Effect of uncertainties in climatologic wind, ocean pCO2, and gas transfer algorithms on the estimate of global sea-air CO2 flux

This paper addresses the uncertainties of global sea‐air CO2 flux estimated on the basis of the sea‐air pCO2 differences and sea‐air CO2 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 CO2 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 CO2 (–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 CO2. All the remaining regions have relatively weak net yearly exchanges of CO2. Recent estimates (Takahashi et al., 2009) of sea‐air CO2 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 CO2 flux with the winter release of CO2 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.