Hongjie Wang

Ocean alkalinity enhancement (OAE) has the potential to mitigate ocean acidification (OA) and induce atmospheric carbon dioxide (CO 2 ) removal (CDR). We evaluate the CDR and OA mitigation impacts of a sustained point‐source OAE of 1.67 × 10 10 mol total alkalinity (TA) yr −1 (equivalent to 667,950 metric tons NaOH yr −1 ) in Unimak Pass, Alaska. We find the alkalinity elevation initially mitigates OA by decreasing p CO 2 and increasing aragonite saturation state and pH. Then, enhanced air‐to‐sea CO 2 exchange follows with an approximate e ‐folding time scale of 5 weeks. Meaningful modeled OA mitigation with reductions of >10 μatm p CO 2 (or just under 0.02 pH units) extends 100–100,000 km 2 around the TA addition site. The CDR efficiency (i.e., the experimental seawater dissolved inorganic carbon (DIC) increase divided by the maximum DIC increase expected from the added TA) after the first 3 years is 0.96 ± 0.01, reflecting essentially complete air‐sea CO 2 adjustment to the additional TA. This high efficiency is potentially a unique feature of the Bering Sea related to the shallow depths and mixed layer depths. The ratio of DIC increase to the TA added is also high (≥0.85) due to the high dissolved carbon content of seawater in the Bering Sea. The air‐sea gas exchange adjustment requires 3.6 months to become (>95%) complete, so the signal in dissolved carbon concentrations will likely be undetectable amid natural variability after dilution by ocean mixing. We therefore argue that modeling, on a range of scales, will need to play a major role in assessing the impacts of OAE interventions.