Detecting and 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 glacier's 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. 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 analyze the potential and limitations of remote sensing for slow-flowing glaciers. Standard remote sensing techniques did not provide reliable results due to the combination of low ice flow and high ablation, and the associated difficulty in establishing coherence and identifying stable features in the remote sensing products. Instead, manual feature tracking based on a combination of stake measurements and the investigation of unpiloted aerial vehicle (UAV) surveys, and airborne imagery was used to generate a reference 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.