Excess Ground Ice Profiles in Continuous Permafrost Mapped From InSAR Subsidence

Excess ground ice formation and melt drive surface heave and settlement, and are critical components of the water balance in Arctic soils. Despite the importance of excess ice for the geomorphology, hydrology and biogeochemistry of permafrost landscapes, we lack fine-scale estimates of excess ice profiles. Here, we introduce a Bayesian inversion method based on remotely sensed subsidence. It retrieves near-surface excess ice profiles by probing the ice content at increasing depths as the thaw front deepens over summer. Ice profiles estimated from Sentinel-1 interferometric synthetic aperture radar (InSAR) subsidence observations at 80 m resolution were spatially associated with the surficial geology in two Alaskan regions. In most geological units, the estimated profiles were ice poor in the central and, to a lesser extent, the upper active layer. In a warm summer, units with ice-rich permafrost had elevated inferred ice contents at the base of the active layer and the (previous years') upper permafrost. The posterior uncertainty and accuracy varied with depth. In simulations, they were best (less than or similar to 0.1) in the central active layer, deteriorating (greater than or similar to 0.2) toward the surface and permafrost. At two sites in the Brooks Foothills, Alaska, the estimates compared favorably to coring-derived profiles down to 35 cm, while the increase in excess ice below the long-term active layer thickness of 40 cm was only reproduced in a warm year. Pan-Arctic InSAR observations enable novel observational constraints on the susceptibility of permafrost landscapes to terrain instability and on the controls, drivers and consequences of ground ice formation and loss. Permafrost soils can contain substantial quantities of ice. When the ice melts due to warming or disturbance, the ground becomes unstable, threatening infrastructure, water resources and ecosystem services. We lack fine-scale ground ice maps across essentially the entire Arctic, limiting our ability to sustainably plan in the Arctic. Here, we develop the first satellite-based technique for mapping ground ice near the surface, including seasonal ground ice in that part that freezes and thaws every year and also perennial ground ice in soil that had previously remained frozen for long times. The central idea is to measure the subsidence over the summer from satellites and relate this to the ice profile near the surface. In exceptionally warm summers, layers that had previously been frozen for a long time may thaw; where they are ice-rich, elevated but limited subsidence toward the end of this summer can be used to identify areas whose large ice contents render them susceptible to long-term terrain instability. The regional to pan-Arctic maps our novel method can provide promise to bolster sustainable planning and stewardship in the Arctic and to help us understand how these landscapes are changing. Remotely sensed subsidence provides insight into near-surface excess ice profiles The estimated excess ice profiles were largely consistent with in-situ observations and associated with the surficial geology In warm summers, maps of ice contents in the (previous years') upper permafrost constrain the susceptibility to thaw instability