Brief communication: Everest South Col Glacier did not thin during the last three decades

Brun, Fanny; King, Owen; Réveillet, Marion; Amory, Charles; Planchot, Anton; Berthier, Etienne; Dehecq, Amaury; Bolch, Tobias; Fourteau, Kévin; Brondex, Julien; Dumont, Marie; Mayer, Christoph; Wagnon, Patrick

doi:https://doi.org/10.5194/tc-2022-166

Preprints

https://doi.org/10.5194/tc-2022-166

Preprints

08 Sep 2022

| 08 Sep 2022

Status: a revised version of this preprint is currently under review for the journal TC.

Brief communication: Everest South Col Glacier did not thin during the last three decades

Fanny Brun, Owen King, Marion Réveillet, Charles Amory, Anton Planchot, Etienne Berthier, Amaury Dehecq, Tobias Bolch, Kévin Fourteau, Julien Brondex, Marie Dumont, Christoph Mayer, and Patrick Wagnon

Abstract. The South Col Glacier is an iconic small body of ice and snow (approx. 0.2 km²), located on the southern ridge of Mt. Everest. A recent study proposed that South Col Glacier is rapidly losing mass. This seems in contradiction with our comparison of two digital elevation models derived from aerial photographs taken in 1984 and a stereo Pléiades satellite acquisition from 2017, from which we measure a mean elevation change of 0.01 ± 0.07 m a^-1. To reconcile these results we investigate wind erosion and surface energy and mass balance, and find that melt is unlikely a dominant process, contrary to previous findings.

Received: 18 Aug 2022 – Discussion started: 08 Sep 2022

Fanny Brun et al.

Status: final response (author comments only)

CC1:
'Comment on tc-2022-166', Paul Andrew Mayewski, 28 Sep 2022

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-166/tc-2022-166-CC1-supplement.pdf

Citation: https://doi.org/10.5194/tc-2022-166-CC1
- AC1: 'Reply on CC1', Fanny Brun, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-166/tc-2022-166-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC1
- AC2: 'Reply on all RCs and CCs', Fanny Brun, 13 Jan 2023
  
  We would like to thank the editor and the reviewers for their detailed reviews of this paper, which helped a lot to improve it. We also want to acknowledge anyone who contributed to the open discussion. The discussion was rich and clearly contributed to increasing our knowledge about this South Col glacier and more broadly about all glaciers located at extremely high elevation.
  Attached is a full response to all the RCs and CCs, as well as a summary of the main changes to the manuscript.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC2
RC1:
'Brun et al. show convincing evidence for zero net mass change at South Col Glacier', Ann Rowan, 10 Oct 2022

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-166/tc-2022-166-RC1-supplement.pdf

Citation: https://doi.org/10.5194/tc-2022-166-RC1
- CC2:
  'Reply on RC1', Tom Matthews, 17 Oct 2022
  
  Please see the attachment.
  
  Citation: https://doi.org/10.5194/tc-2022-166-CC2
  - RC5: 'Reply on CC2', Ann Rowan, 27 Oct 2022
    
    I would like to thank Tom Matthews for making a careful reply to my review and I agree with the corrections to my comments. I am less familiar than Tom is with the avaialble AWS data from the South Col, and some of this information was not clear from the two manuscripts and the response from Mayewski et al. Their efforts to collect AWS data around the South Col have great potential to address some of the questions raised in this discussion.
    
    Citation: https://doi.org/10.5194/tc-2022-166-RC5
    
    AC4: 'Reply on CC2', Fanny Brun, 13 Jan 2023
    
    The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-166/tc-2022-166-AC4-supplement.pdf
    
    Citation: https://doi.org/10.5194/tc-2022-166-AC4
- AC3: 'Reply on RC1', Fanny Brun, 13 Jan 2023
  
  We thank Ann Rowan for her detailed and helpful review of our work. Our response is attached as a pdf file. Note that there are some cross-responses among the different reviews and community comments, and we recommend to read the full response posted in a single document as AC2.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC3
RC2:
'Comment on tc-2022-166', Anonymous Referee #2, 11 Oct 2022

This brief communication by Brun et al. is an important and timely response to the recent paper by Potocki et al. (2022) reporting dramatic ice loss from the Everest South Col Glacier. From differences in photogrammetric DEMs rather than dating an ice core, Brun et al. concluded that the 1984-2017 surface elevation change did not statistically differ from zero. Both studies then attempt surface mass balance modelling to interpret their results. The authors of Potocki et al. (2022) have already responded in the discussion. Implications of their comment that Brun et al. have given the wrong elevation for the ice core should be considered. Rather than the comment that the modelling by Brun et al. challenges the possibility of such large thinning rates, however, I would say that that they have demonstrated that uncertainty precludes firm conclusions from modelling in this case.

