the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Aug 2025
Accelerated lowland thermokarst development revealed by UAS photogrammetric surveys in the Stordalen mire, Abisko, Sweden
Abstract. The estimation of greenhouse gas (GHG) emissions from permafrost soils is challenging, as organic matter propensity to decompose depends on factors such as soil pH, temperature, and redox conditions. Over lowland permafrost soils, these conditions are directly related to the microtopography and evolve with physical degradation, i.e., lowland thermokarst development (i.e., a local collapse of the land surface due to ice-rich permafrost thaw). A dynamic quantification of thermokarst development – still poorly constrained – is therefore a critical prerequisite for predictive models of permafrost carbon balance in these areas. This requires high-resolution mapping, as lowland thermokarst development induces fine-scale spatial variability (~50 – 100 cm). Here we provide such a quantification, updated for the Stordalen mire in Abisko, Sweden for the Stordalen mire, Abisko, Sweden (68°21'20"N 19°02'38"E), which displays a gradient from well-drained stable palsas to inundated fens, which have undergone ground subsidence. We produced RGB orthomosaics and digital elevation models from very high resolution (10 cm) unoccupied aircraft system (UAS) photogrammetry as well as a spatially continuous map of soil electrical conductivity (EC) based on electromagnetic induction (EMI) measurements. We classified the land cover following the degradation gradient and derived palsa loss rates. Our findings confirm that topography is an essential parameter for determining the evolution of palsa degradation, enhancing the overall accuracy of the classification from 41 % to 77 %, with the addition of slope allowing the detection of the early stages of degradation. We show a clear acceleration of degradation for the period 2019 – 2021, with a decrease in palsa area of 0.9 – 1.1 %·a‑1 (% reduction per year relative to the entire mire) compared to previous estimates of ~0.2 %·a‑1 (1970 – 2000) and ~0.04 %·a‑1 (2000 – 2014). EMI data show that this degradation leads to an increase in soil moisture, which in turn likely decreases organic carbon geochemical stability and potentially increases methane emissions. With a palsa loss of 0.9 – 1.1 %·a‑1, we estimate accordingly that surface degradation at Stordalen might lead to a pool of 12 metric tons of organic carbon exposed annually for the topsoil (23 cm depth), of which ~25 % is mineral-interacting organic carbon. Likewise, average annual emissions would increase from ~ 7.1 g‑C·m‑2·a-1 in 2019 to ~ 7.3 g‑C·m‑2·a‑1 in 2021 for the entire mire, i.e., an increase of ~1.3 %·a-1. As topography changes due to lowland thermokarst are fine-scaled and thus not possible to detect from satellite images, circumpolar up-scaling assessments are challenging. By extending the monitoring we have conducted as part of this study to other lowland areas, it would be possible to assess the spatial variability of palsa degradation/thermokarst formation rates and thus improve estimates of net ecosystem carbon dynamics.
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Status: open (until 31 Oct 2025)
- RC1: 'Comment on egusphere-2025-3788', Anonymous Referee #1, 18 Aug 2025 reply
Data sets
Unmanned Aerial Imagery over Stordalen Mire, Northern Sweden, 2014 M. Palace et al. https://doi.org/10.7910/DVN/SJKV4T
Unmanned Aerial Imagery over Stordalen Mire, Northern Sweden, 2015 M. Palace et al. https://doi.org/10.7910/DVN/NUXE30
Unmanned Aerial Imagery over Stordalen Mire, Northern Sweden, 2016 M. Palace et al. https://doi.org/10.7910/DVN/IAXSRD
Unmanned Aerial Imagery over Stordalen Mire, Northern Sweden, 2017 J. DelGreco et al. https://doi.org/10.7910/DVN/NZWLHE
Unmanned Aerial Imagery over Stordalen Mire, Northern Sweden, 2018 M. Palace et al. https://doi.org/10.7910/DVN/2JXWVW
UAV - RGB orthomosaic from Stordalen, 2019-08-16 Abisko Scientific Research Station, Swedish Infrastructure for Ecosystem Science (SITES) https://hdl.handle.net/11676.1/U4o8KrPkEiKw5RsfiCJZeEgX
RGB orthomosaic, digital surface model and slope over Stordalen Mire, Northern Sweden, 2021 M. Thomas et al. https://doi.org/10.14428/DVN/MGNYNN
Unmanned Aerial Imagery over Stordalen Mire, Northern Sweden, 2022 M. Palace et al. https://doi.org/10.7910/DVN/G9Y8WC
Model code and software
Accelerated lowland thermokarst development revealed by UAS photogrammetric surveys in the Stordalen mire, Abisko, Sweden M. Thomas et al. https://doi.org/10.14428/DVN/SX6TYV
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- 1
The authors used UAS surveys to quantify rates of palsa degradation at the Stordalen mire in Sweden from 2019 to 2021, within the context of longer-term change from almost annual UAS surveys from 2014 to 2022. EMI measurements were also collected to characterize the soil properties in stable, degrading, and degraded sections of a study transect.
