Open-ocean cultivation of seaweed (macroalgae) biomass, termed ocean afforestation (OAFF), is a nature-based method currently in the planning stage for carbon dioxide removal (CDR) at scales relevant to climate intervention. Bach et al.1 cast doubt on its potential efficacy by claiming that “two biogeochemical feedbacks, nutrient reallocation and calcification by encrusting marine life, reduce the CDR efficacy of Sargassum by 20–100%” which has negatively influenced the perception hence evaluation of OAFF compared to the unspecified, technological methods favoured by the authors. In addition to the repudiation of these points by Wang et al.2, we show that the green and golden tides3 of Ulva prolifera and Sargassum horneri, respectively, that build up dense mats of free-floating biomass in the millions-of-tonnes range in the eutrophic, central Yellow Sea are more appropriate “natural analogues” for future OAFF than the diffuse Great Atlantic Sargassum Belt (GASB), chosen by Bach et al.1. The dense Yellow Sea algal rafts demonstrate that disparate macroalgal species are capable of building up prodigious, free-floating, harvestable biomass at rates depending on the supplies of seeding stocks (from macroalgal thalli fragments) and nutrients (by eutrophication) that can serve as potential crops for seaweed farms based on artificial upwelling of nutrient-rich water in the vast spaces of the open ocean. |
