Quantifying air–sea re-equilibration-implied ocean surface CO2 accumulation against recent atmospheric CO2 rise

To understand better the role of ocean surface chemical buffering capacity in mitigating the recent atmospheric CO2 rise, we investigated potentials of wintertime mixed-layer dissolved inorganic carbon (DIC) to increase after re-equilibration with decadal atmospheric CO2 rise and the corresponding anthropogenic CO2 accumulation rates, based upon carbonate system chemistry over the global ocean surface above the wintertime thermocline. This is an idealized study to quantify the isolated effect of atmospheric CO2 rise on the DIC content of the global mixed layer, assuming all else constant. Our results show that the potentials of wintertime DIC over the global open ocean surface to rise after re-equilibration with the elevated atmospheric CO2 mol fraction in a reference year 2000 ranged from 0.28 to 0.70 μmol kg−1 ppm−1 (ppm = parts of CO2 per million dry air), while the global mean wintertime sea surface DIC increase rate was close to 1.0 μmol kg−1 year−1. From 1995 to 2005, the decadal mean atmospheric CO2 rise implies an anthropogenic CO2 accumulation rate of 0.40 × 1015 g C year−1 within the global ocean surface. From the 1960s to 2000s, the air–sea re-equilibration-implied ocean surface anthropogenic CO2 accumulation rate may have increased by 46 % due to the accelerated atmospheric CO2 rise. However, the chemical buffering capacity within the ocean surface may have declined by 16 % during the same period.