Carbon removal and climate change mitigation by seaweed farming: A state of knowledge review

The pressing need to mitigate the effects of climate change is driving the development of novel approaches for carbon dioxide removal (CDR) from the atmosphere, with the ocean playing a central role in the portfolio of solutions. The expansion of seaweed farming is increasingly considered as one of the potential CDR avenues among government and private sectors. Yet, comprehensive assessments examining whether farming can lead to tangible climate change mitigation remain limited. Here we examine the results of over 100 publications to synthesize evidence regarding the CDR capacity of seaweed farms and review the different interventions through which an expansion of seaweed farming may contribute to climate change mitigation. We find that presently, the majority of the carbon fixed by seaweeds is stored in short-term carbon reservoirs (e.g., seaweed products) and that only a minority of the carbon ends up in long-term reservoirs that are likely to fit within existing international accounting frameworks (e.g., marine sediments). Additionally, the tiny global area cultivated to date (0.06 % of the estimated wild seaweed extent) limits the global role of seaweed farming in climate change mitigation in the present and mid-term future. A first-order estimate using the best available data suggests that, at present, even in a low emissions scenario, any carbon removal capacity provided by seaweed farms globally is likely to be offset by their emissions (median global balance net emitter: -0.11 Tg C yr