Minor corrections

Abstract

No need to be so cautious: “This is in contradiction”

line 202

“melt that immediately refreezes within the same time step could occur”

line 276

“we suggest that the core”

line 370

“E_p is” or “E_p is given by”

line 382

“u_{*t}”

line 395

“(one hour)”

line 399

“the former thicknesses of each layer”

line 436

“is the ice thickness”

Figure 1

The inset showing the location of Mt Everest is not referred to and is not necessary.

Citation: https://doi.org/10.5194/tc-2022-166-RC2
- AC5: 'Reply on RC2', Fanny Brun, 13 Jan 2023
  
  We thank the reviewer for their detailed and helpful review of our work. Our response is attached as a pdf file. Note that there are some cross-responses among the different reviews and community comments, and we recommend to read the full response posted in a single document as AC2.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC5
RC3:
'Comment on tc-2022-166', Anonymous Referee #3, 17 Oct 2022

Review comments on "Brief Communication: Everest South Col Glacier did not thin during the last three decades" by Fanny Brun et al.

1. General comments:

This paper reports the surface elevation change of a small (0.2 km²) Himalayan glacier located at a high elevation (~8000 m a.s.l.) for a period from 1984 to 2017. The analysis was performed by comparing two DEMs constructed from aerial photographs taken in 1984 and satellite images acquired in 2017. The motivation of the study is a recent publication (Potocki et al., 2022), which estimated an ice thinning rate of ~2 m a⁻¹ based on the analysis of an ice core drilled from this glacier and surface mass balance modeling. In contrast to the rapid thinning rate reported by Potocki et al., the DEM differencing showed little change in the surface elevation. To explain the two inconsistent results, numerical experiments on wind erosion of snow and surface mass balance were performed, as well as inspections of glacier surface conditions with satellite images. Based on the series of analyses, the authors concluded that ablation due to melting was overestimated by the numerical experiment by Porocki et al. (2022).

Considering the importance of glacier changes in the Himalayas as well as the unique location of the studied glacier, the estimate of ~2 m of ice loss every year at 8000 m a.s.l. has a large impact on the research community and society. Therefore, I appreciate the authors’ effort to inspect the glacier change with a different approach. I think the DEM analysis is reliable enough to exclude the possibility of such rapid thinning. Therefore, I support the swift publication of this manuscript on Cryosphere.

2. Concerns

(1) Numerical modeling

It is a good idea to report the result of DEM differencing as a short article. However, the manuscript is not really a “Brief Communication”. The effort of the authors is acknowledged, but in my opinion, this glacier is not suitable for numerical experiments using a model developed somewhere else. Moreover, the importance of snow erosion is clear on such a location even without numerical simulations. The satellite images tell us a lot more than the erosion model. My suggestion to the authors is to keep the modeling part as simple as possible. For example, experiments with shorter spatial and temporal resolutions of the COSIPY mass balance model nicely showed that heat conduction into the ice was possibly missed in Potocki et al. (2022). However, I am worried about the use of Crocus because the model is not validated in the extreme environment of the studied glacier. Why not simply compare the two COSIPY models to discuss possible shortcomings?

(2) Retention, refreezing and superimposed ice

I am wondering if the authors consider retention of meltwater in a firn layer or ice crucks, and subsequent refreezing and superimposed ice formation. I believe these are important processes related to melt in cold environments. Isn’t it likely that melt happens, but it refreezes and does not leave the glacier?

(3) Setting an “ablation area” (Line 273–289, Fig. A6)

The authors set a boundary of ablation and accumulation areas to assess the importance of glacier flow in the ice thickness change. However, it is odd to set such an imaginary boundary because the idea of accumulation and ablation zones does not work on such a small glacier. Further, the assumption of uniform emergence velocity (or thickening due to vertical straining) over the “ablation area” is not realistic. My suggestion is to estimate the velocity and its gradient from the ice thickness and temperature to confirm 2 m of thickening due to vertical straining is not possible at the coring site.