While the study is interesting, I worry that the findings, as they are currently presented, are not particularly novel. The authors find that topography is important for identifying palsa degradation, that palsa degradation leads to more open water and increased soil moisture, and that palsa degradation rates have increased in recent years. These findings are useful, but they have already been demonstrated in other studies. What is potentially novel, however, is the updated estimate of emissions from Stordalen, and the integration of EMI and UAS field methods. I encourage the authors to really highlight these elements in this manuscript.
GENERAL COMMENTS
This manuscript could be greatly improved by separating the Results and Discussion sections. As it currently reads, this section is quite long, and it contains a mix of reporting of results, reminders of methodology, and discussion and links to literature. I would strongly recommend separating out the Results and Discussion sections and focusing the Discussion primarily on the rate of palsa degradation compared to the literature, the implications for organic carbon stability and greenhouse gas emissions, and the scaling up of the results. A stronger Discussion would better justify the suitability of this manuscript in a multi-disciplinary journal like The Cryosphere.
The EMI results are interesting, but they currently feel a bit out of place with the rest of the study that is more focused on UAS-based rates of change. As above, I would suggest separating out the Results and Discussion and focusing in on the hydrological changes that were identified between stable, degrading, and degraded areas from the EMI surveys and what this means for the permafrost carbon feedback and greenhouse gas emissions. Currently, the manuscript has a separate section for the EMI results that discusses an increase in open water and soil moisture, but it would be more effective if the EMI results were properly integrated with other elements of the study, such as elaborating on what increases in ponds and soil moisture would mean for carbon stocks, emissions, etc.
I find it a bit difficult to follow the flow of the study, as it is currently written. I think it would be worth considering re-structuring the Methods section to first describe the data processing for the 2019-2021 model, then the EMI work, then the data processing for the 2014-2022 dataset. This would help to highlight the novel data collection/work (2021 UAS flight, EMI survey, etc.) within the context of a longer study period (2014-2022).
Overall, the figures are nice and the authors have taken care to ensure that the colour schemes are accessible. The text formatting in some of the tables may need to be reviewed, as there are some terms that are capitalized and others that are not.
SPECIFIC COMMENTS
INTRODUCTION
P2 L53, Remove “excess” from this sentence.
P2 L57, There is a new paper that has just come out on the use of the term “abrupt thaw” by Webb et al. 2025 that can be used to replace Turetsky et al. 2020.
P2-3 L53-78, This background information on thermokarst landform types and development is interesting, but it takes away from the purpose of the study itself, which is to quantify palsa degradation in the Stordalen mire using UAS surveys. The Introduction could be improved by introducing palsas as peatland permafrost landforms, discussing the importance of peatland permafrost landscapes for permafrost carbon feedbacks, and then diving into the benefits of UAS imagery over satellite imagery and aerial photographs.
P3 L90-92, I agree that reported rates of degradation are extremely variable, but it would be helpful to highlight to the reader what area or approximate time period these studies are from. In P3 L95-97, the authors state that there is accelerated degradation in more recent years, but it is difficult to understand this relative to the previous statement that does not provide a time reference/study period.
P3 L93-94, Wang et al. 2024, Verdonen et al. 2023, Zuidhoff and Kolstrup 2000, Thie 1974, Payette et al. 2004 are some other studies that also present lateral palsa degradation rates. These may be helpful for further contextualizing the results of this study on P15, L359-363.
P4 L98, What is meant here by “revisit”? This is the first mention of the Stordalen mire, and the authors do not provide examples of previous studies of degradation at the Stordalen mire, other than to say that 55% of Sweden’s largest palsa peatlands are currently subsiding in the previous paragraph. Please clarify.
