Brief communication: Increased glacier mass loss in the Russian High Arctic (2010–2017)

Sommer, Christian; Seehaus, Thorsten; Glazovsky, Andrey; Braun, Matthias H.

doi:https://doi.org/10.5194/tc-16-35-2022

Articles | Volume 16, issue 1

https://doi.org/10.5194/tc-16-35-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-16-35-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 1

Brief communication

|

06 Jan 2022

Brief communication |

| 06 Jan 2022

Brief communication: Increased glacier mass loss in the Russian High Arctic (2010–2017)

Christian Sommer, Thorsten Seehaus, Andrey Glazovsky, and Matthias H. Braun

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Final revised paper (published on 06 Jan 2022)
Supplement to the final revised paper
Preprint (discussion started on 28 Dec 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Comment on tc-2020-358', Anonymous Referee #1, 28 Feb 2021
- AC1: 'Reply on RC1', Christian Sommer, 11 Apr 2021
RC2: 'Review', Anonymous Referee #2, 04 Mar 2021
- AC2: 'Reply on RC2', Christian Sommer, 11 Apr 2021

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (12 Apr 2021) by Etienne Berthier

Dear authors,

Two referees evaluated your manuscript (MS), and I would like to thank both of them for their thoughtful and timely comments. In the online discussion, you provided preliminary responses and suggestions of how to address them.

I feel that your responses could be further improved:

1/ An important comment from reviewer#1 was “Although these issues are addressed along the paper they are not clearly separated and the effects on the results are confusing”. (See his review for the description of the two issues.) I did not find a clear response to this comment in your rebuttal letter. I also would like to remind you that this comment echoes the main comment of my access review (copied after this decision).

2/ In general, I find your answers too short/laconic. I do not ask you to fully quote the revised text in the rebuttal letter but I expect more detailed answers than short statements such as "we changed the correction". The rebuttal letter should be almost self-consistent so that it would already give a clear picture of how the methods/results evolved.

To continue with the evaluation process, I would like to ask you to upload the improved response letter, the revised MS, both clean and track-changed, for a full assessment by me and likely the reviewers.
I am looking forward to read this revised MS.
Best wishes,
Etienne Berthier

Comments from my access review :

Acquisition data correction. It seems to me that two effects need to be distinguished, correct me if I am wrong. First, the fact that measurements are not made at the same of the year and thus, because of the strong elevation/mass balance seasonal cycle, this may lead to overestimation of mass loss (because the changes are computed from an annual elevation maximum in winter to a minimum in Autumn). The second effect is radar penetration. I think it would help/convince the reader/reviewers if these two effects were well separated.

Comparison to earlier studies. GRACE studies provide a continuous time series. Contacting the first authors of recent studies (Wouters/Ciraci), authors could get their time series and extract the exact same time period as them and make a more convincing comparison. I looked at the Wouters et al. time series (Their Figure 2) and, visually, did not find an obvious signal of acceleration.

The 900 kg/m3 density need to be justified. It implies that the authors entirely neglect firn compaction. An assumption not so straightforward in the context of rapid warming of high latitude ice caps.

Figure 1. color blind people may not be able to distinguish the "no coverage" color from the (rare) positive dh/dt values.
Maybe for Severnaya Zemlya (panel e) it would be good to separate the signal from the surging glacier in Vavilov from the rest of the region. So that Severnaya Zemlya can be compared to the other regions. Otherwise the signature of the Vavilov collapse is really strong in the dh/dt curve with elevation.

Table 1. For Franz Josef land and Severnaya (and not Severnaja as mis-written), I do not understand why the error bars are changing between the columns. Can the authors clarify this?

Figure 2. The rational for separating marine/land terminating glaciers for this section (penetration effect) was not clear to me. Can the authors also clarify this point in the text? Why such a separation is needed.

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AR by Christian Sommer on behalf of the Authors (21 May 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (26 May 2021) by Etienne Berthier

RR by Anonymous Referee #2 (11 Jun 2021)

Suggestions for revision or reasons for rejection

This manuscript has been heavily revised with a new SAR-signal penetration correction based on an empirical relation between seasonal SAR backscatter and measured elevation differences. The correction scheme lacks proper validation (e.g. by satellite altimetry), but the authors provide both corrected and uncorrected results which are not so different that they alter the main findings and conclusions. So all in all, I think they have succeed to address the main issues at an appropriate level, providing robust results of recent glacier changes across the Russian High Arctic at a detailed level not presented before.

The manuscript has also been restructured by moving most of the standard methodology (established in earlier publications) to the supplement instead of the confusing and partly repetitive split-up of the previous manuscript version. This has allowed more details on the penetration issue to be included in the main manuscript. Both aspects are clear improvements, but as indicated in the chronological comments below, I think the manuscript can still benefit from some smaller restructuring and inclusion of essential methodological details such as density conversion factors and the parallel calculation of mass-change rates with and without penetration correction. There are also some unclear sentences and inconsistent terminology, so I recommend the authors to do a careful language read-through and edit for the final manuscript version.

