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 CO 2 capture that causes net negative CO 2 emissions. We use CO 2 pathways with CO 2 increasing from 285 ppm (1 × CO 2 ) to 4 × CO 2 and then returning to 1 × CO 2 with various rates of CO 2 removal. Simulations are performed to examine the biogeochemical, radiative, and the full effect of atmospheric CO 2 on ocean CO 2 uptake. When atmospheric CO 2 starts to decrease, the global ocean gradually turns from a CO 2 sink to a source with the North Atlantic and the Southern Ocean showing intense CO 2 outgassing. However, ocean carbon storage shows substantially delayed response to CO 2 decrease. When atmospheric CO 2 returns to 1 × CO 2 , about 60% excess CO 2 absorbed by the ocean at 4 × CO 2 still remains in the ocean. At 4 × CO 2 , in term of magnitude, radiatively effect is equivalent to 7% of the biogeochemical effect. When atmospheric CO 2 returns to 1 × CO 2 , radiatively effect is equivalent to 12%–21% of the biogeochemical effect depending on the rate of CO 2 decrease. Marked nonadditivity of biogeochemical and radiatively effect is found. At 4 × CO 2 , 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 CO 2 starts to decrease. Our results further show that rough linearity of concentration‐carbon and climate‐carbon feedbacks breaks down under the scenario of CO 2 removal.