Tracking Frozen Debris Lobe ground deformation in the Brooks Range (Alaska) using Sentinel-2 optical image time series validated by long-term GPS

Frozen Debris Lobes (FDLs) are slow-moving, permafrost-related landslides affecting hillslopes in the Brooks Range (Alaska). Their recent acceleration, driven by climate warming, is increasingly relevant for the long-term management of key Arctic infrastructure, notably the Dalton Highway—the only road access to the North Slope oil fields—and the trans-Alaska pipeline. FDL activity already has required operational responses within this corridor, including the realignment of the Dalton Highway in 2018 to provide more distance from FDL-A. While detailed field monitoring (RTK-GPS surveys and in situ instrumentation) has been conducted on a limited number of lobes since 2012, corridor-scale assessment of FDL dynamics increasingly relies on remote sensing; however, optical observations in the Arctic are hindered by frequent cloud cover, and validation datasets remain scarce at high latitudes. Here we quantify ground-surface displacements for nine FDLs using Ground Deformation Monitoring with OPTical image time series (GDM-OPT-SLIDE; DATA-TERRA/FormaTerre), an automated processing chain that extracts horizontal surface displacements from Sentinel-2 image time series. We validate satellite-derived displacement rates using a unique GPS dataset collected since 2012. The resulting rates agree with ground observations (R² = 0.71) and reveal a marked acceleration in 2020 followed by a slowdown from 2022 onwards. Because FDL surfaces exhibit heterogeneous land cover (trees, shrubs, and bare soil), we assess land-cover effects using high-resolution LiDAR data. Results indicate that agreement with ground observations improves under dense forest cover. Overall, this study highlights the potential of optical satellite monitoring to track periglacial slope dynamics in warming Arctic permafrost terrain, enabling systematic regional mapping of landform displacement and supporting investigation of climatic controls at the regional scale.