Atmospheric inversion of the surface CO₂ flux with ¹³CO₂ constraint

Abstract. Observations of ¹³CO₂ at 73 sites compiled in the GLOBALVIEW database are used for an additional constraint in a global atmospheric inversion of the surface CO₂ flux using CO₂ observations at 210 sites for the 2002–2004 period for 39 land regions and 11 ocean regions. This constraint is implemented using the ¹³CO₂/CO₂ flux ratio modeled with a terrestrial ecosystem model and an ocean model. These models simulate ¹³CO₂ discrimination rates of terrestrial photosynthesis and respiration and ocean-atmosphere diffusion processes. In both models, the ¹³CO₂ disequilibrium between fluxes to and from the atmosphere is considered due to the historical change in atmospheric ¹³CO₂ concentration. For the 2002–2004 period, the ¹³CO₂ constraint on the inversion increases the total land carbon sink from 3.40 to 3.70 Pg C yr⁻¹ and decreases the total oceanic carbon sink from 1.48 to 1.12 Pg C yr⁻¹. The largest changes occur in tropical areas: a considerable decrease in the carbon source in the Amazon forest, and this decrease is mostly compensated by increases in the ocean region immediately west of the Amazon and the southeast Asian land region. Our further investigation through different treatments of the ¹³CO₂/CO₂ flux ratio used in the inversion suggests that variable spatial distributions of the ¹³CO₂ isotopic discrimination rate simulated by the models over land and ocean have considerable impacts on the spatial distribution of the inverted CO₂ flux over land and the inversion results are not sensitive to errors in the estimated disequilibria over land and ocean.