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 pCO(2) 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 degrees S-62 degrees 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.