the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Mar 2026
Surface mass balance projections until 2100 for Folgefonna, a Norwegian ice cap
Abstract. Robust glacier projections are essential for mountain communities adapting to climate change, yet current projections are limited by climate data that either have coarse spatial resolution or span only a narrow range of future scenarios, thereby obscuring the true scale of predictive uncertainty. In this study, we quantify the cascading impact of uncertainty in climate projections on glacier surface mass balance (SMB) projections. We simulate the SMB of the Folgefonna glacier complex in western Norway for 1970–2100 using the energy-balance and snowpack model BESSI. To represent future climate forcing, we use the EURO-CORDEX ensemble, but show that the dataset is systematically too cold and too wet and exhibits unrealistic precipitation patterns for the western Norway region. We therefore develop a downscaling framework in which each EURO-CORDEX member is represented by analogs drawn from a high resolution convection-permitting model (NorCP). This enables SMB projections that account for a plausible spread in climate models and are based on high-resolution climate data with explicitly resolved physical processes. Using this method, we present the most detailed SMB projections of Folgefonna to date for three emission scenarios until 2100. Whether the glacier complex retains any accumulation zone by 2100 depends on the emission scenario. Ensemble medians indicate that Midtfonna loses its accumulation zone in all scenarios, Nordfonna does so in RCP4.5, and no accumulation zone remains on Folgefonna in RCP8.5. However, cumulative SMB (2026–2100) has an uncertainty of 65–75 m w.e. within each scenario due to climate model spread (25th to 75th quantiles). We furthermore find that the choice of global circulation model has a stronger influence on Folgefonna's SMB than the choice of regional climate model. These findings underscore the need for improving upon agreement between climate models. Detailed glacier mass change projections based on only a subset of the available and hence plausible climate projections underestimate uncertainty and should be considered with caution.
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Status: open (until 15 May 2026)
- RC1: 'Comment on egusphere-2026-1090', Anonymous Referee #1, 08 Apr 2026 reply
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RC2: 'Comment on egusphere-2026-1090', Josephine Lindsey-Clark & Larissa van der Laan (co-review team), 07 May 2026
reply
Review of ‘Surface mass balance projections until 2100 for Folgefonna, a Norwegian ice cap’
Frøystad and Born
Summary
This work by Frøystad and Born looks at the Norwegian ice cap Folgefonna, projecting its mass balance to the end of the century, using BESSI, forced with a EURO-CORDEX ensemble, downscaled using an author-developed downscaling framework. The results focus on the ensemble spread and note the substantial uncertainty, stemming from the climate forcing. Overall, the work is well contextualized and robustly executed. The genuine novelty of the work is found in point 1 of contributions, rather than the added dataset.
General comments
- The paper is quite long, and there are several passages with repetition and some over-explanation which could be shortened. Some examples of lengthy passages which could be condensed include: L151-153, L163-169, L195-199, L212-215, L253-26. Some suggestions of how to cut down are provided in specific comments.
- There are many short sentences which sometimes interrupt the flow and fragment the text, making it less concise than it could be. E.g. L234-235 – these sentences could be merged. Other examples in specific comments.
- The short sentences can also make it unclear whether or not the following sentence is linked to the previous, e.g. a statement is made and the reader is left wondering if this will be evidenced in the following or if it has already been addressed previously. Examples in specific comments.
- It is sometimes unclear in which figure/table the text is referring to. Ensure figures are clearly referenced early on within each discussion of a new idea/observation.
- Mixture of present and past tenses are used e.g. L151 ‘we make’, L191 ‘we determine’ vs L138 ‘we compared’, L301 ‘we fitted’. In most cases these would read better in present tense.
- Mix of UK vs US spellings e.g. ‘colour’ vs ‘theorize’, ‘utilize’
- Some figures contain excessive white space which could be removed. Some full page figures could also be formatted to be more compact. Examples in specific comments.
Specific Comments
Abstract
- L7: ‘show that the dataset is systematically too cold [...]’ relative to observations?
