Ocean Alkalinity Enhancement Efficiency and Responses of Recalcitrant Dissolved Organic Carbon Under Future Emissions Scenarios

Ocean alkalinity enhancement (OAE) is increasingly regarded as a promising ocean-based carbon dioxide removal strategy because it can enhance oceanic carbon dioxide (CO 2 ) uptake while alleviating ocean acidification. However, regional differences in OAE efficiency and the associated responses of recalcitrant dissolved organic carbon (RDOC) remain poorly constrained. Here, we incorporate a microbial carbon pump (MCP) module into the carbon-centric Grid-Enabled Integrated Earth System Model (cGENIE) to assess the carbon sequestration potential of OAE and the RDOC response under the Shared Socioeconomic Pathway (SSP) scenarios SSP2-45 and SSP5-85 across different regional deployment configurations. By explicitly linking OAE with MCP-mediated RDOC dynamics, our study extends previous carbonate-chemistry-focused assessments to include long-term inorganic–organic carbon feedbacks. In idealized simulations with sustained OAE deployment beginning in 2030 CE, atmospheric CO 2 concentrations are (62–75)×10 −6 lower and global mean surface temperatures are 0.15–0.21 °C lower by 2100 CE than in the corresponding simulations without OAE. Alkalinity deployment in the Northwest Pacific yields the highest long-term efficiency and strongest local surface alkalinity enhancement among the configurations tested. OAE also induces distinct long-term responses among dissolved organic carbon (DOC) fractions. The RDOC response lags behind those of labile DOC and semilabile DOC and evolves more slowly over millennial timescales, with concentrations remaining relatively stable at 44–45 µmol/kg. These results underscore the coupled inorganic–organic feedbacks governing OAE efficiency and highlight the necessity of spatial and temporal optimization for maximizing long-term carbon sequestration.