Economic and Biophysical Limits to Seaweed Farming for Climate Mitigation

Net-zero greenhouse gas (GHG) emissions targets are driving interest in opportunities for biomass-based carbon dioxide removal (CDR) and carbon-neutral products, including from marine sources such as seaweed. Yet the biophysical and economic limits to farming seaweed at scales relevant to the global carbon budget have not been assessed in detail. We use coupled seaweed growth and technoeconomic models to estimate the costs of global seaweed production and its potential climate benefits, systematically testing the relative importance of model parameters. We consider two options for seaweed-based climate benefits: (1) sinking seaweed to the deep sea for CDR, and (2) replacing conventional products with seaweed-based products to avoid GHG emissions. We find that our most optimistic modeled costs are dependent on low farming costs, high seaweed yields, and assumptions that almost all carbon in seaweed is removed from the atmosphere and that seaweed products can displace products with substantial embodied non-CO2 GHG emissions. Moreover, the gigaton-scale climate benefits we model would require farming very large areas (>100,000 km2)—a >40-fold increase in the area currently farmed. Our results therefore suggest that seaweed-based climate benefits may be feasible, but targeted research and demonstrations are needed to further reduce economic and biophysical uncertainties.