Losing the accumulation zone: Exploring albedo and ablation in the summit region of Gepatschferner, Austria

Glacier albedo is a key driver of glacier energy and mass balance. In recent years, multi-annual firn and summertime snow cover has decreased on Alpine glaciers, exposing larger areas of ice at increasingly high elevations. This reduces glacier albedo and contributes to feedback mechanisms that lead to increased melt. To understand and better predict mass loss in former accumulation areas under conditions of rapid glacier recession, it is important to constrain the possible range of ice albedo that occurs in these newly snow and firn free regions, the duration of ice exposure, and the correlation and causal connection of these factors with ablation at point and glacier scales. Using a unique dataset from an on-ice weather station (3492 m.a.s.l.), ablation stakes, and remote sensing derived albedo, we provide a quantitative overview of albedo and ablation in the summit region of Weißseespitze, the high-point of Gepatschferner (Austria) from 2018 to 2024. We contextualize the observational data with modeling experiments quantifying the sensitivity of surface mass balance to the observed albedo. In the continuous time series of in situ albedo, the seasonal minimum is reached between late July and early September. From 2018 to 2021, minimum albedo values were about 0.30. In 2022, 2023, and 2024 the minima were considerably lower at 0.16-0.17. Prior to 2022, albedo dropped below 0.4 on 3 to 8 days per year. In 2022, 37 days of low albedo conditions (<0.4) were recorded. Ice ablation at the stakes generally increased with increased duration of ice exposure and ranged from zero ablation in years with mostly continuous summer snow cover (e.g. 2020) to more than -1.5 m w.e. in high-melt years like 2022 and 2024. Sentinel-2 derived albedo captures the range and variability of albedo measured in situ well and shows that ice albedo near the summit of Weißseespitze dropped to values similar to those of the surrounding rock in 2022. For average July-September conditions, an albedo decrease from 0.4 to 0.15 results in 10-15 mm w.e. of additional modeled surface melt per day. The impact of ice exposure on melt varies seasonally, with highest sensitivities early in the season. A five day period of very low albedo conditions (<0.2) results in 26% more modeled surface melt if it occurs in late July compared to early September. The albedo decrease at the AWS since 2022 may be related to the exposure and melting of impurity rich firn and ice layers and the accumulation of impurities at the surface, increased presence of meltwater, and the state of the weathering crust. Our extensive dataset sheds light on upcoming changes to be expected at the highest elevations of alpine glaciers in many regions worldwide and provides a starting point for further studies aimed at linking cause and effect of ice albedo variability across scales.