Review of ‘The Glaciers of the Dolomites: last 40 years of melting’

By Securo, Andrea and others.

Securo and authors present a multi-decadal estimation of surface elevation change for small glaciers in the Dolomites, Italian Alps. Their geodetic data used in this study consists of aerial photographs, uncrewed aerial vehicles (uavs) and LiDAR data. This data and their analysis indicate high rates of glacier mass loss with the Marmolada Glacier accounting for about ⅔ of the region’s volume loss.

Overall, I found this manuscript to be generally well written with methods partially described. The presented data generally supports the conclusions made by the authors. However, like many papers, some clarification of the methods is needed, the English can be substantially improved and manuscript could be shortened. Below, I outline my major points about the paper and follow these with technical comments. 

MAJOR POINTS:

1. Methodology and error analysis needs further description - Given that the authors are using multiple geodetic datasets to calculate volume (and mass) change, the methods section should clearly lay out how the actual uncertainties are propagated to yield the total error budget and final uncertainties. If I understand it correctly, the authors obtained point clouds from all datasets, align these and then use M3C2 for the error analysis. There has been some discussion about the robustness of M3C2 as it assumes planar surfaces and could estimate significance of detectable change (e.g. https://doi.org/10.1016/j.isprsjprs.2021.06.011). I would like to see, for example, how the uncertainty over stable terrain changes due to slope. These are small glaciers in rugged terrain so their slopes will be steep. Also, are there assumptions made for missing data? How is this error source treated?

2. Manuscript needs to be shortened/tightened - I found the ‘Introduction and Previous glaciological research’ sections to be long and would strongly advocate for merging the ‘Previous glaciological section’ with the Intro so that the total length of both sections is about ½ of what it currently is. Also, I think some of the tables could be moved to a supplement as most people rarely need to read each line of these tables (they are, however, useful to have if a reader needs them). 

3. Comparison to previous work - The authors do a commendable job compiling datasets for these small glaciers, but they should explicitly show how their results compare to those, for example, of Hugonent and others (2021). The authors can download data for each of their glaciers (since each glacier has an RGI number this should not be a difficult task). How do their estimates and uncertainties compare to Hugonnet? This is an important test of the reliability of Hugonnet for small glaciers in this region (I would posit that perhaps the present study has better estimates for these small glaciers but I simply don’t know). It would be good to examine this in some detail. 

4. Manuscript should be shortened. I found the length of the discussion section to be somewhat unbalanced with the length of the results section (the former is longer than the latter). I would recommend shortening the discussion section to balance in light of statements that can be backed up with results shown in the results section. Some of the figures/materials in the discussion seem, to me, to be more results and less discussion. 

5. Terminology needs clarification - I would recommend that the authors consult the glossary for the standard definitions used in glacier mass balance (https://wgms.ch/downloads/Cogley_etal_2011.pdf) . There are multiple appearances of mass balance when the authors are referring to ‘geodetic balance’. 

TECHNICAL COMMENTS:

Abstract:

 ‘Use lowercase ‘Alpine’ unless it starts a sentence. 

Pg 1, Line 3: This sentence isn’t technically correct in light of Worldview or Pleiades (very high resolution)

Pg 1, line 11: ‘between used for two items, among for more than two’. Also use of ‘amplitude’ is vague

Pg1, line 13: replace ‘areal reductions’ with ‘area loss’

Pg1, lines 13-15 - This sentence is out of place and likely not needed

P1, line 18: ‘with greatest emphasis on regions of the world’ - unclear what authors are referring to here. 

Pg 2, lines 25-33  - Authors could easily jettison this section to shorten first two sections that need to be put on a diet.

Pg 2, line 50 (and throughout) - ‘unmanned’ is an outdated term these days. Typical use is ‘uncrewed aerial vehicle’ 

pg.3 , line 63. Unclear what authors mean with ‘active’ - deformation, ice flow?

Section 2 - Shorten this section

Figure 1. The figure could use a little bit of work. The inset (upper left) needs at least some lat/lon coordinates for a reader not certain where the Dolomites are. I was initially confused with the color of the glaciers and the colors of the geodetic datasets (numbers). Maybe change the color of glaciers to avoid confusion? (a) - replace ‘position’ with ‘location’

Pg. 6, lines 115-118. Were these photos not available as photogrammetric scans? Also, it’s too bad that the internal orientation (if available) information isn’t used as that might help reduce overall error budget.

Pg, 6, lines 115-126. I had a hard time understanding how GCPs were collected and how they were used. This section should be revised to make it clear exactly what was completed and for which datasets.

Pg. 8, lines 153-154. I’m surprised that there were only small areas of voids. Was hypsometric interpolation attempted?

Pg 11, lines 210. Area change. How were areas of the glaciers digitized? Any uncertainty in area change? Unless I missed it,  planimetric mapping is not described in methods.

Pg. 11, lines 219: Unclear what ‘topographic bounding’ is. Surrounded by rugged terrain?

Pg. 11, lines 228-229: Uncertainties needed for these estimates. 

Pg. 13 lines 215: Terminology needs to be changed to include term ‘geodetic’

Figures 3, 4 - Generally well drafted, but uncertainties would be useful. 

Tables 3, 4- I would recommend moving these perhaps to a supplement. Also it would be good to have a summary line for weighted mean (table 3). Does table 4’s all glaciers line imply this is a weighted mean (by area)?

Figure 5 - I think this is a well drafted figure, but I’m not fond of the color bar. It really should be a standard diverging color (red to blue). The dark ends for both red, blue are problematic for diverging data. Sorapiss Glacier’s mid elevation I presume is debris covered? I would explain before it is brought up in the discussion.  As stated in the major comments, I think some of the discussion and plots should be moved to results section. 

Figure 7. Not certain what color bars on top of the graph refers to. WGMS trend is the dashed line? It’s not evidently clear to me. 

Pg 16, Results - How do these results compare to Hugonnet? Add those values perhaps to one of your tables with both estimates (yours and Hugonnet) including uncertainties.