Warm proglacial lake temperatures and thermal undercutting drives rapid retreat of an Arctic glacier

Determining the characteristics of Arctic proglacial lakes is essential for understanding their current and future influence on glacier mass loss, capacity as a carbon sink and the associated impacts for downstream hydrology and ecology. Here we combine satellite and field observations of Kaskasapakte Glacier (KG) (a lake-terminating glacier in Arctic Sweden) to reveal the interplay between lake parameters and glacier mass loss from 2008–2019. We present the first field evidence of warmer than expected water temperatures (>4 °C at the ice front) at a Scandinavian proglacial lake and illustrate how these drove rapid thermo-erosional undercutting and calving at the terminus, with width averaged retreat rates of up to 23 m per melt year and frontal ablation accounting for 30 % of glacier volume loss between 2015 and 2019. Field observations of how proglacial lake properties influence rates of glacier mass loss remain sparse, yet are increasingly critical for the accurate projection of lake-terminating glacier responses to warming air and lake temperatures, particularly in high-latitude Scandinavia under the influence of Arctic amplification.