Accelerated lowland thermokarst development revealed by UAS photogrammetric surveys in the Stordalen mire, Abisko, Sweden
(2026) The Cryosphere — Vol. 20, n° 9, p. 5271-5301 (2026)
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Thomas et al. - 2026 - Accelerated lowland thermokarst development revealed by UAS photogrammetric surveys in the Stordalen.pdf
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- Organic matter decomposition in permafrost soils depends on factors such as soil pH, temperature, and re-dox conditions. Over lowland permafrost soils, these conditions are shaped by microtopography, which evolves with physical degradation, i.e., lowland thermokarst development. A dynamic quantification of lowland thermokarst development still poorly constrained-is therefore a critical prerequisite for predictive models of permafrost carbon balance in these areas. Here we provide such a quantifica-tion, updated for the Stordalen mire in 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 orthomo-saics and digital surface models from very high resolution (10 cm) unoccupied aircraft system (UAS) photogramme-try 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 using a support vector machine algorithm and derived palsa loss rates. Our findings confirm that topography is a key variable for monitoring palsa loss, nearly doubling the overall accuracy of the classification, while slope enabled the identification of early-stage degradation. We show a clear acceleration of degradation for the period 2019-2021, with a decrease in palsa area of 3.3-3.6 % a −1 (% reduction per year relative to the initial palsa areal extent) compared to previous estimates of ∼ 0.3 % a −1 (1970-2000) and ∼ 0.1 % 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 3.3-3.6 % a −1 , we estimate accordingly that surface degradation at Stordalen might lead to a pool of 12 × 10 3 kg of organic carbon exposed annually within the topsoil (23 cm depth), of which ∼ 25 % is mineral-interacting organic carbon. These results demonstrate that UAS monitoring can robustly quantify rapid thermokarst development that coarser methods miss, and provide a transferable framework for tracking permafrost degradation rates at other low-land sites.
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Thomas, M., Moenaert, T., Hirst, C., Delhez, B., Radoux, J., du Bois d’Aische, E., Lundin, E., Villani, M., Jonard, F., Lambot, S., Van Oost, K., Vanacker, V., Siewert, M., Mörth, C.-M., Palace, M., Varner, R., Sullivan, F., Herrick, C., & Opfergelt, S. (2026). Accelerated lowland thermokarst development revealed by UAS photogrammetric surveys in the Stordalen mire, Abisko, Sweden. The Cryosphere, 20(9), 5271-5301. https://doi.org/10.5194/tc-20-5271-2026 (Original work published 2026)