Effects of Soil Moisture on the Responses of Soil Temperatures to Climate Change in Cold Regions

At high latitudes, changes in soil moisture could alter soil temperatures independently of air temperature changes by interacting with the snow thermal rectifier. We investigated this mechanism with experiments in the Community Land Model 4 (CLM4) with prescribed atmospheric forcing and vegetation state. Under equilibrium historical conditions, increasing CO₂ concentrations experienced by plants from 285 ppm to 857 ppm caused widespread increases in soil water-filled pore space of 0.1-0.2. In permafrost gridcells that experienced this moistening, vertical- and annual- mean soil temperatures increased by 1-2°C. A similar pattern of moistening and consequent warming occurred in simulations with prescribed JJAS rainfall increases of 25% over historical values, a level of increase commensurate with projected rainfall increases. There was a strong sensitivity of the moistening responses to baseline hydrological state. Experiments with perturbed physics confirmed that the simulated warming in permafrost soils was caused by increases in the soil latent heat of fusion per unit volume and in the soil thermal conductivity due to the increased moisture. In transient RCP8.5 scenario experiments, soil warming due to increased CO₂ or JJAS rainfall was smaller in magnitude and spatial extent than in the equilibrium experiments. Active-layer deepening associated with soil moisture changes occurred over less than 8% of the current permafrost area, because increased heat of fusion and soil thermal conductivity had compensating effects on active-layer depth.