Review of “Late Pleistocene glacial terminations accelerated by proglacial lakes” by Scherrenberg et al.

Signed: Niall Gandy

This manuscript presents ice sheet simulations of the past 800 ka BP, using various idealised modelling approaches to better understand the affect of proglacial lakes during deglaciation phases. The authors show that the presence of proglacial lakes accelerate deglaciation, and they attribute this primarily to the negligible basal friction beneath ice shelves. Further simulations also demonstrate the sensitivity of the ice response to the rate of bedrock rebound.

Overall, I think the work presented here will be a significant contribution to understanding palaeo ice sheet behaviour, with the possibility of encouraging further model development and motivating future experimental design to include lake feedbacks. Indeed, this is a mechanism largely ignored in future simulations, and important progress made here could link with future projections. The work is clear and concise. The precision and clarity of the figures are excellent, with the exception of some minor points made below.

The manuscript would benefit from a clearer justification for the experimental design (and perhaps some more analysis), along with some minor points listed below. As such, I recommend publication after minor revisions.

Main points

Experimental design: I appreciate the novel approach of the experimental design, where you have made certain adjustments to better explore the ice sheet behaviour. However, the detailed justification for this is not explained sufficiently in the text, and I am left not fully convinced that your experiments explore the behaviour you intend. For example, the “Rough Water” experiment is designed to show the effect of negligible friction beneath ice shelves, but various feedbacks (surface profile, buttressing effects, a different GIA response to more grounded ice, ect) could be confusing the results. The experimental ethos could be explained in more detail. You could also undertake some offline ice shelf mass budgeting (at each timestep what is the flow over the grounding line, what is lost to surface melt, sub shelf melt, and calving) to disentangle the behaviour.

Specific points

18: linger > remain?

67: I think it is common to conflate susceptibility to MISI/PLISI and sub-shelf melting/calving. Could you clarify the mechanistic difference for the reader before this sentence?

80: Could you comment on the suitability of a Hybrid model to simulate PLISI and grounding line migration? How is this parameterized?

89: In some ways the lacustrine environment might be quite different from the marine environment; the thermal structure may be different, and lakes could become chocked with icebergs. From a practical modelling perspective it is reasonable that you treat marine and lacustrine the same, but you could discuss this further in the text.

93: Do you know (or could you know with some offline calculations) what proportion of the ice sheet margin is missing lakes because the model cannot simulate above sea level lakes?

101: merge > merged

108: Not unfeasible! Millennial scale coupled climate-ice sheet simulation studies do exist, but it is understandable why this is not a reasonable modelling choice here. Please clarify.

126: The use of brackets to describe the reverse behaviour is a tad tricky to follow.

164: A set 30% adjustment assumes that distal ice sheets fluctuate in unison with simulated ice sheets. Is this reasonable?

Figure 4: This figure is challenging to follow, particularly panels b, d, and f. Are the points of deglaciation onset numerically defined? The onset of glaciation curves are difficult to read; I would suggest either removing or replotting. Ideally we shouldn’t need a paragraph (lines 172-176) just to describe how to read a figure, not yet describing the results or discussion the implications.

190: Can you comment on the mechanism for the higher sensitivity of the Eurasian ice sheet?

211: The importance of the simulated ice shelves may depend on their spatial extent and if they are constrained laterally. It would be good to see a figure of simulated ice sheet location and morphometry.

216: I don’t follow the logic here?

Discussions: This section is very limited, it could be incorporated into the Results section.

315: It would be good to develop this point a little further. What are the potential effects of lakes on future Greenland? And do models represent this?

423: It would be preferable for the simulations to be reproducible without contacting the author.

This paper presents a series of numerical simulation of ice sheets over the northern hemisphere for the past 800 thousand years, forced by climate forcing based on GCM.  The main focus of the present paper is the effect of proglacial lakes on glacial terminations, and the authors show that proglacial lakes accelerate the deglaciation of the ice sheets due to the absence of basal friction underneath ice shelves.  Moreover, they show that the time scale of bedrock isostatic response to the variation of ice sheets is a key factor for deglaciation, through the formation of proglacial lakes.  I think this paper is fairly well written with some exception below, and can be accepted with minor revision.

Description of the ice-sheet model: Section 2.1 describes the ice-sheet model briefly, but I feel it too short.  A complete repeat of the previous papers are not necessary, but at least the description of the methods relating to the sensitivity experiments of the present paper should be included in detail.

(i) how to deal the grounding line migration in the model.  it is all right without any special treatment, but mention explicitly.

(ii) how to deal the formation of proglacial lakes.  I suppose that the depth of lake is the difference between the background sea level and the bedrock.

(iii) explicit equation of basal sliding, and the difference in how to apply between base-line and rough-water experiments.

Other points:

Abstract: There are no description of the method used in the present paper, even no explanation that the discussion is based on a series of numerical simulation.  This must be clarified.

BMB experiments: Typical mass balance terms, both surface and base, over ice shelves in the baseline experiment should be mentioned, which will help to get typical ratio of surface/basal mass balance terms.

Figure 2.  Plotting two points corresponds to PI and LGM may help.

Figure 3.  Better to mention in the caption that this is the result of baseline experiment.

Figure 4 and main text.  Please note the definition of the onsets of (de)glaciation in this paper.  Why some points (e.g., 240ka in the total and Eurasia, 290ka in North America) are not detected as the onsets?

Appendix Eq.(1) Please clarify that f_{snow} is no more than 1.  I calculated by hand, and observed that f_snow become 1 when T - T0 is around -10K, and exceeds 1 when -11K.  I am not sure my rough computation is correct, but please clarify the actual computation of f_{snow} (Of course it is all right if T - T0 is always more than -10K).

Eq (3).  Units are not consistent.  Need unit of the coefficient 0.015 before Melt_{prev}.  Again, it is hard to check whether the albedo is between 0 and 1 by this equation.

Eq (4). Need units of the coefficient 0.012.

Eq (21).  Need unit(scale) of P_{PI,corr} to compute log.  Or, is the first term e means exp() function, not e times ()?