Mechanisms modulating ocean carbon uptake beyond 2100

Uptake of anthropogenic carbon by the world oceans is expected to increase throughout year 2100 if CO2 emissions remain unabated, exerting a first order control on atmospheric concentrations and radiative forcing. Several mechanisms contribute to this uptake, but their relative roles remain uncertain, as well as their long term evolution beyond year 2100 once CO2 emissions decline and atmospheric concentrations stabilize. We addressed this question analyzing output from a numerical simulation run under the RCP8.5 high emission scenario extending to year 2300. This simulation was performed with the Community Earth System Model (CESM) that simulates a highly realistic ocean carbon cycle. We found that all regions of the world ocean, tropics, mid- and high-latitude, contribute to the increased uptake throughout year 2100. In the tropics, reduced outgassing is caused by CO2 utilization due to higher biological production and the reduction in CO2-rich water supply with weakening upwelling. The relative influence changes beyond 2100, which gradually weakens the efficiency of carbon uptake as extreme warming causes less solubility. Despite the stabilization of atmospheric CO2 concentrations in 2300, the world oceans continue to uptake carbon, as the dominance of the long-term CO2 absorption in the Southern Ocean by biological production boosting in association with the sea ice retreat. In contrast, the North Atlantic ceases to be a carbon sink by 2300 due to the weakening productivity with the ocean circulation slowdown - contributing to the reduction in uptake efficiency beyond 2100. These results show the key role played by the Southern Ocean, but also the tropics, sustaining the rates of CO2 uptake beyond 2100.