Spatial heterogeneity in post-fire permafrost evolution as revealed by satellite radar observations

Abstract. Wildfires, which have profound impacts on permafrost environments, are expected to increase in frequency and intensity as a result of climate change. This study aims to enhance our understanding of how wildfires affect permafrost by investigating multiple Arctic tundra regions using a space-for-time approach, examining fire events up to approximately 90 years post-occurrence. We employed Synthetic Aperture Radar (SAR) data acquired in L- and C-band to evaluate thaw season deformation rates, associated with soil moisture, as well as annual deformation rates through interferometric (InSAR) retrieval, alongside an analysis of C-band backscatter values. InSAR data offers insights into permafrost degradation, active layer dynamics and soil moisture, while backscatter measurements provide valuable information on land surface roughness, vegetation structure, including surface soil moisture content. Through this approach, we identified increased thaw season subsidence rates, with regional differences in the response to fire. However, when aggregated across all study regions, spanning all permafrost zones, this increased thaw season subsidence persisted for about 50 years post-fire. Elevated annual subsidence rates, on the other hand, were detectable on average only for about 10 years across all regions. However, thaw season deformations were observed to require somewhat longer times to adjust to surrounding values in colder regions. While all regions show similar deformation trend directions, with initially higher subsidence values, backscatter results varied depending on region and ground temperature. We found increased summer backscatter in fire scars within warmer regions likely due to higher soil moisture, while fire scars in colder regions tended to exhibit lower backscatter compared to their unburned surroundings. The study region with average positive annual ground temperatures, where permafrost is still present below 2 m, showed an initial increase in summer backscatter values of about 1 dB, whereas areas < -2 °C featured partially reduced initial backscatter values of, on average, 0.5 dB or less. Examining multiple regions across permafrost gradients revealed the region-specific nature of fire scar characteristics, emphasizing the need to account for a variety of factors when assessing fire scar recovery.