Decadal variations and trends of the global ocean carbon sink

We investigate the variations of the ocean CO2 sink during the past three decades using global surface ocean maps of the partial pressure of CO2 reconstructed from observations contained in the Surface Ocean CO2 Atlas Version 2. To create these maps, we used the neural network-based data interpolation method of Landschutzer et al. (2014) but extended the work in time from 1998 to 2011 to the period from 1982 through 2011. Our results suggest strong decadal variations in the global ocean carbon sink around a long-term increase that corresponds roughly to that expected from the rise in atmospheric CO2. The sink is estimated to have weakened during the 1990s toward a minimum uptake of only -0.8 +/- 0.5 Pg C yr(-1) in 2000 and thereafter to have strengthened considerably to rates of more than -2.0 +/- 0.5 Pg C yr(-1). These decadal variations originate mostly from the extratropical oceans, while the tropical regions contribute primarily to interannual variations. Changes in sea surface temperature affecting the solubility of CO2 explain part of these variations, particularly at subtropical latitudes. But most of the higher-latitude changes are attributed to modifications in the surface concentration of dissolved inorganic carbon and alkalinity, induced by decadal variations in atmospheric forcing, with patterns that are reminiscent of those of the Northern and Southern Annular Modes. These decadal variations lead to a substantially smaller cumulative anthropogenic CO2 uptake of the ocean over the 1982 through 2011 period (reduction of 7.5 +/- 5.5 Pg C) relative to that derived by the Global Carbon Budget.