Thermal regime and geometry of a hanging glacier and its interaction with permafrost: the Pointes du Mourti, Swiss Alps

Abstract. Hanging glaciers are diagnostic features of high-mountain permafrost, but their response to climate forcing and links with surrounding permafrost remain poorly understood. We investigated the hanging glacier at Pointes du Mourti (3653 m a.s.l., Swiss Alps; mean annual air temperature = -4.6 °C) between 2020 and 2025, a period that covers Switzerland’s four warmest years on record. Geometric changes were quantified using uncrewed aerial vehicle photogrammetry and ice-based ground-penetrating radar, while permafrost conditions were assessed using rock surface temperatures, thermal modelling (CryoGrid2), and electrical resistivity tomography. Between 2021 and 2024, the ice thinned by up to 7 ± 0.12 m (20–45 %), with peak losses in 2022 (6.8 ± 0.5 % volume loss). Accelerated melt exposed pre-existing crevasses, enabling more efficient meltwater infiltration, which advanced basal temperature maxima and minima by ~2 months. Despite this, basal temperatures in the upper glacier remain low and continue decreasing due to enhanced winter cooling associated with ice thinning. The lower glacier exhibits temperate conditions with prolonged zero-curtain periods (146–193 days), indicating significant volumes of stored meltwater. The surrounding rock wall permafrost is discontinuous and strongly controlled by topography: only the shaded north-facing slope where the glacier lies maintains negative mean annual rock surface temperature (-3.67 °C), while sun-exposed aspects reach +3.05 °C. Model results indicate active-layer thickening that has doubled since 2015. Our observations suggest a transition toward polythermal conditions driven by rapid ice thinning, meltwater infiltration, and subsequent water storage, increasing the potential for destabilisation of small Alpine hanging glaciers under current climate conditions.