- L14: No accumulation zone remains on any of the glaciers in Folgefonna in RCP8.5? This statement could be clarified by e.g. ‘any of the three glaciers in Folgefonna’, or alternatively mentioning the names of the three glaciers in e.g. lines 5-6.
Introduction
- L34: ‘climate forcing data it’ - missing word
- L46: ‘Such climate models often have large biases when considering specific regions’ can you support this with citations
Data and methods
- L87-98: This passage reads more as introductory material than data and methods
- L113-114: Can you provide a reference for ‘unrealistic precipitation patterns’ which would ‘likely give unphysical data’, or specify why they would be unrealistic in this context
- L117-121: This outline of the remaining subsections of the methods and data section seems a little out of place in the middle of the section. As section 2 contains a lot of information (and spans 11 pages), suggest providing an outline at the beginning of section 2 or separate this into 2 sections, e.g.: Data (2.1, 2.3) and Methods (2.2, 2.4-2.5)
- L138: ‘Our analysis of the western Norway area echoes this.’ – it is not immediately clear that you will show this in the following. To clarify this you could e.g. replace the full stop by a colon to indicate that you do evidence this statement
- L140: ‘unreasonably’ -> ‘unrealistically’
- L150: How are these values (0.1, 10) chosen?
- L156: ‘Not resolved as well’ is ambiguous/informal
- L160: ‘and Folgefonna by extension’ is implied and therefore unnecessary. Consider cutting or replace by e.g. ‘including Folgefonna’
- L168-169: ‘Obviously, an observation-based dataset lacks future data’ is implied and the tone is conversational. These last 2 sentences could be combined e.g.: ‘However, as SeNorge is limited to the observation period and does not provide radiation and humidity variables required by BESSI, it cannot be used directly as climate forcing’
- L185-186: Repetition of lines 178-180
- Fig 4: What do the numbers 1-7 signify here?
- Fig. 4 provides a very informative diagram to illustrate the method, which is accompanied by overexplanation of e.g. choosing the minimum RMSD and the ±15 day window. A short explanation to accompany the diagram in the text is necessary but could be cut down.
- L195-199: Some repetition and overexplanation could be condensed
- L210-213: This detail about interpolating onto different grids is not central to the argument and could be condensed and/or moved to the figure caption
- L216: Validation section missing section number
- L239: This sentence could be replaced by a reference to table to in line 234 or 235 e.g. ‘...and bare ice (Table 2)’.
- L253-254: ‘we therefore tested a few, which are...’ is grammatically incorrect. Consider rephrasing to e.g. ‘We therefore evaluated model(?) performance using RMSD, …’
- L253-263: This section is a bit hard to follow: several statements are vague and/or undersupported e.g. ‘only minor differences were found’ - differences in what? And what value is considered ‘minor’? Consider rephrasing this paragraph to highlight the key points and clarify ambiguous statements.
- L259: ‘Along with the identity line’ is unnecessary as this is explained in the figure caption
Results
- L274: First sentence can be cut and replaced with a reference to (Fig. 8c) in the following sentence
- L266-267: It is unclear if this text (page 16) is referring to the Sørfonna plateau (as stated in L266) or the whole of Folgefonna (as stated in L267) - or are you using these synonymously due to the exclusion of Nord and Midtfonna which is later explained?
- L278-280: Explanation about the static mask for Sørfonna and exclusion of Nord and Midtfonna: does this apply to exclusively to Fig. 8 or to all discussed on page 16? Consider placing this explanation at the start of the section to clarify previous comment above. If this treatment is applied in multiple analyses then a short discussion of this and the associated uncertainties are warranted in the methods section.
- Fig. 8: Full page figure could be rearranged to smaller figure. The caption could be more concise, e.g. cutting statements which are obvious from the legend and labels.
- Fig. 9: Add legend
- L298-300: Which figure is being referenced here?
- L300: ‘To test if this is indeed the case, we conducted additional analysis’ – does this mean that the observations made in L298-300 are illustrated in Fig. 11b?
- L306: Is this relevant to the analysis? If so why?
