Future permafrost degradation under climate change in a headwater catchment of central Siberia: quantitative assessment with a mechanistic modelling approach

Abstract. Permafrost thawing as a result of climate change has major consequences locally and globally, both for the biosphere and for human activities. The quantification of its extent and dynamics under different climate scenarios is needed to design local adaptation and mitigation measures and to better understand permafrost climate feedbacks. To this end, numerical simulation can be used to explore the response of soil thermal and hydrological regimes to changes in climatic conditions. Mechanistic approaches minimise modelling assumptions by relying on the numerical resolution of continuum mechanics equations, but they involve significant computational effort. In this work, the permaFoam solver is used, along with high-performance computing resources, to assess the impact of four climate scenarios of the Coupled Model Intercomparison Project Phase 6 (CMIP6) on permafrost dynamics within a pristine, forest-dominated watershed in the continuous-permafrost zone. Using these century-timescale simulations, changes in the soil temperature, soil moisture, active layer thickness, and water fluxes are quantified, assuming no change in the vegetation cover. The most severe scenario (SSP5-8.5) suggests a dramatic increase in both the active layer thickness and annual evapotranspiration, with the maximum values on the watershed increasing by, respectively, +65 % and +35 % by 2100 compared to current conditions. For the active layer thickness, a variable that integrates both the thermal and hydrological states of the near-surface permafrost, this projected increase would correspond to a ∼350 km southward shift in current climatic conditions. Moreover, in this scenario, the thermal equilibrium of near-surface permafrost with the new climatic conditions would not be reached in 2100, suggesting a further thawing of permafrost even in the case in which climate change is halted.