Comparing thaw probing, electrical resistivity tomography, and airborne lidar to quantify lateral and vertical thaw in rapidly degrading boreal permafrost

Abstract. Permafrost thaw across Earth's high latitudes is leading to dramatic changes in vegetation and hydrology. We undertook a two-decade study near Fairbanks, Alaska, to measure permafrost thaw and associated ground surface subsidence via field-based and remote-sensing techniques. Our study focused on transects representing an unburned area and three fire scars (1988, 2001, and 2010). Three types of permafrost quantification were used. First, repeat measurements of ground surface elevation and depth to the top of near-surface permafrost were made over an 8-to-21-year period at different sites. Widespread near-surface permafrost degradation occurred between 2004 and 2020 with top-down thaw of near-surface permafrost doubling from 18 % to 36 %. Permafrost aggradation was almost completely absent by 2020. Second, we calculated rates of top-down versus lateral thaw using airborne lidar from 2014 and 2020. Lateral thaw of tabular permafrost bodies and development of unfrozen zones between the bottom of the seasonally frozen layer and the top of near-surface permafrost (taliks) were evident. Third, electrical resistivity tomography (ERT) measurements from 2012 and 2020 supported surface-based thaw observations and allowed subsurface permafrost mapping up to 20 m deep. No single method provides all the information needed to adequately assess permafrost change. For example, frost probing yields insight into top-down thaw, lidar allows identification of vertical and lateral subsidence, and ERT identifies the presence/absence of permafrost at tens of meters depth. Future applications of these methods should focus on relating surface and subsurface variables measured on the ground to information that can be remotely sensed across broad regions.