An improved estimate of the isotopic air‐sea disequilibrium of CO₂: Implications for the oceanic uptake of anthropogenic CO₂

We reevaluate the isotopic air‐sea disequilibrium of CO 2 on the basis of a new high‐quality global data set of surface observations of the reduced isotopic ratio (δ 13 C) of dissolved inorganic carbon (DIC). We find a global mean isotopic air‐sea disequilibrium of 0.62±0.10 permil for 1990. Inserting our estimate into an anthropogenic 13 C budget for the period from 1985 to 1995 yields an oceanic uptake of 1.5±0.9 Pg C yr −1 , within the range of most other estimates. The large uncertainty of this estimate could be reduced with (i) better characterization of the surface ocean variability in δ 13 C, (ii) improved knowledge of the air‐sea gas exchange coefficient, and (iii) a significant reduction in the uncertainty associated with the air‐land biosphere disequilibrium.