Is seaweed really a climate solution?

Seaweed has excellent public relations. It needs no irrigation, no fertiliser and no farmland, it grows fast, and it absorbs carbon dioxide as it does. It ends up in food, thickeners, fertilisers and packaging. On paper it looks like the climate fix everyone has been waiting for.

Farming it is already big business. The FAO counts 36.5 million tonnes of farmed algae in 2022, nearly all of it grown in Asia. The question is what that harvest does for the climate.

Every growing seaweed takes up carbon; that part is settled. Removal is a higher bar. To count as carbon dioxide removal, extra carbon has to be kept out of the atmosphere for a long time (buried in deep sediments, say, or locked into long-lived products), and the emissions from boats, processing and transport have to be subtracted. Kelp that is harvested and eaten returns most of its carbon to the air within weeks or months.

The most thorough look at this so far is a 2024 state-of-knowledge review in Science of the Total Environment, which drew on more than 100 publications. Its findings were sobering. Most of the carbon fixed by farmed seaweed currently ends up in short-term reservoirs such as the products themselves, and only a minority reaches long-term stores like marine sediments. The global area under cultivation is tiny, about 0.06% of the estimated extent of wild seaweed, and the authors' first-order estimate suggested that, at today's scale, emissions from farming may cancel out whatever removal takes place.

That is a verdict on seaweed as a carbon sink, not on seaweed farming. Its stronger climate case lies elsewhere: as food and raw material that need no land or fresh water, and as a replacement for more emissions-intensive products. A 2024 modelling study of British Columbia reached the same conclusion: the climate benefits came mainly from seaweed products displacing higher-emission alternatives, because sequestration in the sea was relatively inefficient. Avoided emissions are real and worth having. They just shouldn't be sold as carbon removal, and the two shouldn't be traded as if they were the same credit.

Farms also interact with the ecosystems around them, positively and negatively, in ways that depend on species, site and method. Those effects need measuring locally rather than assuming.

The industry doesn't need the carbon-sink story. Seaweed is a low-input crop that feeds people, supplies materials and can cut emissions elsewhere in the economy. That is a strong enough case on its own, and it has the advantage of being backed by evidence.