Economic and biophysical limits to seaweed farming for climate change mitigation

Abstract

Net-zero greenhouse gas (GHG) emissions targets are driving interest in opportunities for biomass-based negative emissions and bioenergy, 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 related climate benefits, systematically testing the relative importance of model parameters. Under our most optimistic assumptions, sinking farmed seaweed to the deep sea to sequester a gigaton of CO₂ per year costs as little as US$480 per tCO₂ on average, while using farmed seaweed for products that avoid a gigaton of CO₂-equivalent GHG emissions annually could return a profit of $50 per tCO₂-eq. However, these costs depend on low farming costs, high seaweed yields, and assumptions that almost all carbon in seaweed is removed from the atmosphere (that is, competition between phytoplankton and seaweed is negligible) 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 (>90,000 km²)-a >30-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.

Fig. 1. Seaweed production costs.

a–f, Estimated seaweed production costs vary considerably depending on assumed costs of farming capital, seeded lines, labour and harvest (excluding transport of harvested seaweed). Across ambient-nutrient simulations, average farming cost in the 1% of global ocean areas with lowest cost ranges from $190 tDW⁻¹ (a) to $2,790 tDW⁻¹ (c), with a median of $880 tDW⁻¹ (b). Regional insets (d–f) reveal small-scale features in particularly low-cost areas. Supplementary Fig. 2 shows maps for limited-nutrient simulations.

Fig. 2. Net cost of potential seaweed climate benefits.

a–d, Costs of using farmed seaweed to sequester carbon or avoid GHG emissions vary in space according to estimated production costs as well as spatially explicit differences in the costs and net emissions of transportation, sinking or conversion, and replacement of conventional market alternatives with seaweed products. Differentiation between seaweed product groups (b–d) is based on emissions avoided by seaweed products and market value for each product type. Maps show costs when propagating the most optimistic assumptions (5th percentile costs) from ambient-nutrient simulations. Average cost in the 1% of global ocean areas with lowest cost ranges from $20 per tCO₂-eq avoided when seaweed is used for food (b) to $540 per tCO₂ sequestered by sinking seaweed (a). Supplementary Figs. 6 and 7 show maps for limited-nutrient simulations and median costs, respectively.

Fig. 3. Key cost sensitivities of seaweed production and climate benefits.

a–c, Across Monte Carlo simulations in the 2% of ocean grid cells where costs are lowest, estimated seaweed production cost is especially sensitive to the seaweed yield amount and seeded line cost (a), whereas costs of carbon sequestration (b) and GHG emissions avoided (c) are strongly influenced by the fraction of seaweed carbon that corresponds to an equivalent amount removed from the atmosphere and the assumed emissions avoided by seaweed products, respectively, in addition to seaweed yield and seeded line cost. d–f, Kernel density plots for the most important parameters in the cheapest 1% ocean areas, showing that the lowest production and climate benefit costs depend upon seaweed yield being at or above the median of potential seaweed yields (d), an assumed atmospheric removal fraction of >0.6–0.8 (e) and avoided emissions >2.5 tCO₂-eq tDW⁻¹ (f). Supplementary Fig. 9 shows cost sensitivities in limited-nutrient simulations.

Fig. 4. Cumulative potential climate benefits of large-scale seaweed farming.

a,b, Total GHG emissions avoided (a) or carbon sequestered (b) each year could reach gigaton scales if seaweed were farmed over large areas of the ocean. Bars show the potential climate benefits as a function of the lowest-cost ocean area (0.1% of ocean area is roughly 360,000 km², nearly the area of Germany and 130 times the total area of current seaweed farms), and colours indicate the average cost (or profit) per tCO₂-eq emissions avoided or tCO₂ sequestered using optimistically low net costs (5th percentile) from ambient-nutrient simulations. Supplementary Figs. 10 and 11 show cumulative potential and costs at the median and in limited-nutrient simulations.