- L308: ‘following up on the discussion about’ has a conversational tone
- L312-313: Clarify if these experiments will be shown, if so reference the figure
- L316: Unclear what is meant by ‘being the sum of the two’
- L315: What is the complicated picture suggested by Andreassen et al. and Nesje? Comparisons to previous studies should be saved for the discussion
- Section 3.3: It is unclear in the text and table 3 what is being correlated with what, and why P_summer is omitted. Consider adapting the column names so it is clear to the reader and explain why P_summer is excluded
Discussion
- L340: And under RCP8.5?
- L354-355: ‘respectively’ -> move to the end of the sentence
- L368-375: This paragraph misses references to figures where you have evidenced each statement
- L408-409: Reasonably well compared to?
- L431-433: The phrasing in these sentences makes it unclear whether you have done this or would do this as a next step
- L459: ‘spring melt [onset]’
Summary and Conclusions
- L467: This concluding sentence seems a little out of place as your work is not directly studying public trust. Suggest ending with a clear implication of your work for the field, or simply cut this sentence.
Citation: https://doi.org/10.5194/egusphere-2026-1090-RC2
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- 1
Review of “Surface mass balance projections until 2100 for Folgefonna, a Norwegian ice cap”
This study investigates the range of projected surface mass balance changes of the Folgefonna ice cap using a novel downscaling approach to drive an SMB model with a wide range of ESM projections. The study has two main aims: first, to develop a new framework in which RCM output is downscaled using a new “analog” method; and second, to apply this framework to force the SMB model BESSI and generate projections up to 2100 under three emission scenarios. The study is relevant and fits well within the scope of The Cryosphere. The authors demonstrate a substantial amount of work, incorporating a large number of climate simulations, multiple data products, a downscaling method, and SMB model calibration. Given this methodological complexity and the large number of simulations and data products used, it is particularly important that the methods are presented clearly. However, in the current stage this is lacking, and it constitutes my main concern. The presentation of the methods requires clarification before publication. I therefore provide general comments followed by a list of specific recommendations below. I recommend that the manuscript be considered for publication after major revisions.
General comments
Section 2.2: More details about the BESSI model are needed for the reader to fully understand and interpret the results. For example, it would be helpful to clarify how turbulent heat fluxes are calculated, how albedo is parameterized, and how melt and runoff are computed.
Section 2.3: The authors begin with a brief overview of the different data products and methods before describing each component in detail, which is a good structure. However, this section is currently difficult to follow because the overview remains too general. Improving this section would provide the reader with a clearer understanding of the overall methodology. More specific suggestions are provided below.
For the downscaling, analogs are created by selecting similar days from NorCP runs forced by two GCMs: EC-Earth and GFDL-CM3. Additional discussion is needed on how the climate of these models compares to the broader CMIP6 ensemble. Are these two models representative of the full range of plausible climates? Or might the spread of CMIP6 projections be underestimated by relying on only two ESMs?
Regarding using the analog downscaled fields as forcing, it is unclear whether all forcing fields are taken from NorCP to drive BESSI. The algorithm selects the most similar day based on precipitation and temperature, but are all variables (e.g., shortwave radiation, longwave radiation, humidity) taken from this selected day, or are some fields retained from the original RCM simulations?
How do BESSI simulations forced by historical EURO-CORDEX simulations compare to the SeNorge-forced BESSI run? Can the model output be trusted when forced by all these RCM–ESM combinations? Why was the decision made to include all EURO-CORDEX simulations rather than selecting only those models that perform well over the historical period?
A related point concerns the NorCP simulations: did you also run BESSI forced with the two NorCP runs over the historical period to assess their performance compared to observations or the SeNorge-forced reference run?
Specific comments
L35: What is meant by “glacier model uncertainty”? Does this refer to SMB, SEB, ice dynamics, or a combination?
L45: Please provide a reference for this statement.
L50: Please specify the regions referred to here.
L 52-54: Can you give the typical resolution of these models? Is the issue mostly in resolution or the fact the assume is hydrostatic or not?
L 62: “Improving SMB estimates”. If this is the goal it would be helpful to discuss the current state of SMB estimates and their limitations. Since this is addressed in Section 2.1, consider moving that discussion to the introduction.
