Feasibility, Cost, and Potential Impacts of Ocean-Based Carbon Dioxide Removal Approaches

“Ocean carbon dioxide removal strategies are already being discussed by scientists, non-governmental organizations, and entrepreneurs as potential climate response strategies,” said Scott Doney, chair of the committee and professor in environmental sciences at the University of Virginia. “Right now, society and policymakers do not have the information they need to evaluate the impacts and trade-offs of these climate responses. If we want to make fully informed decisions about the future of our ocean and climate, we need to complete some very critical research in the next decade.”

Assessments of Six Ocean-Based Carbon Dioxide Removal Approaches
Nutrient Fertilization — This approach adds nutrients such as phosphorus or nitrogen to the ocean surface to increase photosynthesis by phytoplankton, thereby increasing the uptake of CO2 and transfer of carbon to the deep ocean, where it can stay for a century or longer. The report says there is medium to high confidence that this approach will be effective and scalable, with medium environmental risks and with low scale-up costs beyond the costs for environmental monitoring. The report estimates $290 million will be needed for research priorities, including for field experiments and to track carbon sequestered as a result.

Artificial Upwelling and Downwelling — Upwelling moves cooler, more nutrient- and CO2-rich deep water to the surface, stimulating the growth of phytoplankton, which absorb atmospheric carbon dioxide. Downwelling moves surface water and carbon to the deep ocean. The report says there is low confidence in the efficacy and scalability of these approaches, and that they carry medium to high environmental risks, with high costs and challenges for carbon accounting. The report estimates $25 million would be needed for research priorities, such as technological readiness and limited and controlled ocean trials.

Seaweed Cultivation — Large-scale seaweed farming that transports carbon to the deep ocean or into sediments has medium efficacy and medium to high durability for removing atmospheric CO2, the report says, but medium to high environmental risks. The report estimates $130 million for priority research to understand technologies for efficient large-scale farming and harvesting, the long-term fates of seaweed biomass, and the environmental impacts.

Ecosystem Recovery — Carbon removal and sequestration through protection and restoration of coastal ecosystems and the recovery of fish, whales, and other marine wildlife — which the report says could have low to medium efficacy — comes with the lowest environmental risks among the assessed approaches, and high co-benefits. The report estimates $220 million for research priorities, including to study effects on macroalgae, marine animals, and marine protected areas.

Ocean Alkalinity Enhancement — This approach chemically alters ocean water to increase its alkalinity in order to enhance reactions that take up atmospheric CO2. The report says there is high confidence in its efficacy. Ocean alkalinity enhancement carries medium environmental risks and medium to high scale-up costs. The report estimates $125 million to $200 million for priority research, including field and laboratory experiments to explore the impact on marine organisms.

Electrochemical Processes — By passing an electric current through water, these processes can either increase the acidity of seawater in order to release CO2, or increase the alkalinity of seawater to enhance its ability to retain CO2. There is high confidence in its efficacy, and medium to high confidence in its scalability, but this approach carries the highest scale-up cost of any of the approaches assessed — and medium to high environmental risks. The report estimates $350 million for priority research, including for demonstration projects and to develop and assess improved materials.

The study — undertaken by the Committee on a Research Strategy for Ocean Carbon Dioxide Removal and Sequestration — was sponsored by ClimateWorks Foundation.