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

Abstract. The limited representation of soil processes in Land Surface Models (LSMs) contributes to the uncertainty in current state and future projections of permafrost change. In particular, insufficient LSM depths, coarse vertical discretizations, and the omission of hydro-thermodynamic coupling can strongly affect subsurface temperatures, active layer thickness (ALT), and permafrost extent. This work aims to gain knowledge on permafrost sensitivity to changes in the soil hydrology and thermodynamics in permafrost-affected regions. We explore the response of the Max Planck Institute Earth System Model (MPI-ESM) to historical and climate change scenario forcing using an ensemble of fully-coupled simulations under three configurations of permafrost hydrology: 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. Deepening the LSM reduces near-surface soil warming by about 0.1 °C per decade 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). These results indicate that inter-model spread in permafrost responses to climate change can be partly explained by differences in the representation of soil physics. Our findings underscore the importance of refining LSM hydrological and thermodynamic processes in ESMs, with implications for the assessment of risks related to carbon feedbacks and infrastructure vulnerabilities in Arctic regions.