On the emission-scenario dependence of the efficiency of ocean alkalinity enhancement

Ocean alkalinity enhancement (OAE) deliberately modifies the chemistry of the surface ocean to enhance the uptake of atmospheric CO2. Although it is known that the efficiency of OAE (the amount of CO2 sequestered per unit of alkalinity added) depends on the chemical background state of the surface ocean, the consequences of this dependency for simulated OAE scenarios have never been systematically explored. Here we show, using idealized and scenario simulations with an Earth system model (ESM), that under quadrupling of pre-industrial atmospheric CO2 concentrations, the simulated efficiency of OAE increases by about 30% from 0.76 to 0.98. We find that only half of this effect can be explained by changes in the sensitivity of CO2 sequestration to alkalinity addition itself. The remainder is due to the larger portion of anthropogenic emissions taken up by a high alkalinity ocean. Importantly, both effects are reversed if atmospheric CO2 concentrations were to decline due to large scale deployment of land-based (or alternative ocean-based) carbon dioxide removal (CDR) methods. By considering an overshoot pathway that relies on large amounts of land-based CDR, we demonstrate that OAE efficiency indeed shows a strong decline after atmospheric CO2 concentrations have peaked. Our results imply that methodological choices must be made if carbon credits for OAE are to be allocated based on simulated efficiencies.