Seaweed afforestation at large-scales exclusively for carbon sequestration: Critical assessment of risks, viability and the state of knowledge

There have been discussions of scaling up offshore seaweed cultivation and sinking it exclusively for carbon sequestration (‘ocean afforestation’) thereby help mitigate climate change, but is this concept feasible? Here we investigate the feasibility ocean afforestation across five perspectives: 1) Ecological feasibility; 2) Technical 3) Economic 4) Co-benefits risks; 5) Governance social considerations. Optimising ecological factors such as species selection use currents, alongside low-cost biodegradable rafts in theory could see scaled globally. An area 400,000km 2 or 16.4 billion would be needed 1 gigatonne CO fixation given roughly 16 25m each per tonne fixation. However, (calculated from net primary productivity) (carbon permanently removed atmosphere) are fundamentally different processes, yet distinction often overlooked. Quantifying remains elusive several outstanding oceanic biogeochemical For example, displacement phytoplankton communities their associated via nutrient reallocation a critical knowledge gap understanding change mitigation potential afforestation. Ocean also carries complex risks to marine ecosystems, impact on benthic deposition. Additionally, governance challenges exist legality operation relation treaties. The still its infancy, while there large research gaps, further investment into should before can adequately compared against suite ocean-based strategies.