Modelling the role of fires in the terrestrial carbon balance by incorporating SPITFIRE into the global vegetation model ORCHIDEE – Part 2: Carbon emissions and the role of fires in the global carbon balance

Abstract. Carbon dioxide emissions from wild and anthropogenic fires return the carbon absorbed by plants to the atmosphere, and decrease the sequestration of carbon by land ecosystems. Future climate warming will likely increase the frequency of fire-triggering drought, so that the future terrestrial carbon uptake will depend on how fires respond to altered climate variation. In this study, we modelled the role of fires in the global terrestrial carbon balance for 1901–2012, using the ORCHIDEE global vegetation model equipped with the SPITFIRE model. We conducted two simulations with and without the fire module being activated, using a static land cover. The simulated global fire carbon emissions for 1997–2009 are 2.1 Pg C yr⁻¹, which is close to the 2.0 Pg C yr⁻¹ as estimated by GFED3.1. The simulated land carbon uptake after accounting for emissions for 2003–2012 is 3.1 Pg C yr⁻¹, which is within the uncertainty of the residual carbon sink estimation (2.8 ± 0.8 Pg C yr⁻¹). Fires are found to reduce the terrestrial carbon uptake by 0.32 Pg C yr⁻¹ over 1901–2012, or 20% of the total carbon sink in a world without fire. The fire-induced land sink reduction (SR_fire) is significantly correlated with climate variability, with larger sink reduction occurring in warm and dry years, in particular during El Niño events. Our results suggest a "fire respiration partial compensation". During the 10 lowest SR_fire years (SR_fire = 0.17 Pg C yr⁻¹), fires mainly compensate for the heterotrophic respiration that would occur in a world without fire. By contrast, during the 10 highest SR_fire fire years (SR_fire = 0.49 Pg C yr⁻¹), fire emissions far exceed their respiration partial compensation and create a larger reduction in terrestrial carbon uptake. Our findings have important implications for the future role of fires in the terrestrial carbon balance, because the capacity of terrestrial ecosystems to sequester carbon will be diminished by future climate change characterized by increased frequency of droughts and extreme El Niño events.