3. Specific comments

Line 19: “estimated that contemporary thinning rates — or ablation rates,” >> This is confusing. What was estimated by Potocki et al. (2 m a−1) is “negative surface mass balance”, I think.

Line 27: “Automatic Weather Station” >> automatic weather station

Line 32: “1.5 m a−1” >> Here and in other places, please make it clear if it is water equivalent, snow depth, or ice equivalent.

Line 76-77: This is already mentioned in Line 25.

Line 106: The terminology is not clear to me because: (1) erosion occurs after snow deposits on the surface and (2) precipitation includes snow drifting away before deposition. Why not like this?

    - Precipitation: all snow falling on the glacier surface

    - Deposition: snow attached to the glacier surface, a part of the precipitation

    - Erosion: snow removed from the glacier surface after the deposition

    - Accumulation: deposition minus erosion

Line 110-111: “… the most similar …” >> Are you talking about the inland of the Antarctic ice sheet? Isn’t it much drier than the studied glacier? I do not think high elevations in the Himalayan mountains and Antarctica are so similar.

Line 114: “offline nature” >> What do you mean? The erosion model is decoupled from the climate model?

Line 116: “as a function of surface snow density only” >> Wind speed?

Line 127-128: Not clear what “uncorrected precipitation” and “tuned estimates” are. Can you clarify the sentence?

Line 138: “falling snow is not eroded” >> It sounds odd because erosion occurs for deposited snow, but not for falling snow.

Line 142: “191 mm w.e.” >> Is this what you wrote in Line 128? If yes, please avoid repetition. Please also be consistent with the unit.

Line 149: “The wind erosion model is simple and has large limitations.”?

Line 150: “act as a negative feedback” >> It sounds strange to me that density increases as a function of erosion, because Equation A7 is not like that. Maybe, “regulate”?

Line 151: “snowfalls disappear” >> It sounds odd if you mean snow disappears from the glacier surface. “snow on the glacier disappears”?

Line 155: “predicted” >> “reproduced”?

Line 160: “eroded or re-mobilized after deposition” >> This is the correct use of “deposition”, but it is wrong according to the definition by the authors.

Line 162-164: Please revise this sentence because (1) it is self-evident that “deposition efficiency is not constant”, and (2) it does not imply “erosion is a major ablation process”, and (3) the last clause “that is not constant in time” is redundant.

Line 167: “thus integrate the surface energy balance over a much longer period” >> What do you mean?

Line 202: “15 min” >> Isn’t it 1 min as stated in Line 186?

Line 287: “velocity deformation” >> “velocity due to ice deformation”?

Line 293: “continental type” >> This sounds odd. I think ice flows slowly because the glacier is small.

Line 295: “incoming precipitation depositions” >> Is this term usual in glaciology? I have never seen it before.

Line 319: “impossible” >> Maybe “very difficult”?

Figure 4 caption Line 3: “predicted” >> “estimated” or “simulated”?

Citation: https://doi.org/10.5194/tc-2022-166-RC3
- AC6: 'Reply on RC3', Fanny Brun, 13 Jan 2023
  
  We thank the reviewer for their detailed and helpful review of our work. Our response is attached as a pdf file. Note that there are some cross-responses among the different reviews and community comments, and we recommend to read the full response posted in a single document as AC2.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC6
CC3:
'Comment on tc-2022-166', Tom Matthews, 18 Oct 2022

As a co-author on Potocki et al. (2022) and on the comment by Mayewski et al. (this Discussion), I would like to re-iterate the query expressed in the latter for Brun et al. to communicate/explore uncertainty in the 1984 DEM (and hence the DEM of difference) in more detail. For example, does the uncertainty analysis presented by the authors fully consider positional errors in the 1984 image (see comments about the camera's departure from nadir)? Is the uncertainty (in dH) independent of surface slope? Might it be, for example, that the very large dH in the (steep) upper Khumbu and Rongbuk glacier sections reflects such positional errors, rather than a redistribution of mass within the glacier. This request for more attention on uncertainty quantification was perhaps covered by reviewer Ann Rowan who advocated for greater clarity of the methods used, but has not been mentioned again by the other reviews. Given the importance of the DEM analysis -- to reach an important conclusion about an such an "iconic" location -- I hope that Brun et al. will attend to this request in their revision.

