“Efficiency” here refers to the amount of atmospheric CO₂ taken up for a given addition of alkaline substances. It also includes consideration of the rate and temporal evolution of CO₂ uptake, as future uptake (e.g. 500 years from now) has little value for climate change mitigation. “Permanence” relates to the risk that CO₂ stored in the ocean due to OAE might leak back into the atmosphere on climate-change relevant timescales. The two concepts are distinguished mainly by timescale.
The efficiency and permanence of CO₂ removal contribute to the climate mitigation effectiveness and economic viability of OAE. They depend on the method and location of OAE and factors such as the degree of air-sea CO₂ equilibration and the risk of post-delivery carbonate precipitation. Both are sensitive to the biogeochemical and physicochemical features of the application site and the chemical and technological nature of the OAE application. The scale and duration of OAE deployment is also likely important (i.e., some approaches or locations may initially result in effective OAE, but may not be sustainable in the long-term). Increase/reduction of OAE efficiency can also arise from impacts on natural cycling, such as changes to stratification, the carbon dioxide uptake rates, the efficiency of the biological carbon and carbonate pumps or other feedbacks.
Key questions are:
