|Review on “Antarctic dynamics contribution to future sea level constrained by ice discharge observations”|
Dear Eveline van den Linden et al.,
Many thanks for your work on the manuscript and taking into account all the suggestions, I think the manuscript has improved a lot! Given that most of the text is new, there are a few new comments from my side. After that, I think the manuscript is fine to be published and I want to congratulate the authors on this work!
From the replies I understand now that you use the historic observations of ice discharge to constrain the melt parameter (e.g., p3 lines 80). There is however one discrepancy in units which you might want to consider - I think you should at least discuss it. In the LARMIP2 paper, the ice sheet modelers were asked to provide the results in units of sea-level equivalent ice loss, which is calculated based on changes in the volume above flotation of the ice sheet. This does not directly compare to changes in ice discharge. In the extreme case, when an ice stream is just about at the flotation limit and very slightly grounded, it could be that its discharge increases and the grounding line retreats, but the sea-level contribution of this is quasi negligible.
- p 1, line 15-16, and p28, lines 587-589: I do not understand your argumentation so far that your results support this statement. If I understand it correctly, it is based on how your calibrated melt parameters compare to the ISMIP6 median parameter. However, the ISMIP6 experiments also included the PIGL calibrations which is much more sensitive, and this is not included in your argumentation, or? So maybe re-calibrating parameters would also reduce the upper range of the ISMIP6 projections based on this parameter calibration, or am I missing something? Please explain your reasoning for this statement better.
- p 2, line 35: that the range of uncertainties appears to be increasing is arguably not because the we know less as implied by this formulation, but because more models and processes are included, i.e., the uncertainties become “visible”
- p 3, line 62: I was a bit surprised by calling this a “melt parameterisation” since in my head this is usually a 2-dimensional field of melt rates, but I think this is fine, maybe add a short explanation to make this clear.
- p 7, line 125: this sentence still sounds weird to me as it is not the water that is changing its temperatures in the cavity.
- equation (4) do you also use the ice shelf cavity mean depth when testing the deeper ocean layers?
- p 9, line 166: show also the equations over which you are optimizing, this would make it easier to understand what you are describing here
- p 10, 180-181: give the median values, so that they are somewhere in the manuscript.
- p 10, section 3.1, please show the discharge curves you use for calibration in the Amundsen Sea and for the whole Antarctic ice sheet
- p 10, line 196: if correct, add “..to sea level while CMIP models indicate an increase in ocean forcing,…”
- p 10, line 200: add citations that support this attribution
- Fig 5: add % to the numbers in the top of the panels
- p 13, line 244-246: not sure I understand this sentence, please clarify
- p 14, line 255-256: earlier you stated that you include the linear parameterisation for comparison with LARMIP2?
- p 21, line 384-398: reformulate this argument, you base your reasoning here on a comparison between projections, but we do not know which projection is correct and hence a conclusion about which methodology is better cannot be drawn. Instead you could use papers that support the quadratic relationship (e.g., Holland et al., 2008).
- p 24, line 466-467: Please explain more. Which numbers do you compare to conclude this?
- p 26, line 598: this could also indicate an insensitivity of discharge to basal melt (in the case of no buttressing)
- p 26, line 508-514: I am not sure I understand what you mean. Are you basically saying that FRIS cannot be calibrated at the moment?
- p 26, line 514: it could be misread at the moment that you calibrate with basal melt (not discharge), maybe be clearer here
- p 26: one point that is missing in your discussion is that you consider a constant basal melt rate increase over the entire ice shelf, no spatial patterns and effects are taken into account
- p 26, line 532: “physically correct” – I do not think that you can derive this from your previous reasoning.
- p 27, line 559-560: LARMIP2 did not mainly focus on the future, it did compare to historic ice loss and found their projections to be consistent
In the manuscript, van den Linden et al. use linear response functions from Levermann et al. (2020) which were derived from ice sheet model responses in sea-level contribution to perturbations by uniform sub-shelf melt rate increases. Sea-level projections are updated by using CMIP6 instead of CMIP5 models and by recalibrating the sensitivity of melt rates to ocean temperature changes using observed mass changes. The authors conclude that with the new calibration, sea-level projections are lowered in comparison to LARMIP2 (Levermann et al., 2020) and ISMIP6 (Seroussi et al. 2020; Edwards et al. 2021).
First of all, I want to thank the authors for this well written manuscript which is easy to understand and follow. Unfortunately, I think that the approach presented in the manuscript cannot be applied this way. However, the approach could be interesting and informing further studies, so I suggest two possible modifications that would make it applicable in a methodologically correct way.
The central issue is that the calibration factor gamma, which relates ocean temperature changes at depth to sub-shelf melt rates changes, is fitted over the historic period in the Weddell Sea, Ross Sea and in East Antarctica, all three regions where very little mass gains or losses have been observed (see Rignot et al. 2019; Figure A1 in the manuscript), in particular in comparison to the overall volume of the regions (if the methodology can be applied to the Antarctic Peninsula should also be checked). This makes a sound calibration with changes in ocean temperatures from CMIP6 models over the historic period basically impossible, due to a number of issues: (1) the changes in mass in the respective region might not even be causally linked to ocean forcing but explainable through, e.g., surface mass balance changes; (2) changes in ocean temperatures in CMIP6 models show a wide spread and how close they are to real changes and if they can actually capture subtleties in the historic record that can be linked to the small changes in ice discharge, is questionable.
This means that the calibration factor is fitted between two numbers that are zero or quite small, but have large uncertainties, so that in the end the calibration factor is not really constrained. And this is physically correct, because if there is no enhanced ice discharge due to changes in ocean forcing in the historic record, as for example in the Weddell Sea, the melt sensitivity to ocean forcing cannot be deduced from observations. This problem shows for example in the result that most calibration parameters are zero in many ice-sheet-ocean-model combinations (section 3.1). And that, even if the parameters are fitted to represent past discharge, they largely underestimate the observed mass loss (Fig. 6).
Two suggestions to avoid this problem are:
1) focus the study on the Amundsen Sea (potentially also the AP), where actually enhanced ice discharge has been documented extensively and linked to enahnced ocean-driven melting. This would allow you to derive a sound fit for that region. Then you could compare the projections for the Amundsen Sea with ISMIP6 and LARMIP2 for that region. Alternatively,
2) if you want to include the whole of Antarctica, you could use your proposed method to fit gamma in the Amundsen Sea (and AP), where substantial discharge occurred, and assume an uncertainty distribution of gamma (based on LARMIP, your fit, ISMIP6 calibration,..) for the other regions.
Since these would mean major changes to the manuscript and potentially the central findings, in the following I mostly omitted comments on the methodology and results that will be affected by the major comment above: