A global efficiency map of ocean alkalinity enhancement (OAE) for CO2 removal

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 a large scale. However, OAE deployments are subject to slow or incomplete air-sea CO2 exchange, reducing the efficiency of carbon removal, defined as the excess CO2 uptake per mol of alkalinity addition. We used a coupled ocean circulation-biogeochemistry model to generate the first global, time-resolved map of OAE efficiency across four different seasons and investigated its controlling factors. An ensemble of alkalinity pulse injections in the global ocean were simulated with the global 1-degree ocean component of the Community Earth System Model version 2 (CESM2). Alkalinity was added to the surface ocean for 1 month in a total of 690 patches and in 4 different seasons of a year. Each simulation was run for 15 years for each patch and season to compute OAE efficiency, residence time of excess alkalinity retained in the mixed layer, and CO2 re-equilibration timescales - all referenced to the geographic location of the induced perturbation. OAE efficiency showed large spatial and seasonal variations. The highest seasonal mean OAE efficiency achieved after 15 years, ranging from 0.7 to 0.9, were found in the subpolar oceans, the semi-closed regions, such as the Gulf of St. Lawrence and the North Sea, as well as the coastal zones along the Pacific and South Atlantic. The lowest seasonal mean, ranging from 0.3 to 0.5, was found in the high latitudes Atlantic and Southern Ocean where deep water forms. The intermediate values, ranging from 0.5 to 0.7, were found predominantly in the subtropical gyres, as well as western and eastern boundary currents. Seasonally, higher maximum OAE efficiency could generally be achieved when alkalinity is released in the summer rather than in winter. Accurate understanding of the CO2 response curves, as provided by our maps, is critical for choosing suitable OAE deployment sites and is central to the MRV (Measurement, Reporting & Verification) challenge faced by all marine CDR methods.