Effect of uncertainties in climatologic wind, ocean p CO 2 , and gas transfer algorithms on the estimate of global sea‐air CO 2 flux

This paper addresses the uncertainties of global sea‐air CO 2 flux estimated on the basis of the sea‐air p CO 2 differences and sea‐air CO 2 gas transfer rate. Uncertainties in the global sea‐air flux estimates are identified from different gas transfer algorithms and monthly wind speeds (NCEP‐2 and SSM/I). The net global sea‐air flux estimates for the contemporary oceans range from 0.9 ± 0.5 Pg‐C a −1 to 1.3 ± 0.8 Pg‐C a −1 for reference year 2000. Including the carbon transferred from rivers in the preindustrial steady oceans, the uptake rates for anthropogenic CO 2 range from 1.3 ± 0.6 Pg‐C a −1 to 1.7 ± 0.8 Pg‐C a −1 . For all three gas exchange parameterizations used, the difference between global fluxes using SSM/I and NCEP‐2 winds ranged between 3% and 37%. The subtropical regions are the largest sinks of CO 2 (–0.8 Pg‐C a −1 ), and the equatorial Pacific is the largest source (0.4 Pg‐C a −1 ). The North Pacific (–0.4 Pg‐C a −1 ) and the North Atlantic (–0.3 Pg‐C a −1 ) are also significant sinks of CO 2 . All the remaining regions have relatively weak net yearly exchanges of CO 2 . Recent estimates (Takahashi et al., 2009) of sea‐air CO 2 flux in the ice‐free zone of the Southern Ocean (50°S−62°S) reveal a small mean annual flux (–0.06 Pg‐C a −1 ) resulting from the cancellation of the summer uptake CO 2 flux with the winter release of CO 2 caused by deepwater upwelling. Our study results for the same region range from –0.03 to –0.06 Pg‐C a −1 , which are within the uncertainties of the estimates.