Combining ground-penetrating RaDAR, unmanned aerial vehicle photogrammetry and borehole thermal data to investigate the evolution of hanging glaciers in the western European Alps

The evolution of hanging glaciers in a context of changing climate has significant implications because their stability is particularly sensitive to changes in their basal thermal regimes. Projections indicate that by the end of this century, all glaciers below 4000 m altitude in the European Alps will likely transition from a cold-based to a temperate-based state due to climate forcing. Unstable hanging glaciers already threaten villages, transport routes and ski infrastructure in the Alps. Given the high density of settlements, infrastructure and access for recreation, the evolution of hanging glaciers must be well understood. However, modelling the thermal regimes of hanging glaciers is often difficult because of their complex geometries, and the difficulties associated with data acquisition. Our study utilised ground-based ground-penetrating radar (GPR) techniques in a novel application to investigate the bedrock geometries of four hanging glaciers at two sites at the Pointes du Mourti (3563 m a.s.l.), Pennine Alps, Switzerland, and the Aiguille du Midi (3842 m a.s.l.), Mont-Blanc Massif, France. By combining a GPR survey with two years of thermal data recorded in two boreholes, and two annually spaced UAV photogrammetric surveys, we investigated the geometry and the current evolution of the Pointes du Mourti hanging glacier. We were able to extract subglacial bedrock geometry and ice depths covering an area of 9791 m2, representing 16.2% of the glacier area in 2022. We demonstrated that this hanging glacier, with a mean inclination angle of 43°, resides in a concave feature on the mountain slope with a rugged subglacial surface. Basal temperatures in the upper part of the hanging glacier are below -2.5 °C, indicating cold-based conditions, whereas the central part may be in a more temperate-based condition. Furthermore, the surface topography of all the investigated glaciers underwent substantial changes during the unusually hot summer of 2022, which followed the very dry winter of 2021/2022, exceeding historical norms. The Pointes du Mourti hanging glacier lost 0.86 m of thickness on average and more than 7% of its surface area between October 2021 and September 2022. Similarly, the Jumeau Ouest hanging glacier, at the Aiguille du Midi, lost 0.92 m between June and September 2022. Our study shows that ground-based GPR can be successfully employed in challenging topographical environments to determine sub-glacial geometries of hanging glaciers. This method can be a valuable tool in a multi-faceted approach to hanging glacier investigations, effectively providing necessary ice depths and bedrock configurations. This research contributes valuable insights for both scientific and administrative communities invested in comprehending the consequences associated with the evolution of hanging glaciers.