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[1] We synthesize estimates of the contemporary net air-sea CO 2 flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007 and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO 2 (pCO 2 ) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO 2 for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a À1 . This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in the high latitudes. Both estimates point toward a small ($ À0.3 Pg C a À1 ) contemporary CO 2 sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO 2 and a strong uptake of anthropogenic CO 2 . A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO 2 -based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO 2 is estimated to be À1.7 ± 0.4 Pg C a À1 (inversion) and À1.4 ± 0.7 Pg C a À1 (pCO 2 -climatology), respectively, consisting of an outgassing flux of river-derived carbon of $+0.5 Pg C a À1 , and an uptake flux of anthropogenic carbon of À2.2 ± 0.3 Pg C a À1 (inversion) and À1.9 ± 0.7 Pg C a À1(pCO 2 -climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between À0.2 and À0.3 Pg C a À1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO 2 fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.

[1] Regional air-sea fluxes of anthropogenic CO 2 are estimated using a Green's function inversion method that combines data-based estimates of anthropogenic CO 2 in the ocean with information about ocean transport and mixing from a suite of Ocean General Circulation Models (OGCMs). In order to quantify the uncertainty associated with the estimated fluxes owing to modeled transport and errors in the data, we employ 10 OGCMs and three scenarios representing biases in the data-based anthropogenic CO 2 estimates. On the basis of the prescribed anthropogenic CO 2 storage, we find a global uptake of 2.2 ± 0.25 Pg C yr À1 , scaled to 1995. This error estimate represents the standard deviation of the models weighted by a CFC-based model skill score, which reduces the error range and emphasizes those models that have been shown to reproduce observed tracer concentrations most accurately. The greatest anthropogenic CO 2 uptake occurs in the Southern Ocean and in the tropics. The flux estimates imply vigorous northward transport in the Southern Hemisphere, northward cross-equatorial transport, and equatorward transport at high northern latitudes. Compared with forward simulations, we find substantially more uptake in the Southern Ocean, less uptake in the Pacific Ocean, and less global uptake. The large-scale spatial pattern of the estimated flux is generally insensitive to possible biases in the data and the models employed. However, the global uptake scales approximately linearly with changes in the global anthropogenic CO 2 inventory. Considerable uncertainties remain in some regions, particularly the Southern Ocean.