A joint global carbon inversion system using both CO₂ and ¹³CO₂ atmospheric concentration data

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 (62 collocated with ¹³CO₂ sites) for the 2002–2004 period for 39 land regions and 11 ocean regions. This constraint is implemented using prior CO₂ fluxes estimated with a terrestrial ecosystem model and an ocean model. These models simulate ¹³CO₂ discrimination rates of terrestrial photosynthesis 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. This joint inversion system using both¹³CO₂ and CO₂ observations is effectively a double deconvolution system with consideration of the spatial variations of isotopic discrimination and disequilibrium. Compared to the CO₂-only inversion, this ¹³CO₂ constraint on the inversion considerably reduces the total land carbon sink from 3.40 ± 0.84 to 2.53 ± 0.93 Pg C year⁻¹ but increases the total oceanic carbon sink from 1.48 ± 0.40 to 2.36 ± 0.49 Pg C year⁻¹. This constraint also changes the spatial distribution of the carbon sink. The largest sink increase occurs in the Amazon, while the largest source increases are in southern Africa, and Asia, where CO₂ data are sparse. Through a case study, in which the spatial distribution of the annual ¹³CO₂ discrimination rate over land is ignored by treating it as a constant at the global average of −14. 1 ‰, the spatial distribution of the inverted CO₂ flux over land was found to be significantly modified (up to 15 % for some regions). The uncertainties in our disequilibrium flux estimation are 8.0 and 12.7 Pg C year⁻¹ ‰ for land and ocean, respectively. These uncertainties induced the unpredictability of 0.47 and 0.54 Pg C year⁻¹ in the inverted CO₂ fluxes for land and ocean, respectively. Our joint inversion system is therefore useful for improving the partitioning between ocean and land sinks and the spatial distribution of the inverted carbon flux.