Understanding slow glacier flow under climate change: A case study on Vernagtferner, Austria

Abstract

Long-term surface velocity observations of glaciers reflect the dynamics of glacier ice and its interaction with the mass balance, including variations due to climate change. In this study, we investigate the surface velocities of a slow-flowing glacier which is influenced by strong surface melt and negative mass balance during the last decades. The annual stake measurements date back to 1966 and allow the study of ice dynamics for more than five decades. We observed a strong relationship between the surface velocity and ice thickness, especially in the case of the glacier0s response to thinning. A series of slightly positive mass balances led to a minor glacier advance around 1980, associated with a considerable speed-up of the glacier. With the onset of the negative mass balances, the velocity has decreased steadily until today. Our results suggest that the displacement of ablation stakes response relatively direct to changes in surface mass balance. The surface mass balance is currently the dominant control for geometry change, while local dynamic contributions to elevation change remain secondary. Based on recent in situ measurements, a seasonal variation of surface velocities can be identified, with around 30 % higher summer velocities in relation to the annual average. In order to investigate the current ice surface flow, we aimed to derive a spatially distributed velocity map. The combination of low ice flow and high ablation challenges the use of standard remote sensing techniques, as ablation induced changes overlay the displacement of surface features. We therefore relied on manual feature tracking based on unpiloted aerial vehicle (UAV) surveys, and airborne imagery, and combined the results with stake measurements to generate a dataset for the period 2018–2023. With an average velocity of 1 m yr−1 and a maximum displacement rate of 4 m yr−1 in the central part of the glacier, it gives a clear picture of the low present-day glacier flow.