Abstract. The globally integrated sea–air anthropogenic carbon dioxide (CO2) flux from 1990 to 2009 is determined from models and data-based approaches as part of the Regional Carbon Cycle Assessment and Processes (RECCAP) project. Numerical methods include ocean inverse models, atmospheric inverse models, and ocean general circulation models with parameterized biogeochemistry (OBGCMs). The median value of different approaches shows good agreement in average uptake. The best estimate of anthropogenic CO2 uptake for the time period based on a compilation of approaches is −2.0 Pg C yr−1. The interannual variability in the sea–air flux is largely driven by large-scale climate re-organizations and is estimated at 0.2 Pg C yr−1 for the two decades with some systematic differences between approaches. The largest differences between approaches are seen in the decadal trends. The trends range from −0.13 (Pg C yr−1) decade−1 to −0.50 (Pg C yr−1) decade−1 for the two decades under investigation. The OBGCMs and the data-based sea–air CO2 flux estimates show appreciably smaller decadal trends than estimates based on changes in carbon inventory suggesting that methods capable of resolving shorter timescales are showing a slowing of the rate of ocean CO2 uptake. RECCAP model outputs for five decades show similar differences in trends between approaches.

Abstract. Air-sea CO 2 fluxes over the Pacific Ocean are known to be characterized by coherent large-scale structures that reflect not only ocean subduction and upwelling patterns, but also the combined effects of wind-driven gas exchange and biology. On the largest scales, a large net CO 2 influx into the extratropics is associated with a robust seasonal cycle, and a large net CO 2 efflux from the tropics is associated with substantial interannual variability. In this work, we have synthesized estimates of the net air-sea CO 2 flux from a variety of products, drawing upon a variety of approaches in three sub-basins of the Pacific Ocean, i.e., the North Pacific extratropics • N), the tropical Pacific (18 • S-18 • N), and the South Pacific extratropics (44.5-18 • S). These approaches include those based on the measurements of CO 2 partial pressure in surface seawater (pCO 2 sw), inversions of ocean-interior CO 2 data, forward ocean biogeochemistry models embedded in the ocean general circulation models (OBGCMs), a model with assimilation of pCO 2 sw data, and inversions of atmospheric CO 2 measurements. Long-term means, interannual variations and mean seasonal variations of the regionally integrated fluxes were compared in each of the sub-basins over the last two decades, spanning the period from 1990 through 2009. A simple average of the long-term mean fluxes obtained with surface water pCO 2 diagnostics and those obtained with ocean-interior CO 2 inversions are −0.47 ± 0.13 Pg C yr the North Pacific extratropics, +0.44 ± 0.14 Pg C yr −1 in the tropical Pacific, and −0.37 ± 0.08 Pg C yr −1 in the South Pacific extratropics, where positive fluxes are into the atmosphere. This suggests that approximately half of the CO 2 taken up over the North and South Pacific extratropics is released back to the atmosphere from the tropical Pacific. These estimates of the regional fluxes are also supported by the estimates from OBGCMs after adding the riverine CO 2 flux, i.e., −0.49 ± 0.02 Pg C yr −1 in the North Pacific extratropics, +0.41 ± 0.05 Pg C yr −1 in the tropical Pacific, and −0.39 ± 0.11 Pg C yr −1 in the South Pacific extratropics. The estimates from the atmospheric CO 2 inversions show large variations amongst different inversion systems, but their median fluxes are consistent with the estimates from climatological pCO 2 sw data and pCO 2 sw diagnostics. In the South Pacific extratropics, where CO 2 variations in the surface and ocean interior are severely undersampled, the difference in the air-sea CO 2 flux estimates between the diagnostic models and ocean-interior CO 2 inversions is larger (0.18 Pg C yr −1 ). The range of estimates from forward OBGCMs is also large (−0.19 to −0.72 Pg C yr −1 ). Regarding interannual variability of air-sea CO 2 fluxes, positive and negative anomalies are evident in the tropical Pacific during the cold and warm events of the El Niño-Southern Oscillation in the estimates from pCO 2 sw diagnostic models and from OBGCMs. They are consistent in phase with the Southern Oscillation In...

The Southern Ocean (SO) carbon sink has strengthened substantially since the year 2000, following a decade of a weakening trend. However, the surface ocean pCO 2 data underlying this trend reversal are sparse, requiring a substantial amount of extrapolation to map the data. Here we use nine different pCO 2 mapping products to investigate the SO trends and their sensitivity to the mapping procedure. We find a robust temporal coherence for the entire SO, with eight of the nine products agreeing on the sign of the decadal trends, that is, a weakening CO 2 sink trend in the 1990s (on average 0.22 ± 0.24 Pg C yr −1 decade −1), and a strengthening sink trend during the 2000s (−0.35 ± 0.23 Pg C yr −1 decade −1). Spatially, the multiproduct mean reveals rather uniform trends, but the confidence is limited, given the small number of statistically significant trends from the individual products, particularly during the data-sparse 1990-1999 period. Plain Language Summary The Southern Ocean plays an important role in regulating Earth's climate as it takes up a substantial amount of carbon dioxide from the atmosphere, thereby limiting the effect of global warming. However, this part of the global ocean is also the least well observed and observational data are sparse. Therefore, to study Southern Ocean carbon uptake, data interpolation methods are used to estimate the variability of the carbon uptake from the few existing observations. This poses the question on how reliable these estimates are. The Surface Ocean CO 2 Mapping intercomparison project aims to do exactly that, that is, test how reliable current estimates are by comparing results from different methods. Here we compare the results from nine data interpolation methods in the Southern Ocean from 1990 to 2010 and find a broad and encouraging agreement regarding decadal carbon uptake signals, whereas a spatially more refined analysis reveals much less agreement locally, illustrating the need to continue the measurement effort in the Southern Ocean.