Transient snow line altitudes of glaciers in the European Alps from multi-mission remote sensing data (2000–2025)

Abstract. Observations of glacier snow line altitude (SLA) provide important information for estimating glacier surface mass balance and glaciological modelling. Optical satellite remote sensing enables the repeated measurement of SLA at glacier-specific to regional scales, yet producing continuous multi-temporal SLA time series requires the processing and analysis of large volumes of medium-resolution imagery. In addition, traditional image brightness thresholding may suffer from temporal changes in illumination, variations in reflectivity from evolving snow and ice surface properties, and cross-sensor radiometric differences. Here, we present a fully automated cloud-processing workflow for SLA retrieval based on optical image segmentation and adaptive, scene-specific thresholding, to map the snow-ice boundary on glaciers under various acquisition conditions and based on multi-mission data. Our method uses atmospherically corrected reflectance data from the Landsat-5 to -9 and Sentinel-2 satellite missions and accounts for cloud cover, non-glacier pixels, terrain shadows and temporal glacier surface elevation change. Our SLA dataset comprises a total of ∼200,000 observations across 408 glaciers in the European Alps between 2000 and 2025. In addition, we provide estimates of end-of-summer SLAs and multi-annual change trends for most glaciers and years. Validation against visually delineated SLAs at selected reference glaciers demonstrates high agreement, with a root mean square error of ∼100 m vertical deviation. Applying the workflow to glaciers in the Alps reveals a mean regional SLA rise of 145 m since 2000 (+6 m year⁻¹), characterized by pronounced interannual variability. Particularly during the last decade, seasonal SLA change can be tracked due to the short revisit times of the current Landsat-8, -9 and Sentinel-2 earth observation missions. All data are publicly available at Zenodo (https://doi.org/10.5281/zenodo.18223929; Sommer et al. (2026)).