Specific comments and edits (line numbers refer to the version with tracked changes):

L21: Clarify: interpolation -> spatial interpolation (or: interpolation to unmeasured areas)

L26: Since SAR penetration has become a large (and important) part of the manuscript, the introduction could be expanded with a paragraph on that topic using some of the material and references that are now in the data&methods section (L58-62, L89-91).

Section 2.1: The main methods have now been moved to the supplement and only a brief summary remains here. I think this makes a much clearer distinction although it forces the reader to look up the supplement or the more methodology-oriented previous papers. The important density assumption should still be mentioned here with regards to the conversion from elevation change to mass change. And it would only take another sentence to also mention that you did separate elevation/mass rate calculations for land- and marine-terminating glaciers based on RGI. Also important for the Results/Discussion. And finally, it should be made clear that you make mass-change estimates both with and without the penetration correction (as in Table 1) and the associated terms should be clear and consistent to avoid confusion.

Section 2.2: This is a new subsection about X-band penetration. It reads fine, but is somewhat awkward in a data&methods section as it partly discusses SAR/X-band penetration in general (like introductory or discussion material) and partly describes the timing of the data without any further methodological description than “When comparing elevations of different TanDEM-X scenes the relative difference in penetration depths is determined by the acquisition dates and seasons”, which really says nothing about how it was done. Instead the penetration-related methods come in the next Section (2.3) which is narrowly named “SAR backscatter intensity analysis. I think it would be clearer to combine these two sections with a title that fits both, e.g. “Correction of seasonal penetration bias”. Actually, since a penetration correction is added to the autumn DEMs before calculating mass change rates, I think it would be even better if this section comes first and describes both the TanDEM-X data and the correction, e.g. “2.1 TanDEM-X elevation data and penetration correction” and “2.2 Glacier elevation- and mass rates”.

Section 2.3: Nice add-on, but the topic is really penetration, not backscatter itself, so a revised title or a merge with Sect 2.2, as mentioned above, should be considered.

Eqs. 1-2: Any references for these equations if they have been used in a similar context?

L122: smaller as -> smaller than

L148: add “average: ...“ to the parentheses to be clear. What is “...confined to a smaller number of glaciers”? I think you mean that strong thinning is confined to a smaller number of glaciers, not the “general less negative” elevation changes.

L150: “Slight thickening is also observed...”. What does “also” refer to? Delete.

L152: The term “adjusted mass change” should have been clearly defined beforehand in the methods section, otherwise one has to guess what you mean here. Personally, I think “uncorrrected” and “corrected” would be clearer terms than “measured” and “adjusted”, but either is fine as long as they are clearly defined at an early stage.

L159: I understand what you mean, but the sentence is not technically clear. Rephrase.

L156-168: I suggest to move this text about the penetration issue ahead of the glaciological results, similar to what I suggested for the data&methods section and what is already done in the Discussion. First you build confidence in the data and corrections, then you look at glacier changes.

L194: What is “relatively large”? Based on your calculations and relevant studies, you can provide an approximate meter-range here to make it more informative.

L195: An important reference study, but it is not really a “similar observation” because your relative penetration differences indicate penetration beyond previous summer surface which is not expected to be deeper than 1-2 m in your case. If the summer-surface indeed plays a role here, it can be troublesome because there are large variations from year-to-year in how much melt and refreezing there is in the higher accumulation area. Some years almost no melt and refreezing, other years thick ice layers can form. I miss a brief discussion of this issue in relation to the findings of Rott et al.

L221: More negative than what? Franz Josef Land or the other studies. Unclear sentence.

L224: Sudden transition from discussing mass changes to elevation changes. Please reread the Discussion and try to be more consistent with terms and quantities discussed.

L224: Marine-terminating only or land-terminating also? Fig. S3 indicate little difference between the two types at NZ, which I think should have been discussed too. But here you maybe talk about a smaller number of glacier with even larger changes, please specify.

L229: If terms have been well explained earlier, you wouldn’t need a long add-on like “...to the signal penetration adjusted mass change rate derived by TanDEM-X”.

L230: “also measured by Strozzi”. What does “also” refer to? Your results show few indications of accelerated flow on NZ as far as I can see, so it’s unclear how your results relate to those of Strozzi et al. Please clarify the relevance in the manuscript.

L233: “does not seem to be related to potential SAR penetration” -> “do not seem to be related to differences in SAR penetration” (there is penetration, but it can vary...)

L236: Or it can be related to a long-term dynamic imbalance (too low ice-flux velocities) with cyclic fast-flow/surging as seen for some glaciers. Your results are not conclusive.

