| Summary: | 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 ( p CO 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 -1 . This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in thehigh latitudes. Both estimates point toward a small (~ -0.3 Pg C a -1 ) 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 p CO 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 -1.7 ± 0.4 Pg C a -1 (inversion) and -1.4 ± 0.7 Pg C a -1 ( p CO 2 -climatology), respectively, consisting of an outgassing flux of river-derived carbon of ~+0.5 Pg C a -1 , and an uptake flux of anthropogenic carbon of -2.2 ± 0.3 Pg C a -1 (inversion) and -1.9 ± 0.7 Pg C a -1 ( p CO 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 -0.2 and -0.3 Pg C a -1 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.
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