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 long study on the Tanana Flats near Fairbanks, Alaska to measure permafrost thaw and associated ground surface subsidence via field-based and remote-sensing techniques. Our study focused on four transects that included 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 between 1999 and 2020. Widespread near-surface permafrost degradation was evident between 2004 and 2020 with top-down thaw of near surface permafrost doubling from 18 % to 36 % over the study period. Multi-year frost and repeat thin permafrost, two types of permafrost aggradation, were almost completely absent by 2020. Second, we calculated rates of top-down versus lateral thaw using airborne lidar measurements collected in 2014 and 2020. Lateral thaw of tabular shaped permafrost boundaries and development of unfrozen zones between the bottom of the seasonally frozen layer and the top of near-surface permafrost (taliks) were evident. Third, repeated electrical resistivity tomography measurements in 2012 and 2020 supported surface-based thaw observations and allowed subsurface mapping of permafrost morphologies up to 20 m deep. The study identified strengths and limitations of the three methods we used to quantify permafrost thaw degradation. Future applications of these methods should apply geospatial analyses to identify variables relating surface and subsurface conditions to project finer scale field-based spatial assessments across broader regions.