Permafrost thermal response to improved soil hydro-thermodynamics in historical and scenario simulations with a modified version of the MPI-ESM

Soil warming is particularly sensitive in Arctic regions, underlain by permafrost. Permafrost degradation with warming enhances the release of substantial amounts of carbon into the atmosphere, which acts as a positive radiative feedback. However, the increasing temperature is not the only factor affecting permafrost degradation. Water availability changes affecting the Arctic, induced by changes in the atmospheric general circulation considerably affect the soil moisture and ice presence and subsequently thermal structure in permafrost regions. The interaction between soil hydrology and thermodynamics is still poorly represented by most of the CMIP6 land surface models (LSMs), mainly in terms of the soil depth, vertical resolution, and coupling between hydrology and thermodynamics.

This work explores the response of the Max Planck Institute Earth System Model (MPI-ESM) in historical and scenario simulations to changes in the hydrological and thermodynamic features of its LSM, JSBACH, in permafrost-affected regions. An ensemble of experiments was performed with varying soil depth and vertical resolution under two configurations of the hydro-thermodynamical coupling, which generate comparatively drier or wetter conditions over permafrost areas. Results show that deepening JSBACH reduces the intensity of near-surface warming, reducing the deep permafrost degradation area by ca. 2 million km² and constraining the active layer thickness deepening by the end of the 21^st century in high radiative forcing scenarios. Nevertheless, the largest impacts on permafrost extent and active layer thickness are produced by the dry and wet settings, which yield diverging soil moisture and warming conditions during the 21st century. These two configurations show differences in near-surface and deep permafrost extent of up to 5 million km² by the end of the 21^st century.