Feasibility of Using Biologically-based Processes in the Open Ocean and Coastal Seas for Atmospheric CO2 Removal

Abstract

This chapter assesses the feasibility of six ways to achieve greenhouse gas removal by using marine biological processes to increase CO2 drawdown from the atmosphere. Four approaches involve increasing the supply of nutrients to the open ocean: adding iron directly; adding iron indirectly via the atmosphere; adding macronutrients; and by enhancing upwelling to increase natural nutrient recycling. The intention is to stimulate primary production and CO2 uptake by marine phytoplankton, and hence deep-water carbon storage via the ocean biological carbon pump. Two coastal methods are also considered to increase CO2 drawdown: restoring blue carbon ecosystems (mangroves, salt marsh and seagrass habitats) as a ‘nature-based solution’, and large-scale cultivation of seaweeds (macroalgae) and subsequent carbon storage, primarily by the marine equivalent of BECCS (bioenergy with carbon capture and storage). For each approach, information is given on scientific understanding of key processes, environmental consequences, cost-effectiveness, carbon accounting, governance and public acceptability. Major uncertainties are identified regarding the quantification of carbon removal, the permanence of storage, and the possible emissions of other greenhouse gases. Governance-related constraints are particularly important for the open ocean approaches. Marine BECCS using seaweed theoretically offers high potential, but its large-scale deployment has not been well investigated. Coastal ecosystem restoration is already being used for small-scale carbon offsetting, and can be considered a no-regrets mitigation option, on the basis of its non-climatic benefits. The scalability and effectiveness of this approach for increasing CO2 removal are, however, uncertain, with many associated risks.