Equilibrium Responses of Soil Carbon to Climate Change: Empirical and Process-Based Estimates

Abstract

We use a new version of the Terrestrial Ecosystem Model (TEM), which has been parameterized to control for reactive soil organic carbon (SOC) across climatic gradients, to evaluate the sensitivity of SOC to a 1?C warming in both empirical and process-based analyses. In the empirical analyses we use the steady state SOC estimates of TEM to derive SOC-response equations that depend on temperature and volumetric soil moisture, and extrapolate them across the terrestrial biosphere at 0.50 spatial resolution. For contemporary climate and atmospheric C02, mean annual temperature explains 34.8% of the variance in the natural logarithm of TEM-estimated SOC. Because the inclusion of mean annual volumetric soil moisture in the regression explains an additional 19.6%, a soil moisture term in an equation of SOC response should improve estimates. For a 1?C warming, the globally derived empirical model estimates a terrestrial SOC loss of 22.6 1015 g (Pg), with 77.9% of the loss in extra-tropical ecosystems. To explore whether loss estimates of SOC are affected by the spatial scale at which the response equations are derived, we derive equations for each of the eighteen ecosystems considered in this study. The sensitivity of terrestrial SOC estimated by summing the losses predicted by each of the ecosystem empirical models is greater (27.9 Pg per ?C) than that estimated by the global empirical model; the 12.2 Pg loss (43.7%) in tropical ecosystems suggests that they may be more sensitive to warming. The global process-based loss of SOC estimated by TEM in response to a 1?C warming (26.3 Pg) is similar to the sum of the ecosystem empirical losses, but the 13.6 Pg loss (51.7%) in extra-tropical ecosystems suggests that they may be slightly less sensitive to warming. For the modelling of SOC responses, these results suggest that soil moisture is useful to incorporate in empirical models of SOC response and that globally derived empirical models may conceal regional sensitivity of SOC to warming. The analyses in this study suggest that the maximum loss of SOC to the atmosphere per?C warming is less than 2% of the terrestrial soil carbon inventory. Because the NPP response to elevated CO2 has the potential to compensate for this loss, the scenario of warming enhancing soil carbon loss to further enhance warming is unlikely in the absence of land use or changes in vegetation distribution.