Biophysical potential and uncertainties of global seaweed farming

Abstract

International climate goals require over 5 gigatons/year (Gt/year) of CO2 to be removed from the atmosphere by midcentury. Macroalgae mariculture has been proposed as a strategy for such carbon dioxide removal (CDR). However, the global potential for seaweed cultivation has not been assessed in detail. Here, we develop and use a dynamic seaweed growth model, the Global MacroAlgae Cultivation MODeling System (G-MACMODS), to estimate potential yields of four different types of seaweed worldwide, and test the sensitivity of these estimates to uncertain biophysical parameters under two nutrient scenarios (one in which the surface ocean nutrient budget is unaltered by the presence of seaweed farms, and another in which seaweed harvest is limited by nutrients that are resupplied by vertical transport). We find that 1 Gt of seaweed carbon could be harvested in 0.8% of global exclusive economic zones (EEZs; equivalent to ~1 million km2) if farms were located in the most productive areas, but potential harvest estimates are highly uncertain due to ill-constrained seaweed mortality and nitrogen exudation rates. Our results suggest that seaweed farming could produce climate-relevant quantities of biomass carbon and highlight key uncertainties to be resolved by future research.

Economic and biophysical limits to seaweed farming for climate change mitigation

“…G-MACMODS is a nutrient-constrained, biophysical macroalgal growth model with inputs of temperature, nitrogen, light, flow, wave conditions and amount of seeded biomass 30,53 , that we used to estimate annual seaweed yield per area (either in tons of carbon or tons of dry weight biomass per km 2 per year) 33,34 . In the model, seaweed takes up nitrogen from seawater, and that nitrogen is held in a stored pool before being converted to structural biomass via growth 54 .…”

“…The conversion from stored nitrogen to biomass is based on the minimum internal nitrogen requirements of macroalgae, and the conversion from biomass to units of carbon is based on an average carbon content of macroalgal dry weight (~30%) 55 . The model accounts for farming intensity (sub-grid-scale crowding) and employs a conditional harvest scheme, where harvest is optimized on the basis of growth rate and standing biomass 33 .…”

“…For seaweed CDR, after the seaweed is harvested, it can either be sunk in the same location that it was grown, or be transported to a more economically favourable sinking location where more of the seaweed carbon would remain sequestered for 100 yr (see 'Distance to optimal sinking point' below). Immediately post-harvest, the seaweed still contains a large amount of water, requiring a conversion from dry mass to wet mass for subsequent calculations 33 :…”

“…Seaweed biomass harvested. We used spatially explicit data for seaweed harvested globally under both ambient and limited-nutrient scenarios from the G-MACMODS seaweed growth model 33 .…”

“…Details of our analytic approach are described in Methods. In summary, we use outputs from a newly developed biophysical model (G-MACMODS) 33,34 to estimate potential harvest of four different seaweed types (tropical red, tropical brown, temperate red and temperate brown; Supplementary Fig. 1) at a resolution of 1/12° (~9 km at the equator) globally.…”