Review to Basantes-Serano et al. (2022): "New insights into the decadal variability in glacier volume of an iconic tropical ice cap explained by the morpho-climatic context, Antisana (0°29' S, 78°09' W)" submitted to The Cryosphere.

The authors present a study of the photogrammetrically derived surface elevation changes of the Antisana ice cap. The surface elevation changes since the mid-20^th century are separated into five subsets, each roughly representing a decade, converted into mass changes, and correlated to their morpho-climatic setting.

Overall, this manuscript is well structured and presents a very sound study. The research questions are laid out clearly, the methods are explained and applied properly. The significance of the study is high, because it extends our knowledge of one of the best studied glaciers in the tropics, a region where long-term glacier observations are particularly rare but highly demanded for calibration of glaciological and hydrological models.

I have some comments to the text, datasets and methods, which I assume to be a major revision.

My main points of concern are:

The coverage of dh-samples (L 178-184): I suggest adding a figure (supplement) showing the data coverage across the glacier hypsometry. I just wonder if the coverage is evenly distributed or if certain elevation bands or larger areas are systematically missing and the authors possibly introduce a bias.

The density used to drive elevation changes into mas changes (eq. 5). I'd like the authors to elaborate a bit more on the density used. I know, it is convenient and mostly enough to use the approach of Huss (2013), especially in periods of mass loss. However, the second period in this study (1965-1978) is characterized by mass gain and this raises the question of possible lower densities than 850 kg/m³, because it needs several decades to compact snow to ice. It would be worth to look into possible other sources of density (e.g. Williams et al., 2002).

Gridded precipitation: It would be good to have some explanation why ERA5 precipitation was selected for the analyses. There are other gridded precipitation data sets like the PSL South America daily gridded precipitation, GPCC, or a recently published data set for the Peruvian and Ecuadorian water sheds (Fernandez-Palomino et al., 2021). Ideally, the authors add a figure (supplement) relating stations M003 and M188 to the gridded data. For example, show in two panels the time series of monthly air temperature and precipitation from a gridded data set (monthly box plots or shading ±1 standard deviation from a reference period) and the station measurements as lines.

Comments:

Title: I suggest rewording the title omitting "new insights" and "iconic". The first, because apart from the surge of G8 I speculate nothing is really new, and I assume most results are an upscaling or a validation of assumptions based on earlier studies of G15 or other tropical glaciers. The latter, because it reads more lurid than scientific.

L 43: Here you could connect to Nicholson et al. (2013), who compare the micrometeorological conditions of small tropical glaciers.

L 99: At what altitude is the maximum precipitation rate recorded? On tropical mountains the positive vertical precipitation gradient often reverses above a certain altitude. Is this the case for Antisana as well?

L 154: Could you explain in a half sentence why the geometric characteristics were optimal?

L 180: Could you explain this spatial optimisation?

L 206: I assume Scor needs a capital S.

L 248: I think you should state how many ERA5 grid cells cover your domain (Fig. 2).

L 313: termini (plural)

L 350 and whole chapter 4.2. I think the two groups from Fig. 5 are confused with the two groups in the text. The figure caption says group I are the glaciers at the Pacific side. In the text it is the other way round. Please correct.

L 364: Is this mass gain (1998-2009) also detectable in the precipitation time series? (Another reason to add a figure about precipitation).

L 369: Please, elaborate on the role of subsurface heating as possible reason for the surge. L78 suggests that Antisana is an active volcano.

L 371/372: This is an odd sentence and consider deleting it. I even doubt the message is true, because the ice flow dynamics are a consequence of the climate variations, and especially at longer time scales glacier response times will be met. Then ice flow dynamics reflect climate variations. The reference (Thompson) is missing in the reference list.

L 400: This sentence is difficult to read. Consider restructuring.

Table 5: Explain Bm and 'Bm.

L 409: I suggest adding a few sentences on the concept of the reference surface balance (Elsberg et al., 2001; Harrison et al., 2005; Huss et al., 2012).

L411: I don't fully understand why the mass balance should be overestimated in this case. When referring to a larger surface area the specific mass balance should become a smaller number, thus an underestimation as shown in L 414 and Fig. 7. Same problem in L 435 (underestimation). Maybe a clearer wording does the job.

L 432: … many regional studies: Please add the references.

L 470 et seq. and Fig. 8: Very interesting figure. Consider adding a note, that tropical glaciers are known to be particular sensitive on moisture/precipitation/clouds (Mölg et al., 2009; Prinz et al., 2016; Sicart et al., 2005).

L 680: Consider deleting Hastenrath 1981.

