Global ocean carbon uptake: magnitude, variability and trends

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.