Polythermal conditions in small glaciers in the Swiss Alps

Abstract. Englacial temperature measurements in the Alps remain sparse and biased toward high-elevation accumulation areas, leaving the thermal state of ablation zones poorly constrained. Here, we present borehole thermometry and ground-penetrating radar surveys from six small Swiss glaciers (< 0.5 km², 2700–3800 m a.s.l.). Polythermal conditions are confirmed in three glaciers, with the cold-temperate transition surface at depths of 17–38 m and ice temperatures ranging from temperate to −2.1 °C. A fourth site, Glacier du Sex Rouge, likely retains its historically documented polythermal structure, but borehole measurements were limited to the near-surface cold layer and do not reach the temperate ice below. A consistent spatial pattern emerges, with temperate ice at higher elevations transitioning into fully or partially cold-based glacier termini. Ground-penetrating radar retrievals are generally consistent with borehole-derived thermal conditions, with low scattering in cold ice and enhanced scattering in temperate zones. The observed thermal structures are closely linked to the history of firn cover loss, reconstructed from long-term mass balance records, with sites that lost their firn cover earliest showing the most advanced cooling. Our findings suggest that polythermal conditions among small Alpine glaciers may be more widespread than previously recognised, with important implications for glacial hazard assessments and highlighting the need for systematic regional-scale thermal observations and modelling.