Quantifying the Near-term Response of the Ocean CO₂ Sink to Emissions Mitigation

Abstract. The ocean plays a critical role in sequestering carbon, yet how the air-sea carbon sink will respond to rapid reductions in atmospheric carbon dioxide (CO2) concentrations remains a key uncertainty for climate mitigation and carbon accounting. In this study, we utilize the ECCO-Darwin ocean biogeochemistry model to simulate the ocean’s response to a range of near-term CO2 mitigation scenarios. Our results demonstrate an immediate but spatially heterogeneous weakening of the ocean carbon sink following reduced atmospheric forcing. Global air-sea carbon uptake decreases substantially within the first several years of mitigation, with the magnitude of the reduction scaling with the strength of the mitigation scenario. We find that the mitigation signal is primarily confined to the upper-500 m, reflecting dominant decadal-scale ventilation pathways. The most pronounced reductions in uptake occur in subtropical thermocline regions, western boundary currents, and the subpolar North Atlantic — regions characterized by intense ventilation and rapid air-sea equilibration. These findings align with recent CMIP-based long-term projections and observation-based inversions, confirming that the areas currently dominating anthropogenic carbon uptake are also the most sensitive to atmospheric forcing changes. Our results suggest that observational efforts to track mitigation impacts should be prioritized in these high-latitude and boundary current systems, where signals emerge earliest and most strongly. Ultimately, this study underscores the rapid sensitivity of the ocean carbon sink to changes in atmospheric forcing and highlights the necessity of sustained, strategically-placed observations to detect and attribute changes in the global carbon budget under future climate strategies.