Enhanced carbon loss in the Southern Ocean under mitigation scenarios.

The Southern Ocean (SO) plays a pivotal role in the global carbon cycle by absorbing anthropogenic carbon dioxide (CO2). Yet, it remains uncertain whether this role will persist under future mitigation efforts. Using idealized emission-driven simulations with the Community Earth System Model version 2 (CESM2), we find that the SO transitions from net CO2 sink to a substantial source during zero- and negative-emission phases. This outgassing stems from a persistent regional increase in sea surface temperature and a globally uniform decline in surface alkalinity. Together, these changes maintain elevated oceanic partial pressure of CO2 ([Formula: see text]) despite decreasing atmospheric CO2, reversing the air-sea [Formula: see text] gradient ([Formula: see text]). Moreover, the SO exhibits a high climatological gas-transfer coefficient, primarily driven by the region's strong wind speeds and high CO2 solubility, which amplifies the [Formula: see text]-driven flux and enhances CO2 outgassing. These results indicate that the present-day SO CO2 sink may lose its effectiveness during climate mitigation, emphasizing the need to consider regional feedbacks for long-term climate policy effectiveness.