Evolution of the seasonal dynamics of the lake-terminating glacier Fjallsjökull, southeast Iceland, inferred using high-resolution repeat UAV imagery

Proglacial lakes are becoming ubiquitous at the termini of many glaciers worldwide, leading to increased glacier mass loss and terminus retreat, yet an understanding of the key processes forcing their behaviour is lacking. This study utilised high-resolution repeat uncrewed aerial vehicle (UAV)-Structure from Motion (SfM) imagery to provide insights into the changing dynamics of Fjallsjökull, a large lake-terminating glacier in southeast Iceland, across the 2019 and 2021 summer melt seasons. We show that the overall dynamics of the glacier are controlled by the ~120 m deep bedrock channel under the study region, which has caused the glacier to flow faster as it enters deeper water, leading to increased ice acceleration, thinning and retreat, with the glacier decoupled from local climate as a result. The close correspondence between ice velocity and surface thinning suggests the implementation of the dynamic thinning feedback mechanism, with such a response likely to continue in future until the glacier recedes out of the bedrock channel into shallower water. As a result, these findings clearly indicate the complex nature of the calving process, highlighting the need for continued monitoring of lake-terminating glaciers in order to better understand and predict how they may respond in future.