A 1-D examination of decadal air–sea re-equilibration induced ocean surface anthropogenic CO₂ accumulation: present status, changes from 1960s to 2000s, and future scenarios

Abstract. Based upon the well-understood carbonate system chemistry over global ocean surface (above the wintertime thermocline and shallower than upper 100 m), we investigated potentials of wintertime ocean surface DIC (dissolved inorganic carbon) to rise in response to the decadal air–sea re-equilibration, and the corresponding anthropogenic CO₂ accumulation rates. For a reference year 2000, the potentials of wintertime DIC to rise in response to the rising atmospheric CO₂ mole fraction ranged from 0.28 to 0.70 μmol kg⁻¹ ppm⁻¹ (ppm = parts of CO₂ per million dry air) over the global open ocean surface, while the global mean wintertime surface DIC increase rate was close to 1.0 μmol kg⁻¹ yr⁻¹. The decadal anthropogenic CO₂ accumulation rate within the surface ocean was estimated at 0.31 × 10¹⁵ g C yr⁻¹ around the reference year 2000, accounting for a non-negligible component (likely 12 to 14%) of the recent oceanic sink for anthropogenic CO₂. From 1960s to 2000s, this rate likely increased by 47% due to the accelerated atmospheric CO₂ rise. However, the ocean surface anthropogenic CO₂ accumulation potential under a unit atmospheric CO₂ rise may have declined by 16% during the same period.