The paper “Rapid warming and degradation of mountain permafrost in Norway and Iceland” by Etzelmüller et al. presents results of permafrost monitoring through borehole temperature and geoelectrical surveys coupled with heat flow modeling to discuss the main heat transfer processes responsible for the reported observations.

In general, the paper is very well written and is based on a relevant balance between the core of the MS and the Appendixes to complete the observations. The scientific content is sound and timely as there is currently an international effort in publishing permafrost trends over the past decades.

However, despite it is concise and well-written, I miss some detailed explanation about the physical processes that could explain specific observations (notably about the snow, vegetation and ice content effects) as well more comparison with recently published data. In the same way, some sections (e.g. site descriptions, modeling approach, conclusions) are very general and lack detailed explanations. Therefore, I recommend to detail a little bit more the MS and take into account a few minor recommendations before accepting the manuscript.

GENERAL COMMENTS

Trends: Trends are given for SAT, GTS, etc., but no information about their calculation is given. Could you explain how the trends are calculated? Are there comparable to other trends recently published in the literature?

Section. 2. More details could be given to better discuss the results afterwards. For example, there is no information about the regional limits of permafrost or about the type of permafrost (discontinuous…) for each area. This information is always interesting because it is often inferred from modeling of a steady state and comparison with existing temperature data that represent a transient state give valuable information to assess ground thermal conditions and their disequilibrium with current climate conditions.

Furthermore, it is not clear how BH that are presented in the study are chosen and why some are left out. Could you explain BH choice?

Sect. 3.5. Detailed information about the model setting and parameters (spatial resolution, thermal propertied, geothermal heat flux, ground ice content…) are missing. One important factor to reproduce ground temperature evolution is the ice content. How is the ice content set up in the modeling experiment for the different sites? The ice content is sometimes mentioned when presenting the data (e.g. L 410) but it is not explicitly discussed. It may have a primary influence on talik development and I think this would deserve clearer description and discussion.

Conclusions are really short and very general. This would be worth detailing a little bit more the statements: e.g. what is meant by a “high” rate?  There is no word about differences between Iceland and Norway while they are discussed in the core of the MS, etc.

DETAILED COMMENTS

L 17: Seismic data are mentioned only at this line in the whole MS. Either remove this mention or give more detail about these data in the paper.

L 25: What do you mean by « increasing » snow cover? Duration? Height?

L106: In title of 2.3. coordinates are not given while they are given for other areas.

L137: Title is repeated

L143-145: This level of information about hydraulic characteristics is only given for this area. Try to homogenize the different sub-sections.

L160-16: Could you provide information about the quality of the regressions? Why regressions based on monthly data to predict daily values? This mismatch has likely an influence on the accuracy of the final results.

L199ff: what do you mean by “high” or “low” resistivity? Could you provide a range of values?

L220: how is this wind drift factor defined and applied?

L223: What is the method of lines?

L 229ff: Trends given in this paragraph are not clearly linked to Fig 2a. For example, I do not see that Iceland has a much lower warming than Northern Norway in Fig. 2a. This highlight the need to clarify how trends are calculated.

L263-265: a few explanations about how the vegetation cools the ground would be interesting.

L 284: active layer thickness here is mentioned but not related to a figure or data, which makes it difficult to follow the statement contained in this sentence. Reversing 4.3 and 4.4 is maybe a solution to mention ALT when discussing GT.

L 310: ice-richness is mentioned but it is not clear how the ice content is determined. This might be worth adding a few word about that in Tab. 1 and sect. 2.

L323: this is not the talik that thawed, but the permafrost.  

L 356: “Geophysical changes” here means geoelectrical as the seismic data are not presented. Include the seismic data or rephrase throughout the MS to make it clear that this is only geoelectrical data.

L 409: the statement is interesting but this needs to be a little bit more detailed. Could you better explain or illustrate how the snow controls GT evolution?

Fig. 1: report name of the area in 1c. Write boreholes that are not used in the study differently (if I understand well this is for example the case at 1e (Juvflye) as it is stated L83-84 that there are 7 BH but only 5 are used in the study.

Tab. 1: What blue and red texts mean? Those trends are already some result and it is not explained how they are calculated

Fig. 2: graphs c and d are barely readable. Make sure that names reported in these graphs are also reported in Fig. 1.

Fig. 3b: it is interesting to note that at Gu1, permafrost disappeared in 20113-2014 but formed again in 2017 and after. Does this deserve more discussion?

Fig. 4: add a grid in the background of the graphs, especially a line at 0°C.

Fig. 5: it is interesting to note that the talik at BH2 is reversible.

Fig. 7: what are the black lines at the bottom of the graphs? What is the R² between modeled and observed values?