References:

Elsberg, D. H., Harrison, W. D., Echelmeyer, K. A. and Krimmel, R. M.: Quantifying the effects of climate and surface change on glacier mass balance, J. Glaciol., 47(159), 649–658, doi:10.3189/172756501781831783, 2001.

Fernandez-Palomino, C. A., Hattermann, F. F., Krysanova, V., Lobanova, A., Vega-Jácome, F., Lavado, W., Santini, W., Aybar, C. and Bronstert, A.: A novel high-resolution gridded precipitation dataset for Peruvian and Ecuadorian watersheds – development and hydrological evaluation, J. Hydrometeorol., 23(3), 309–336, doi:10.1175/JHM-D-20-0285.1, 2021.

Harrison, W. D., Elsberg, D. H., Cox, L. H. and March, R. S.: Different mass balances for climatic and hydrologic applications, J. Glaciol., 51(172), 176, 2005.

Huss, M.: Density assumptions for converting geodetic glacier volume change to mass change, Cryosph., 7(3), 877–887, doi:10.5194/tc-7-877-2013, 2013.

Huss, M., Hock, R., Bauder, A. and Funk, M.: Conventional versus reference-surface mass balance, J. Glaciol., 58(208), 278–286, doi:10.3189/2012JoG11J216, 2012.

Mölg, T., Cullen, N. J., Hardy, D. R., Winkler, M. and Kaser, G.: Quantifying climate change in the tropical midtroposphere over East Africa from glacier shrinkage on Kilimanjaro, J. Clim., 22(15), 4162–4181, doi:10.1175/2009JCLI2954.1, 2009.

Nicholson, L. I., Prinz, R., Mölg, T. and Kaser, G.: Micrometeorological conditions and surface mass and energy fluxes on Lewis Glacier, Mt Kenya, in relation to other tropical glaciers, Cryosph., 7(4), 1205–1225, doi:10.5194/tc-7-1205-2013, 2013.

Prinz, R., Nicholson, L., Mölg, T., Gurgiser, W. and Kaser, G.: Climatic controls and climate proxy potential of Lewis Glacier, Mt. Kenya, Cryosph., 10(1), 133–148, doi:10.5194/tc-10-133-2016, 2016.

Sicart, J. E., Wagnon, P. and Ribstein, P.: Atmospheric controls of the heat balance of Zongo Glacier (16°S, Bolivia), J. Geophys. Res., 110(D12), D12106, doi:10.1029/2004JD005732, 2005.

Williams, M. W., Francou, B., Hood, E. and Vaughn, B.: Interpreting Climate Signals from a Shallow Equatorial Core: Antisana, Ecuador, in The Patagonian Ice Fields, edited by G. Casassa, F. V. Sepulveda, and R. M. Sinclair, pp. 169–175, Kluwer Academic / Plenum Publishers, New York., 2002.

Review of "New insights into the decadal variability in glacier volume of an iconic tropical ice-cap explained by the morpho-climatic context, Antisana, (0°29’ S, 78°09’ W)" by Basantes-Serrano et al., (2022).

General comments

This article describes the decadal changes in glacier volume in the Antisana ice cap located in the tropical Andes, Ecuador. The authors have used photogrammetric and remote sensing techniques to provide a long-term geodetic mass balance for the Antisana ice cap. Overall, there has been a lack of long-term glacier mass balances studies in this region. For this reason, additional information and novel insights into the past and current state of tropical glaciers are very welcome. In general, I think this is a well-presented and worthwhile piece of research and could help increase our knowledge about the spatiotemporal patterns of glacier volume changes. The topic is timely and highly relevant for various research branches including glaciology, hydrology, and climatology. I am very much in favor of seeing this manuscript published, and would like to make the following suggestions.

 Methods

• The authors used state of- the art remote sensing and photogrammetric techniques to generate digital elevation models to estimate volume changes. The authors also applied state-of-the-art post-processing techniques (including co-registration, gap filling, outliers filtering, etc.) to provide a complete series of glacier elevation, volume, and area changes for the whole massif-volcano. However no information about the glacier area estimation.
• They also evaluate the effect of the morpho-topographic and climatic variables on glacier volume changes. However, in some sections, they mixed morpho-topographic-climate or vice-versa. In the title the use morpho-climatic. I suggest being consistent with the terms and clearly stating the variables evaluated.

 Volume to mass changes conversion:

• The authors used one conversion factor (density) of ice volume change (850 kg m^-3). However, very little discussion is associated with the choice of this number. Why just this value? Are the uncertainty ranges sufficiently? (±60 kg m^-3). The authors also report that during the period 1965-1978 all the glaciers gain mass (moderate). Maybe it is possible to present density scenarios (e.g. Seehaus et al., 2019). For instance, a second scenario of two different conversion factors for areas below and above the ELA (e.g. Kääb et al. 2012).

