Multi-annual temperature evolution and implications for cave ice development in a sag-type ice cave in the Austrian Alps

Ice caves are, similar to mountain glaciers, threatened by the warming climate. To better understand the response of perennial ice in caves to a changing climate, we analysed the thermal characteristics of a sag-type ice cave in the Austrian Alps (Hundsalm ice cave), based on long-term temperature measurements for the period 2008-2021. Observations show a warming trend in all parts of the cave as well as a distinct seasonal pattern with two main regimes, i.e. an open (winter) and a closed (summer) period. During the closed period, a persistent stable stratification prevails that largely decouples the cave from the external atmosphere. The open period is characterised by unstable to neutral stratification, which is an effect of convection during episodes when cold air can penetrate into the cave. Criteria to detect corresponding periods are investigated. Vertical temperature profiles also provide hints on corresponding circulation patterns and the spatial temperature variability in the cave. The positive air temperature trend is reflected in a decrease in perennial cave ice, derived from stake measurements. Besides surface melting, we find compelling evidence of basal melting of ice. The observed ablation rates can be well reproduced by applying a modified degree-day model, which, however, is less feasible regarding mass balance. Overall, we conclude that Hundsalm ice cave is highly impacted by regional warming, which will lead to the disappearance of its perennial ice deposits within the next decades.