Polythermal conditions in small glaciers in the Swiss Alps

Glacier thermal conditions directly influence ice mechanics, meltwater storage, and drainage, thereby governing glacier stability and hazard potential. Polythermal glaciers, in particular, can create conditions that promote ice break-offs, ice avalanches, water pocket outbursts, and even large-scale glacier detachments. Understanding their distribution is therefore critical for hazard assessment. Yet englacial temperature measurements in the Alps remain sparse and are biased toward high-elevation accumulation areas, while the thermal state of mid- and lower-elevation ablation areas is largely unknown. Extended melt seasons and refreezing of meltwater in cold firn have been associated with warming at high elevations, whereas firn loss at lower elevations may reduce meltwater retention and latent heat input. Modeling studies suggest that this imbalance can lead to cooling in ablation areas, an effect that may be particularly pronounced for very small glaciers, where internal heat production from glacier dynamics is minimal.

Here, we present a new englacial temperature dataset from six small Swiss Alpine glaciers (3000–3800 m a.s.l.), directly addressing the lack of observations in mid- to lower-elevation ablation areas that are poorly constrained by existing measurements. The dataset combines borehole thermometry with ground-penetrating radar surveys. Polythermal conditions were identified in three glaciers, with the cold–temperate transition surface (CTS) occurring at depths of 14–25 m. Below the seasonal surface layer, ice temperatures generally ranged from temperate conditions to –2.1 °C. Two of the glaciers exhibit a recurring pattern in which temperate ice at higher elevations transitions downslope into fully or partially frozen glacier tongues. At the third polythermal site, the CTS was detected at several locations between 17 and 22 m depth, while basal thermal conditions remain partly unresolved. One glacier appears predominantly cold, and at two additional sites, shallow thermistors recorded year-round cold conditions within the seasonal layer, while temperate ice at depth cannot be ruled out. Ground-penetrating radar reflectivity is generally consistent with borehole-derived thermal conditions, characterized by low reflectivity in cold ice and enhanced reflectivity in temperate zones. Our findings suggest that polythermal-type glaciers in the European Alps may be more widespread than previously recognized, with important implications for glacial hazard assessment and for understanding climate-driven changes in smaller Alpine glaciers.