L241-242: This is your conclusion based on a single observation period. The term “increasingly negative” infers a more continuous process (acceleration) which you cannot conclude from your data and is not supported from gravimetry time series either. To be on the safe side, you should limit your conclusion to an increased mass loss in your period versus other studies from the 2000s. Please keep this in mind elsewhere too.

L243: As pointed out in the previous review, this sentence is unclear. Please revise according to the explanation in the author response letter which makes sense.

Fig 1: Nice figure, but the data coverage of each panel should be made clear by statements for panel a and b (all data or only overlapping?) and by referring to Fig. S2 for aerial extents. This important aspect can also be made clearer in the manuscript text.

Fig. 2, caption: Nice figure. To be more precise I would say: “...corrected for differences in seasonal SAR-signal penetration (Fig. S2).”

Supp-L55: How was the co-registration done?

Fig. S2b: Penetration-bias corrections look very different from scene to scene, which I suppose is due to different backscattering conditions for the acquisitions. Although this can be a large local issue, it has only a small impact on the regional change rates which is the main scope of this paper. Still, a brief discussion of this matter would be good.

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RR by Anonymous Referee #1 (07 Jul 2021)

ED: Publish subject to revisions (further review by editor and referees) (07 Jul 2021) by Etienne Berthier

AR by Christian Sommer on behalf of the Authors (15 Sep 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Sep 2021) by Etienne Berthier

RR by Anonymous Referee #1 (21 Oct 2021)

ED: Publish subject to minor revisions (review by editor) (25 Oct 2021) by Etienne Berthier

AR by Christian Sommer on behalf of the Authors (04 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (05 Nov 2021) by Etienne Berthier

Dear Authors,

You considered carefully the last comments of the referee and I am happy to accept your manuscript for publication.

However, before final publication and copy-editing, I ask you to take into account carefully my comments below. There are still a few typos in the text and some statements need further clarifications. In particular, a non specialist like me has a bit of a hard time to make sense of the different values quoted for the penetration depth of the radar signal.

To facilitate and speed-up my assessment (I do not want to delay further publication), I to ask you to upload a point-by-point response, the revised manuscript both cleaned and track-changed.

Best regards,

Etienne Berthier

L42: "over most of our study area" sounds better to me (the study area has already been defined just above)

L47 "respective" not needed

L66. in “beta_1”, 1 should be a subscript (like done for 0)

L67. Does "autumn" corresponds to September? Not necessarily obvious (officially “autumn” starts 21 September). Maybe stick to September (everywhere in the text), or define "autumn".

L108. I find "measured" ambiguous. I think "uncorrected" (as you defined it above) would be best.

L134. I think I would rather write "the occurrence of melt or presence of fresh snow" (presence of melt does not sound right or rather “presence of meltwater”)

L150. See my comment above about "autumn", a bit ambiguous. Check everywhere.

L169. "smaller" is not clear. Do you want to say "less negative"? And then “less negative” than what? To be improved. Maybe quote the value of the mass change rate you want to compare to (the sum for the 4 glaciers you listed above, maybe?).

L180. A thought (not necessarily leading to changes in the MS). Can you exclude a melt/refreezing event between your two set of measurements (for example in summer 2015 or 2016) that would create an effecient scattering surface not so deep in the firn and would induce this apparent uplift ?

Figure 1a : I could not find the curve for the 2016-12 backscatter. Were these data included? Maybe I missed them, sorry of this is the case.

Fig 1d: maybe I missed something but the mean difference in signal penetration in the cruve appears much larger than the ~2 m values quoted in the text (rather around 3 m here on average). See also comments below on the values in the supplement. In the end a reader would have a hard time quoting a mean penetration depth from your study.

Supplement

L132. Add space between 150 and "m"

L136. Can you help the non SAR specialist reader and compare these numbers with the values of the main text which are 2.13 m (L100) and 2.3 m (L108). In the end it is a bit confusing.

L140. Penetration of 5.4 m or 3.1 m (or 2.13 m in the text) seems to translate in a larger difference over a total of 6 years. Maybe clarify why the corrections are so similar.

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AR by Christian Sommer on behalf of the Authors (08 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (10 Nov 2021) by Etienne Berthier

AR by Christian Sommer on behalf of the Authors (15 Nov 2021) Author's response

Short summary

Arctic glaciers have been subject to extensive warming due to global climate change, yet their contribution to sea level rise has been relatively small in the past. In this study we provide mass changes of most glaciers of the Russian High Arctic (Franz Josef Land, Severnaya Zemlya, Novaya Zemlya). We use TanDEM-X satellite measurements to derive glacier surface elevation changes. Our results show an increase in glacier mass loss and a sea level rise contribution of 0.06 mm/a (2010–2017).