L79-81: Please provide a reference. It would also help to include approximate values for average snowfall and melt rates.
L89–90: Please specify which glacier/SMB projections are currently available and why they are insufficient.
L92: What type of model was used to derive these estimates?
L109: Consider using “spread” instead of “width”.
L111-112: What resolution was NorCP run at? Also clarify here that it is forced by only two ESMs rather than the full CMIP6 ensemble.
L 113: A reference (figure or citation) is needed for the statement that EURO-CORDEX has unrealistic precipitation patterns over western Norway.
L119: Include the typical resolutions of the datasets here and clarify that SeNorge is a gridded observational product.
Figure 2: Given the complexity of the methods, this figure could be improved to better clarify the workflow. For example, indicate the number of NorCP simulations, the number of EURO-CORDEX RCM–ESM combinations, the role of ERA-Interim simulations, and which variables are subject to bias correction and analog downscaling.
L124: “Forced by dynamically downscaled GCMs”: Which model performed the dynamical downscaling? Or do the authors mean that NorCP is used to downscale EC-Earth and GFDL-CM3?
Table 1: first row; is this a mixed historical forcing of the two ESMs or one historical simulation for each? What version of EC-Earth is used?
L140: More prior explanation of the SeNorge dataset is needed here. Consider placing the section describing SeNorge before this section.
L141: ‘display similar patterns’: which patterns? Please specify and indicate where this is shown.
L 151: Can you comment on whether the bias is stable over the historical period? It would also be helpful to indicate the typical magnitude of the bias corrections.
Section 2.3.2: The manuscript would benefit from a table summarizing the ESMs, the RCMs used for downscaling, and their spatial resolutions.
L171-175: This section is unclear. If the bias correction is derived from the historical period and assumed constant, does it affect interannual variability in the SSP scenarios?
L 172: do you mean bias correction applied to the EURO-CORDEX instead of NorCP?
L 174: do you mean ‘climate variability’ in bias of the RCM?
L 178: ‘between models’: does this refer to RCMs? ESMs?
L 181: can you be more specific already what is meant with ‘most similar day’ in terms of what?
L 192: consider using ‘target day of the year’
L215: use ‘rectangular’ instead of square domain
L 219: ‘to not choose itself’: please clarify
L 234: It is unclear why an analog is constructed for SeNorge if it already represents the observational reference dataset.
L 247: How is this SMB for the basins used for calibration?
L253: ‘criteria’: do you mean evaluation criteria or performance metrics?
L 278: are these spreads based on the same number of RCM-ESM combinations within each scenario? How do the spreads change when considering only models available across all scenarios?
Fig 8a: can you explain why the inter-annual variability in the SeNorge-forced BESSI run is much larger than those forced by historcial RCM-ESM simulations? How does that affect your projections?
L 296. Please revise this statement, as historical simulations do not correspond to emission scenarios.
Table 3: What about summer precipitation? How is rainfall treated in the model?
L 329-330: this is unclear to me, please clarify statement
Section 3.4: did you assess how well BESSI performs in estimating runoff? Have SeNorge-forced simulations been compared to observed streamflow (if available)? It would also be useful to indicate what fraction of melt contributes to runoff.
L 355-257: this statement could be tested/demonstrated by plotting annual SMB against annual temperature
L 359-360: Is the precipitation gradient discussed here also present in the forcing datasets?
First paragraph of 4.2: Instead of the elevation explaining the stronger SMB response of the west, it seems from Fig 11b that for the same elevation, the west shows a stronger SMB decline compared to the same elevation on the west. Can you explain that?
L370: Is the relationship with winter temperature simply reflecting overall warming trends, or is there also a direct relationship with winter SMB?
L 383: What explains the large differences between model simulations? Could this be related to variables not included in the analog downscaling (e.g., radiation, humidity)?
L 400-402: How this SeNorge* was produced was not clear until now, this should be clarified earlier on in the methods.
Section 4.3: What is the advantage of the analog method compared to alternative approaches, such as training an emulator to reproduce NorCP simulations and applying it to the RCM–ESM ensemble?