by atmospheric warming,” and the similar statement on L538, should add model-specific caveats.
Unfortunately, this model dependency of the result makes the broader conclusion that the EIS (or parts of it) cannot be analogues for the West-Antarctic Ice Sheet (WAIS) unjustified. I also think this is poorly laid out in the manuscript, appearing only in the conclusions, rather than given proper thought in the discussion. Perhaps a section on “potential comparison to the WAIS” in the discussion is required, rather than appearance in the conclusions. Note that all of the below are not criticisms of the model, but rather reasons why the results may be model dependent.
The model setup for basal sliding is inadequately described and the choice of sliding law seems at odds with the norm within ice sheet modelling. The sliding law used is linear (L138), but most models use a non-linear sliding law (e.g. a Columb “pseudo-plastic” law or the newer Zoet and Iverson (2020) law). The method of calculating N for equation 3 is inadequately described also – does this depend on a hydrology parameterisation? The reason this leads to a model specific result is that both factors will affect the pattern of ice streaming simulated by the model, and thus the thickness of ice at the grounding line which will influence retreat rates.
MISI and grounding lines:
An improvement to the model presented here is the inclusion of an analytical treatment of the grounding line. However, the coarse resolution of the model (20 km grid size) means that the ice thickness at the grounding line is likely poorly represented. Though there is subgrid treatment of ice velocity at the grounding line, this does not incorporate different values of elevation. Likely, this means an underestimate of the ice thickness variations across the grounding line, and hence a dampening of the marine ice sheet instability. Thus, a model that resolves the bed topography at the grounding line at higher resolution (e.g. BISICLES) may produce different results. This is especially the case for narrow fjord areas of Norway and the main troughs, which will be represented by a few pixels only at the mouth of the ice stream in the model.
This raises another point – are the differences between this paper and previous results (Petrini et al., 2019) a consequence of including the grounding line parameterisation or spin up procedure (the latter is the focus of the authors in the discussion, but the former is a big change to the model)?
The sea level lowering of 120 m is also likely an under representation – is the bed depressed by isostatic loading?
Sensitivity to other parameters:
My reading is that the authors use a single set of parameters in their model, which are then exposed to different forcings. These parameters need listing, perhaps as a table in the supplement. However, this also raises a question of whether the results are dependent upon the choice of parameters, which are presumably left at some default value. For example, if calving rates, or positive degree day parameters were different then the response of the simulated ice sheet may be different. For example, lower positive degree day melt rates might lead to less sensitivity to climate. Similar changes may happen for a wide variety of parameters. Ideally, a wider set of parameter values would be used to make an inference about the EIS behaviour. But in the absence of a perturbed parameter ensemble experiment, this limitation should at least be noted.
Both the spin up in this work and the previous (Petrini et al., 2019) assume some sort of (quasi?) steady-state at the LGM. However, the LGM was likely a snapshot in time, and in many places across the EIS was not achieved synchronously (see Clark et al., 2022 for an example across the BIIS). If a transient spin-up was applied, growing the LGM ice sheet to a non-stable extent, then a model may produce different results.
I think for those who study palaeo-ice sheets from evidence (i.e. not modellers) the paper needs more explanation. Perhaps explicitly say you are not aiming to get the right timing/pattern of deglaciation, but rather explore the sensitivity of this model. This should come around L89, and would help this other community engage with your work.
Overall, I would like to see this manuscript published, as I am sympathetic to the aims. But substantial revisions are required to improve the robustness of the results. What is really required is an inter-model comparison. I am not suggesting the authors conduct such an approach, but perhaps this is something the authors might suggest in the discussion. Instead, a refocussing towards the specifics of this model is recommended. I hope my comments help in improving the manuscript.
Clark, C.D., Ely, J.C., Hindmarsh, R.C., Bradley, S., Ignéczi, A., Fabel, D., Ó Cofaigh, C., Chiverrell, R.C., Scourse, J., Benetti, S. and Bradwell, T., 2022. Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE‐CHRONO reconstruction. Boreas, 51(4), pp.699-758.
Lucas K Zoet and Neal R Iverson. A slip law for glaciers on deformable beds. Science, 368(6486):76–78, 2020.