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 CO2 on the basis of a new high-quality global data set of surface observations of the reduced isotopic ratio (delta C-13) 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 C-13 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 delta C-13, (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.