The efficiency and ocean acidification mitigation potential of ocean alkalinity enhancement on multi-centennial timescales

Abstract. Carbon dioxide removal (CDR) strategies such as ocean alkalinity enhancement (OAE) are likely required in addition to rapid emissions reductions to limit global warming to well below 2 °C. However, the long-term efficiency of OAE and its potential to mitigate climate change and ocean acidification remain uncertain. Here, we investigate efficiencies, climate and ocean acidification responses of idealized OAE using a fully coupled, emission-driven Earth system model across three global warming stabilization scenarios (1.5 °C, 2 °C, and 3 °C) spanning 1861–2500. OAE is implemented as a continuous global surface alkalinity addition of 0.14 Pmol yr^-1 following the CDRMIP protocol from 2026 onward. Our results show that OAE reduces atmospheric CO₂ by 73–130 ppm by 2500, with larger reductions under higher warming scenarios and during the first 100 to 200 years of alkalinity addition. In contrast, global surface air temperature decreases nearly linearly by 0.14–0.17 °C per century across all scenarios, indicating that the cooling rate due to OAE is largely insensitive to the emission pathway and background warming level. The interpretation of OAE efficiency depends strongly on the chosen metric. The global gross ocean carbon capture efficiency of 0.79 remains close to the theoretical maximum, reflecting the negative emissions through OAE, whereas the net ocean capture and atmospheric CO₂ reduction efficiencies are substantially lower and decline over time due to carbon cycle feedbacks in response to lowered atmospheric CO₂. OAE mitigates ocean acidification, at the surface as well as in the interior ocean, with most centennial-scale mitigation arising from atmospheric CO₂ drawdown, an effect shared with other CDR approaches. Direct chemical effects of added alkalinity contribute transiently and diminish over time as the ocean–atmosphere system equilibrates. Overall, our results underscore that rapid emission reductions remain the most effective strategy for achieving the Paris Agreement goals and mitigating ocean acidification.