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 idealized 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. 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 (697 PgC) still remains in the ocean. At this time, CO₂-induced ocean carbon storage changes are 694, 607, and 592 PgC, and climate-induced ocean carbon storage changes are -149, -99, and -74 PgC, for CO₂ removal rate of 0.5%, 1%, and 2% per year, respectively. Marked nonlinearity between CO₂ effect and climate effect is found. When atmospheric CO₂ peaks at 4×CO₂, the nonlinearity is 48 PgC, accounting for 7% of the total ocean carbon storage. When atmospheric CO₂ returns to 1×CO₂, the nonlinearity grows to account for 38%, 26%, and 20% of the corresponding carbon storage for 0.5%, 1%, and 2% CO₂ removal rate, respectively. Our results further show that rough linearity of concentration-carbon and climate-carbon feedbacks breaks down under the scenario of CO₂ removal.
 

ocean carbon cycle，,atmospheric CO2 removal,earth system modeling