Permafrost sensitivity to soil hydro-thermodynamics in historical and scenario simulations with the MPI-ESM

Abstract. Global warming is particularly intense in the Arctic, where temperature trends are up to four times higher than those observed globally. Among its multiple consequences, this enhanced warming is especially concerning because it triggers the degradation of permafrost. However, the increasing temperature is not the only factor conditioning permafrost thaw. Changes in water availability induced by disturbances in the permafrost water table considerably affect Arctic soils moisture and ice presence and thermal structure in permafrost regions. Our understanding of the interactions between hydrology and thermodynamics is still limited as well as its representation in the majority of state-of-the-art land surface models (LSMs), mainly in terms of the processes included, LSM depth, spatial resolution, vertical discretization, and hydro-thermodynamic coupling.

This work explores the response of the Max Planck Institute Earth System Model (MPI-ESM) to changes in the soil hydrology and thermodynamics in permafrost-affected regions. An ensemble of fully-coupled historical and climate change scenario simulations was performed under three configurations: the standard model that will be taken as a reference, and two variants that generate rather dry or wet conditions across permafrost areas. Enhanced soil depth and vertical resolution within the LSM, JSBACH, were also incorporated globally to capture fine-scale thermodynamics and allow for deeper heat propagation. Results show that deepening the LSM reduces the intensity of near-surface soil warming by 0.1 °C dec^-1 in high radiative forcing scenarios, reducing permafrost retreat by up to 1.9–3.1 10⁶ km² by the end of the 21st century. However, the greatest influences are produced by the dry and wet configurations, which lead to distinct initial states, historical, and future evolution for permafrost temperatures (offset of 3 °C), active layer thickness (1–2 m) and permafrost extent (5 10⁶ km²). This work underscores the importance of refining hydrological and thermodynamic processes in ESMs to improve projections of permafrost responses under climate change scenarios, with implications for the assessment of risks related to carbon feedbacks and infrastructure vulnerabilities in Arctic regions.