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

Abstract To limit global warming to below 2°C by 2100, carbon dioxide removal (CDR) from the atmosphere will be necessary. Ocean alkalinity enhancement (OAE) is a promising approach to achieving CDR at scale. However, OAE deployments are subject to incomplete air-sea CO2 equilibration — reducing the efficiency of carbon removal. Here, we generated the first global maps of OAE efficiency, including an assessment of the seasonal variation in efficiency. We show that the equilibration kinetics exhibit two characteristic timescales — rapid surface equilibration followed by a slower second phase, representing the re-emergence of excess alkalinity that was initially subducted. These kinetics vary considerably with latitude and the season of alkalinity release, both critical considerations for the placement of prospective OAE deployments. We also calculated histograms quantifying the spatial and temporal scales of the induced CO2 uptake and thus identifying the requirements for modeling OAE in regional ocean models.