Uncertainties:

• Overall, no details are mentioned about glacier area estimation or source. How did you obtain the glacier areas? How was the uncertainty of glacier mapping considered? No details about the uncertainty of the glacier area are included (not included in your error propagation equation).

Specific comments:

Title: I am not fully convinced with your title. I would suggest restructuring the title since this study signifies the first long-term geodetic mass balance /volume change, and also because Antisana ice cap more than “iconic” is a benchmark glacier for the inner tropics.

Abstract: Please provide numbers of volume or mass changes for this section. Strong and slight mass loss can sometimes be subjective.

21 -> what about the climatic variables

80 -> it seems that it was an important eruption in 1800.

115 -> did you scan the negatives? or how was the digitalization process for the aerial photographs?

135 -> did you apply any correction (GCP points) to the Pleiades image? Some of the images present some displacement.

212 - 216 -> Please check this, you have included the internal ablation due to the heat transfer in the subglacial interface layer and due to heat released due to glacier dynamics in your uncertainties. However, I think this is not necessary. To my knowledge, the geodetic mass balance is providing the total glacier mass balance including internal ablation (Cogley et al., 2011). Hence the uncertainty from the geodetic estimation should be enough.

 214 -> include the area error/uncertainty into your propagation equation. No details about the uncertainty of the glacier area mapping are included.

280 -> what do you consider as morpho-topographic features just an elevation profile? Please provide clear detail about the morpho -topographic -climate variables.  In the title of your study, you just included the morpho-climatic. Please be consistent throughout the text.  

320 -> Table 4 -> the periods should be 1956-1965; 1965-1979, 1979-….etc…did you calculate the dhdt using these dates? You stated in line 237 that the time was not adjusted. I think that the results from ⅀period and 1956-2016 should be included in your uncertainty estimation as well (e.g. Brun et al., 2017; Menounos et al., 2019 -systematic errors-).  

370 -> is there any signal of geothermal activity in the Antisana glacier? This could explains the surge event?, at least it is a factor that is should be considered since is an active volcano (although its last eruption was in 1800). Is there any fumarolic activity?

371 -> It is a confusing sentence. Ice flow dynamics are also a response of climate variations.   

405 -> table 5 -> Just morpho-topographic?  Please indicate what is Bm and ‘Bm

I missed a comparison of your results with those from Braun et al., (2019) and Dussaillant et al., (2019). Although they used RGI_V6 glacier outlines to estimate volume change over a limited period, it is a good opportunity to check their number with more high-resolution data as you have shown here.

Figures:

The figures are clearly meant to support the overall study, but they also present some issues for the reader

Figures 1 and 2 -> maybe both images can be merged, and the insert table can be inserted as a normal table.

Figure 3 -> It is difficult to follow the colors. Is it possible to change the brightness of the plot? it is not possible to identify the colors (opaque). In the period 1956-2016, there are data gaps mainly in glaciers from G4 to G7. I was wondering how you managed the samples in these accumulation areas (gray).  The same for 2010-2106 period.

References

Braun, M. H., Malz, P., Sommer, C., Farías-Barahona, D., Sauter, T., Casassa, G., Soruco, A., Skvarca, P. and Seehaus, T. C. (2019). Constraining glacier elevation and mass changes in South America, Nat. Clim. Chang., doi:10.1038/s41558-018-0375-7.

Brun, F., Berthier, E., Wagnon, P., Kääb, A. and Treichler, D. (2017). A spatially resolved estimate of High Mountain Asia glacier mass balances from 2000 to 2016, Nat. Geosci., 10(9), 668–673, doi:10.1038/ngeo2999.

Dussaillant, I., Berthier, E., Brun, F., Masiokas, M., Hugonnet, R., Favier, V., Rabatel, A., Pitte, P. and Ruiz, L. (2019).Two decades of glacier mass loss along the Andes, Nat. Geosci., doi:10.1038/s41561-019-0432-5, 2019.

Menounos, B., Hugonnet, R., Shean, D.,Gardner, A., Howat, I., Berthier, E., et al. (2019). Heterogeneous changes in western North American glaciers linked to decadal variability in zonal wind strength.Geophysical Research Letters,46, 200–209. https://doi.org/10.1029/2018GL080942

Seehaus, T., Malz, P., Sommer, C., Lippl, S., Cochachin, A., and Braun, M. (2019). Changes of the tropical glaciers throughout Peru between 2000 and 2016 – mass balance and area fluctuations, The Cryosphere, 13, 2537–2556, https://doi.org/10.5194/tc-13-2537-2019.