Brief communication: Thinning of debris-covered and debris-free glaciers in a warming climate

Banerjee, Argha

doi:https://doi.org/10.5194/tc-11-133-2017

Articles | Volume 11, issue 1

https://doi.org/10.5194/tc-11-133-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/tc-11-133-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 11, issue 1

Brief communication

|

18 Jan 2017

Brief communication |

| 18 Jan 2017

Brief communication: Thinning of debris-covered and debris-free glaciers in a warming climate

Argha Banerjee

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Final revised paper (published on 18 Jan 2017)
Preprint (discussion started on 15 Jun 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review comments on "Thinning of debris-covered and debris-free glaciers in a warming climate" by A. Banerjee', Anonymous Referee #1, 17 Aug 2016
- AC1: 'Author's response to **Review comments on "Thinning of debris-covered and debris-free glaciers in a warming climate" by A. Banerjee', Anonymous Referee #1**', Argha Banerjee, 22 Aug 2016
RC2: 'Reviewer comments', Anonymous Referee #2, 12 Sep 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Argha Banerjee on behalf of the Authors (23 Sep 2016) Author's response Manuscript

ED: Reconsider after major revisions (29 Sep 2016) by G. Hilmar Gudmundsson

ED: Referee Nomination & Report Request started (27 Oct 2016) by G. Hilmar Gudmundsson

RR by Anonymous Referee #2 (15 Nov 2016)

RR by Anonymous Referee #3 (27 Nov 2016)

Suggestions for revision or reasons for rejection

This is in my view a relevant and interesting paper that investigates the details in dynamic response of debris covered glacier tongues in contrast to debris-free glaciers.
In context of global warming and widespread glacier retreat, elevation change on debris-covered glaciers has in recent years been the focus of many studies. But they provided no clear explanation on the observation that debris and debris-free tongues show similar thinning rates.
This paper shows through simple dynamic flow modelling and a separation of the contributing terms to thinning and how dynamic effects and delay in response may explain why thinning rates of debris covered glaciers in the same range as for debris-free tongues.
Such a ‘dynamic’ consideration and explanation to this topic of changing debris covered glaciers seems a very valuable contribution, particularly, as this topic is otherwise rather dominated by ‘observational’ or ‘melt’ studies.
The use of a simple flowline model is to me fully justified as it reduces the problem to the essential dynamic components and does not obscure the problem by additional processes/feedbacks.

The revised version to my view sufficiently addressed the issues of the reviewers where required: e.g. clarified the relevance of the study, clarified and justified the use of a simple flow model,….

There are however a couple of points that need in my view addressing before publication of this paper which I outline below in some more detail.

More substantial points:

a) In general the writing (English language, punctuation…) needs to be improved (quite a few awkward formulations and wrong order of terms in sentences) and minor editing issues (such as for example, singular/plural, wrong spaces, brackets and commas around citations etc.) need to be eliminated by carefully checking and proof reading the manuscript in a further revision. As there were too many such issues I did not mark them all in the detailed comments below.

b) (p. 4 first paragraph): I struggle to follow the line of argument on line 3 and 4. On what basis the authors comes to the conclusion that why in a later stage thinning rate would in A become smaller than in glacier B and sentence. Is the reason due to the delayed response of B whereas A already is adjusted to the changed climate. I can very well see this conclusion from the later modelling but at this stage it seems not so obvious to me.
Should be clarified and better explained.

c) Figure 1 c and d (and e and f): Figure 1 is very instructive, but unfortunately it is difficult to see the temporal evolution of thing rates or dF/dx which is however very crucial to understand what is going on. One should be able to see which lines are at which time step. A reader who is experienced with such model output can probably read it right but not the general reader.
Either label the different blue lines with the model years or maybe visually easier color them following a easy to read color code (rainbow colors that change with time).

d) If I understand right a crucial point in that the thinning rates from debris covered tongues get similar or higher than on debris-free tongues is the delay in dynamic response of the debris covered tongues. Or in other words the ice-free tongues have already adjusted when the debris covered parts are approaching their highest thinning rates (or do I get this wrong?). This point of delayed response should in my view be made more explicite in paragraph ‘3.1.2. time evolution of thinning rates’ at it seems it is all about timing.

e) More a note to further support this study: Often elevation change assessment are focussed on lower parts of glaciers (ablation areas, as not too steep), but the suggested dynamic effect on average thinning rates would probably be even more amplified and clearer if focussed only on for example the ablation area. This would further support the conclusions of this paper.

Detailed minor comments and editing issues:

p. 1 line 12: I would say ‘…in its DYNAMIC response…’ to flag more the DYNAMIC aspect.
p. 1 line 12: here only modelling studies are cited but surely there are ‘other’ studies that considered ‘dynamic effects’ before.
p.1 line 23: ‘…has emerged FROM the large…’
p. 1 line 25 and p 2 line 1: I would reformulate tis to ‘…of supraglacial debris-cover and may seem counterintuitive.’
p. 2 line 7: ‘…melting FROM thermokarst PROCESSES, namely’
p. 2 line 33: for clarification I would add a ‘initially’ before ‘just the difference
p. 3 line one: something wrong in the sentence at end of line: ‘…over a time scale THAT is short COMPARED to the …’?
p. 3 line 7: singular ‘glacier’
p. 3 line 10 ‘mass-balance’ (double ‘ss’)
p. 3 line 13 : profile should be in plural: ‘…mass-balance profiles…’
p. 3 line 14: ‘…and only CHANGES only by the shift in ELA, and no reference needed here.
p. 3 line 18: I am sure some readers will not agree with the point that changes in ice cliffs/ponds are NOT IMPORTANT. The point is that in this study it makes sense to explicitly exclude it as it wants to see what the ‘dynamic’ effect is. Thus, in my view there is no need to say the ice cliffs etc are not important or the effect is small.
p. 3 line 9: I think even without ‘fast’ advection this simple mass-balance profile is justified.
p. 3 line 21: I would add ‘initially’ again between ‘..responds’ and ‘with a…’
p. 4 line 15, 16: YEARS should be in plural on both lines
p.4 line 17: i assume these decadal rates refer to the first (few) decade and later decline.
p. 4 line 25: I assume these is the ‘…initial AVERAGE thinning rate (averaged along glacier)
p. 5 line 3: ‘… profile of THE thinning rate gets …’

Fig 1, caption: make clear that this figure refers to experiment 1.

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ED: Publish subject to minor revisions (Editor review) (11 Dec 2016) by G. Hilmar Gudmundsson

AR by Argha Banerjee on behalf of the Authors (20 Dec 2016) Author's response Manuscript

ED: Publish as is (25 Dec 2016) by G. Hilmar Gudmundsson

Short summary

Measurements of debris-covered and debris-free glaciers in the Himalaya-Karakoram show similar decadal scale thinning, despite a suppression of melt under the debris. Using physical arguments, supported by simulations of 1-D idealised glaciers, we analyse the evolution of thinning rates on both glacier types under a warming climate. The dynamics of the emergence velocity profile control the thinning rate evolution in general and lead to the observed trends in the thinning rate data.