Citation: https://doi.org/10.5194/tc-2022-166-CC3
- AC7: 'Reply on CC3', Fanny Brun, 13 Jan 2023
  
  We thank Tom Matthews for his comments on our work. Our response is attached as a pdf file. Note that there are some cross-responses among the different reviews and community comments, and we recommend to read the full response posted in a single document as AC2.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC7
RC4:
'H. Machguth and E. Mattea: Review of tc-2022-166', Horst Machguth, 26 Oct 2022

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-166/tc-2022-166-RC4-supplement.pdf

Citation: https://doi.org/10.5194/tc-2022-166-RC4
- AC8: 'Reply on RC4', Fanny Brun, 13 Jan 2023
  
  We thank Horst Machguth and Enrico Mattea for their detailed and helpful review of our work. Our response is attached as a pdf file. Note that there are some cross-responses among the different reviews and community comments, and we recommend to read the full response posted in a single document as AC2.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC8
CC4:
'Glacial Melting over the South Col Glacier: Observations from S1-SAR', Nicholas Steiner, 27 Oct 2022

As a contribution to this discussion, I would like to submit some observations from remote sensing that may aid in interpretation of annual surface melting from modeling in Brun et al. (2002). In Scher et al. (2021) we created a record of surface melting over glaciers in the Himalayas from a time series Sentinel-1 synthetic aperture radar (S1-SAR). Melt is detected where annual backscatter is reduced as liquid surface water obscures the radar scattering from the glacier interior, resulting in a marked reduction in backscatter. For the South Col glacier, we observe radar signatures that indicate surface melting is occurring in 2019 over areas of exposed ice in the southern extent of the glacier (Figure 1, attached). From time series S1-SAR, we observe continuous indications of surface melting from June 26, 2019, until October 6, 2019, with approximately biweekly repeat observations during this period. Since seasonal snow over areas that are exposed on an interannual basis are not deep enough to contribute substantially to radar scattering, we infer that the melting signal originates from structural features (e.g., laying) in the glacier interior that result in enhanced backscatter during colder winter months. It is important to note that at C band frequencies backscatter is extremely sensitive to liquid water and it is difficult to differentiate very small amounts of surface melting from more extensive melting, and therefore our methodologies are not well suited to evaluate the amount of melting that may be occurring. For more details on our methodologies, please refer to Scher et al., (2021).
Scher, C., Steiner, N. C., & McDonald, K. C. (2021). Mapping seasonal glacier melt across the Hindu Kush Himalaya with time series synthetic aperture radar (SAR). The Cryosphere, 15(9), 4465-4482.

Citation: https://doi.org/10.5194/tc-2022-166-CC4
- AC9: 'Reply on CC4', Fanny Brun, 13 Jan 2023
  
  We thank Nicholas Steiner for his interesting contribution to the discussion. Our response is attached as a pdf file. Note that there are some cross-responses among the different reviews and community comments, and we recommend to read the full response posted in a single document as AC2.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC9
CC5:
'Comment on tc-2022-166 -- response to Macguth and Mattea, w/ref to Steiner', Tom Matthews, 28 Oct 2022

Please see the attachment

Citation: https://doi.org/10.5194/tc-2022-166-CC5
- AC10: 'Reply on CC5', Fanny Brun, 13 Jan 2023
  
  We thank Tom Matthews for his comments on our work. Our response is attached as a pdf file. Note that there are some cross-responses among the different reviews and community comments, and we recommend to read the full response posted in a single document as AC2.
  
  Citation: https://doi.org/10.5194/tc-2022-166-AC10

Fanny Brun et al.

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Short summary

The South Col Glacier is an iconic small body of ice and snow located on the southern ridge of Mt. Everest. A recent study proposed that South Col Glacier is rapidly losing mass. In this study, we examined the glacier thickness change for the period 1984–2017, and found no thickness change. To reconcile these results, we investigate wind erosion and surface energy and mass balance, and find that melt is unlikely a dominant process, contrary to previous findings.


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