P4 L103-105, Please provide what years the EMI surveys were conducted.
METHODS
P4 L109, Section 2.1 is lacking information on the climatic conditions over the study period, from 2019-2021 for the primary part of the study, and from 2014 to 2022 for the additional UAS data that was used. This would be critical for contextualizing palsa degradation.
P4 L113, Given that the study is primarily conducted from 2019 to 2021, or even from 2014 to 2022, is there a more recent value for MAAT since 2006? Please update.
P4 L120, Zuidhoff and Kolstrup 2005 and Railton and Sparling 1973 also discuss vegetation associated with different palsa stages.
P4 L123, Is there any available information, either from this study or from previous studies, on the height of the palsas and the thickness of the permafrost at this site? It is helpful to know that the active layer thickness varies from 50 cm in stable areas to >200 cm in degraded areas, but is it possible that a talik has formed and that there is still permafrost present at depth?
P5 L134, The authors state here that the field campaign took place between September 14 and October 10, 2021, but that the UAS flight took place on September 17, 2021. What else occurred during this time period? When were the EMI surveys conducted?
P5 L137, Thanks for providing the forward overlap. What was the side overlap?
P6 L134, Please specify that this is RGB imagery collected from UAS. While this is clear when looking at Table A 1, this should be included in the main text as well.
P6 L161, I think it would be best to present this information in paragraph form and to explain each of the steps and what datasets were used in each step. For example, stating that the slopes were extracted from DSMs where applicable is quite vague, and the reader is likely unsure of what is and what isn’t applicable. Is this trying to convey that slopes were extracted from DSMs for 2019 and 2021, but not for the other years? And how was the area of interest extracted? Was it clipped?
P7 Figure 2, Remove the extra “t” in “literature” in the caption for panel a. The grey and yellow bounding boxes are very similar in colour and are a bit difficult to differentiate.
P7 L183-184, Are there any historical aerial photographs or satellite images that can help to confirm that permafrost was not present in these locations for several decades?
P7 L189, Are there any locations at all where permafrost aggradation and palsa expansion occurred? Having a section that describes climatic conditions from 2014 to 2022 as suggested above would be helpful for this.
P7-8 L190-222, As with P6, I think that it would be best to present much of this information in paragraph form. This could be supported by a figure or table that explains the process more visually and that possibly integrates information from Table A2 and Figure A3.
P11 L257, Hypotheses are usually presented in the Introduction, not the Methods section. Please move this up to the Introduction and provide more information on how the authors expect the electrical properties of the soil to vary along the degradation gradient. Should the EC be higher or lower according to the factors presented (soil texture, clay content, water content, salinity, organic matter type, organic matter proportion, soil structure, soil density, soil temperature, and most importantly, permafrost presence/absence!). Instead, in this section, please focus on describing how the EMI surveys were positioned, how long they were, etc. It is helpful to know that there were 1083 points, but the reader is not informed of how far the points are from each other, whether they are all along the same line, etc.
P11 L271, Is there a reference or any more information available for this custom-made acquisition program?
RESULTS AND DISCUSSION
P13 Figure 4, This figure is very effective, particularly panel b! I would recommend changing the light blue colour of the “degraded areas” in panel a to another colour, as this looks like water at first glance.
P14 L344-355, This is the first instance where the reader can really come to understand the authors’ “revisit” of palsa degradation rates in the Stordalen mire. These past studies should be first presented in Section 2.1 so that the reader is able to keep this information in mind as they read through the results of this study.
P15 L379, The section entitled “Palsa degradation means higher levels of humidity” does not really discuss humidity levels at all. It may be more appropriate to instead name the section something like “Palsa degradation leads to increases in soil moisture and open water”.
P16 Figure 5, I understand that data could not be collected in 2020, so the corresponding bar has a dashed outline. But if data could not be collected in 2020, how is there a bar and a value associated with this year at all?
P15 L385-386, The authors state here and show in Figure 2 that the processing extents for the 2014-2022 comparison and the 2019-2021 comparison are not the same. In addition to the work that has been done, is it possible to clip the results of the 2019-2021 comparison to the 2014-2022 comparison extent, so that the authors can additionally present results that are directly comparable?
CONCLUSION
P20 L481-485, This is a helpful summary of findings that integrates the EMI and 2014-2022 work well.