Simulated Responses of the Ocean Carbon Cycle to Different Rates of Atmospheric CO2 Removal

Carbon dioxide removal could play a key role in limiting future global warming. Here, we use an Earth system model to investigate the responses of the ocean carbon cycle to idealized scenarios of direct atmospheric CO₂ capture that causes net negative CO₂ emissions. We use CO₂ pathways with CO₂ increasing from 285 ppm (1 × CO₂) to 4 × CO₂ and then returning to 1 × CO₂ with various rates of CO₂ removal. Simulations are performed to examine the biogeochemical, radiative, and the full effect of atmospheric CO₂ on ocean CO₂ uptake. When atmospheric CO₂ starts to decrease, the global ocean gradually turns from a CO₂ sink to a source with the North Atlantic and the Southern Ocean showing intense CO₂ outgassing. However, ocean carbon storage shows substantially delayed response to CO₂ decrease. When atmospheric CO₂ returns to 1 × CO₂, about 60% excess CO₂ absorbed by the ocean at 4 × CO₂ still remains in the ocean. At 4 × CO₂, in term of magnitude, radiatively effect is equivalent to 7% of the biogeochemical effect. When atmospheric CO₂ returns to 1 × CO₂, radiatively effect is equivalent to 12%–21% of the biogeochemical effect depending on the rate of CO₂ decrease. Marked nonadditivity of biogeochemical and radiatively effect is found. At 4 × CO₂, the nonadditivity accounts for 7% of total ocean carbon storage in the fully coupled simulation, and the magnitude of nonadditivity grows with time even after atmospheric CO₂ starts to decrease. Our results further show that rough linearity of concentration-carbon and climate-carbon feedbacks breaks down under the scenario of CO₂ removal.