Potential of Ocean Alkalinity Enhancement in Climate Stabilization scenarios at Different Warming Levels

Carbon dioxide removal (CDR) strategies, such as ocean alkalinity enhancement (OAE), are crucial for limiting global warming to below 2°C alongside strong emission reductions. However, the efficiency and temperature mitigation potential of OAE under different stabilization scenarios and on long timescales remain uncertain. This study employs the Adaptive Emissions Reduction Approach within a comprehensive fully coupled Earth System Model to address these gaps. Two sets of five-member ensemble simulations spanning 1861 to 2500 were conducted: (i) stabilization scenarios at 1.5°C, 2°C, and 3°C global warming levels, and (ii) simulations applying idealized and large-scale OAE globally of 0.14Pmol per year at the ocean surface following the CDRMIP-protocol from 2026 onward using the emissions pathways from (i). Our results show that adding alkalinity at the surface lowers surface air temperature by 0.014°C per decade (1.5°C scenario) to 0.018°C per decade (3.0°C scenario). The ocean’s additional carbon uptake per unit of added alkalinity ranges from 0.53 to 0.69, with higher efficiencies in the higher global warming scenarios. However, atmospheric CO2 reduction efficiencies are up to 0.2 lower due to anomalous release of carbon from the land. OAE efficiency remains stable until atmospheric CO2 peaks but declines thereafter, driven by changes in the pCO2 equilibration timescale, which shortens with reductions in buffer capacity before peak CO2, and lengthens during the stabilization phase where buffer capacity increases again as a result of declining atmospheric CO2. These findings highlight the complex dynamics of OAE in response to evolving climate and carbon cycle feedbacks, offering critical insights for the deployment of CDR strategies.