Investigating hanging glaciers and permafrost conditions on an Alpine summit: insights from the Pointes du Mourti, 3563 m a.s.l., Valais Alps, Switzerland
Résumé
Mean annual temperatures in Switzerland have risen by 2.9 °C since the preindustrial reference period (1871–1900), with pronounced impacts on alpine permafrost in the European Alps. Even relative to this long-term trend, the 2021–2024 period was exceptional, including the three warmest years since records began in 1864. Climate warming presents a growing threat to the stability of hanging glaciers (HGs) - which are considered diagnostic features of alpine permafrost - due to their sensitivity to changes in basal thermal regime. Current projections indicate a shift from cold to temperate-based thermal states in all glaciers below 4000 m a.s.l. in the Alps by the end of the century. We present a comprehensive investigation of a HG and permafrost conditions at the summit of the Pointes du Mourti (3653 m a.s.l.), Pennine Alps, Switzerland (mean annual air temperature 2021–2023: – 4.5 °C). Over a three-year period, we applied a multi-method approach combining rock surface temperature (RST) measurements, CryoGRID2 thermal modelling, electrical resistivity tomography (ERT), uncrewed aerial vehicle photogrammetry, and ice borehole temperature monitoring. We also conducted the first known application of ice-based groundpenetrating radar on a HG to resolve subglacial geometry. Despite Alpine Permafrost Index Map based predictions of widespread permafrost (Boeckli et al., 2012), thermal modelling and ERT indicate discontinuous conditions: permafrost is absent along the NW–SE ridge but persists on the N face and SW–NE ridge, where it is better geometrically isolated from lateral heat flux. Active layer thickness has approximately doubled since 2015 at the locations where permafrost was modelled. ERT reveals unfrozen zones at 10–20 m depth influenced by adjacent warm slopes. Maximum HG ice thicknesses are < 29 m, with a 2.91 m mean ice loss and 11.8 % volume loss between 2021–2024. Numerous crevasses have opened; maximum ice losses exceed 6 m. The 2022 heatwaves alone drove a 6.8 % volume loss. While borehole data confirm cold basal ice in the upper HG, the data show meltwater percolation in the lower HG, which may indicate a transition from cold to temperate basal-ice conditions, increasing the risk of mechanical instability and potential collapse. Our findings suggest a strong coupling between permafrost degradation and HG instability. Whether this HG collapses or simply ablates to extinction under continued climate warming remains a critical open question.