Cost-effective ocean alkalinity enhancement confined to coastal regions.

Carbon dioxide removal from the atmosphere through geoengineering technologies is inevitable to limit global warming below 2°C. Ocean alkalinity enhancement (OAE), which consists of increasing the ocean carbon sink by injecting alkaline materials into it, could potentially remove gigatons of carbon dioxide from the atmosphere every year. Limited information is, however, available on how to scale up this technology and where in the ocean it would be economically viable. In this study, we conduct a techno-economic analysis, combining biogeochemical, technical, and economic data, to identify the ocean areas where OAE is both chemically and cost-efficient. We show that net-negative emissions via OAE are only feasible and viable in coastal areas. In coastal areas, the costs and carbon emissions associated with the shipping of alkaline materials are low given the short distance to ports, minimizing the cost per ton of carbon sequestered. In contrast, in most open oceans, the long distances covered by the vessels not only increase the costs of OAE activities but also vessel carbon emissions, which can lead to net positive emissions. Implementing coastal OAE activities has policy implications as it could lead to conflicts of use with other coastal activities (conservation, fisheries, tourism, energy production), other carbon capture activities (enhance rock weathering), and international tensions in case of transboundary harm.