Global oceanic and land biotic carbon sinks from the Scripps atmospheric oxygen flask sampling network

Measurements of atmospheric O₂/N₂ ratio and CO₂ concentration are presented over the period 1989–2003 from the Scripps Institution of Oceanography global flask sampling network.Aformal framework is described for making optimal use of these data to estimate global oceanic and land biotic carbon sinks. For the 10-yr period from 1990 to 2000, the oceanic and land biotic sinks are estimated to be 1.9 ± 0.6 and 1.2 ± 0.8 Pg C yr^-1, respectively, while for the 10-yr period from 1993 to 2003, the sinks are estimated to be 2.2 ± 0.6 and 0.5 ± 0.7 Pg C yr^-1, respectively. These estimates, which are also compared with earlier results, make allowance for oceanic O₂ and N₂ outgassing based on observed changes in ocean heat content and estimates of the relative outgassing per unit warming. For example, for the 1993–2003 period we estimate outgassing of 0.45 × 10¹⁴ mol O₂ yr^-1 and 0.20 × 1014 mol N₂ yr^-1, which results in a correction of 0.5 Pg C yr^-1 on the oceanic and land biotic carbon sinks. The basis for this oceanic outgassing correction is reviewed in the context of recent model estimates. The main contributions to the uncertainty in the global sinks averages are from the estimates for oceanic outgassing and the estimates for fossil fuel combustion. The oceanic sink of 2.2 Pg C yr^-1 for 1993–2003 is consistent, within the uncertainties, with the integrated accumulation of anthropogenic CO₂ in the ocean since 1800 as recently estimated from oceanic observations, assuming the oceanic sink varied over time as predicted by a box-diffusion model.