Mapping the global variation in the efficiency of ocean alkalinity enhancement for carbon dioxide removal

To limit global warming to below 2 °C by 2100, CO₂ removal from the atmosphere will be necessary. One promising method for achieving CO₂ removal at scale is ocean alkalinity enhancement (OAE), but there are challenges with incomplete air–sea CO₂ equilibration, which reduces the efficiency of carbon removal. Here, we present global maps of OAE efficiency, and assess the seasonal variation in efficiency. We find that the equilibration kinetics have two characteristic timescales: rapid surface equilibration followed by a slower second phase, which represents the re-emergence of excess alkalinity that was initially subducted. These kinetics vary considerably with latitude and the season of alkalinity release, which are critical factors for determining the placement of potential OAE deployments. Additionally, we quantify the spatial and temporal scales of the induced CO₂ uptake, which helps identify the requirements for modelling OAE in regional ocean models.