Dear Vincent Peyaud and co-authors,
Your manuscript received 3 detailed and largely positive reviews that highlighted the value of the manuscript from the perspective of glacier modelling (approach and validation/constraining with data) and the interesting results on future glacier evolution of Mer de Glace and indicated that this manuscript would be valuable contribution to TC after some revisions.
However, besides quite a few rather minor editing corrections the referees also raised some more substantial points which mainly concerned in brief:
-the flux gate approach and its sensitivity
-structural issues and details in the flow model/methods section
-more substantial discussion in context of other modelling studies and the wider meaning of the results
-readability of figures
-english language

Given the authors response to the reviews, it seems the minor and more substantial points will be addressable by the authors or have already been addressed (see track change document) and the manuscript will likely get into a state to be accepted for publication.

At this stage I will not give detailed feedback yet as I first have to see the fully revised manuscript but I already have some suggestions that the authors may already address beyond their already suggested revisions in the response/track change document:

1) please carefully proof read the document again, in the track change document there are a couple of issues/mistakes, in particular in the rewritten/edited parts… I marked some of them in the attached pdf-file with yellow color, but there are probably quite a few more.

2) some of the responses (in particular for referee 1) give rather lengthy explanations to the referees but they do not make clear how things have finally been addressed in this respect, I suspect nothing has been done to address it in the manuscript. So be clear in the response on what you have done or not done in the revisions (maybe not all planned revisions mentioned in the response have implemented in the track change document yet).
Further, some aspects of these explanations may be relevant for all readers of the paper not just the referee and hence sometimes some additional clarification can be made in the text, for example briefly explain why some has been or has not been done this way. I am not going through your whole response here but I list here the most important points (likely not complete):
a) Explain briefly in the in an appropriate place in the methods part, why you used the flux gate method (lack of bed data, well constraints on flux etc….)
b) Mention briefly discuss your sensitivity analysis of the lapse-rate you present in the response to ref 1 and maybe add some figure detail to the supplement.
c) Refer also in the main text (and maybe add some of the volume %/numbers results) to the additional new volume evolution figure in the supplements (R3-1) (see response to comment 2 of referee 3.
d) A clarification/adjustment in the manuscript may be needed on the point of the referee 1 (comment 6) of discrepancy between too thick and velocity too high. Currently it is not clear how this is addressed in the manuscript or just explained to the referee.
e) …

3) the additional sensitivity analysis of the flux gate approach is now described at the end of the results (sect. 4.3) but the results are not really described/analysed in the results, there is just a reference to fig 8. And should the description of the experiments not rather be in the methods?

In addition to all the referee comments, the points above should be considered in the revisions and once the revisions are finalized I ask the authors to submit this revised version which will then be looked again in detail by the editor or potentially a referee. Also make sure that the response is clear about what revisions have been undertaken or where nothing has been done and justify why. Then

Andreas Vieli, editor
5 Aug 2020

Dear Vincent Peyaud and co-authors,
Your manuscript received 3 detailed and largely positive reviews that highlighted the value of the manuscript from the perspective of glacier modelling (approach and validation/constraining with data) and the interesting results on future glacier evolution of Mer de Glace and indicated that this manuscript would be valuable contribution to TC after some revisions.
Besides some rather minor editing corrections the referees also raised some more substantial points which in brief mainly concerned:
-the flux gate approach and its sensitivity
-structural issues and details in the flow model/methods section
-more substantial discussion in context of other modelling studies and the wider meaning of the results
-readability of figures
-english language

These points and some additional comments by the editor (based on response) have in the revised manuscript now been addressed satisfactorily or justified., thus this manuscript is very close to be accepted for publication. Unfortunately, mostly in the revised parts of the text, small editing or writing errors occur (see list below by editor) which have to be addressed before publication. When these corrections are done, please carefully proofread the manuscript again.
I congratulate the authors for their very interesting contribution and for their cooperation in the review and revision process.

Andreas Vieli, editor
6 Sept 2020

Minor corrections/editing issues
(lines refer to revised manuscript)

Throughout document: I think something went wrong with the numbers/units of the mass balance gradients. I assume the should mostly be 0.009 /a (or 0.007 /a), but now it says 0.09 m/a /100m which would be 0.0009 /a. please check and correct throughout (p. 6 line 18+20, p. 13 and p. 15 (top).

p. 19: Acknowledgements: please acknowledge the referees as well.

p. 1 Line 9: add a comma after 'velocity'
p.2, line 1: '…models…' (plural)
p. 2 line 3: 'parameterizations' (plural. Same line, '…on a equilibrium
p.2 line 28:'… French Alps and covers…'
p. 2 line 31 : '… at a speed…'
p. 4 line 9: 'large' or 'additional' instead of 'great'
p. 5 caption fig 2, line 2: this should be 'orange' not 'green'.
p. 6 line 6: 'independent' not 'independent'
p. 6 line 18 and 20: 0.009 a^(-1) or 0.9m a^(-1) (100m a)^(-1)
p. 6. Line 18 the '0.02' should also have a unit…
p. 10 line 29: style '…that we slightly overestimated the thickness as well as the volume.'
p. 10 line 30: something wrong in this sentence and repeats 'underestimated' a bit too many times. Rephrase!
p. 11 line 8-9: delete the first 'to': '…drops close to zero…'
p.13 line 8: '…for the debris cover evolution…' (and not 'cover debris evolution')
p. 13 (line 11, 13) and 14 (top): recheck the units of the mass balance gradient
p. 13 line 13: make the variable k_b in mathematical style
p. 13 line 14: '… ice ablation that is reduced by…'
p. 15 line 6: here you mean the 'rate factor' in glen's flow law, why not be more specific and say this.
p. 15 line 27: style, 'In comparison, …' instead of 'Compared to it'.
p. 17 line 9: 'Because the model domain was…' (delete 'of').
p. 18 line 5: 'DEMs' (should be plural).
p. 18 line 15: awkward wording: 'On timescales of decades' (rather than 'On the timescales of the decades'.
p. 18 line 19: not so clear what you refer to here with 'THIS future evolution…'. Is it 'This discrepancy in future evolution…'