Accelerated lowland thermokarst development revealed by UAS photogrammetric surveys in the Stordalen mire, Abisko, Sweden

Organic matter decomposition in permafrost soils depends on factors such as soil pH, temperature, and redox 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 quantification, 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 orthomosaics and digital surface 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 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 lowland sites.