Seaweed absorbs carbon dioxide as it grows. That much is not in dispute. Whether farmed seaweed removes carbon from the atmosphere in a lasting way, and whether it can be counted as a climate benefit, is a much harder question. The honest answer is that it depends on what happens to the seaweed next, and that for most farmed seaweed the case for long-term storage is weak. Companies that source seaweed are increasingly asked about its climate credentials. This article sets out what can be said with confidence and what cannot.
Uptake is not storage
Like land plants, seaweeds use photosynthesis to turn carbon dioxide into organic matter. A growing seaweed farm is therefore taking up carbon. But for that to count as removal from the atmosphere in climate terms, the carbon has to stay out of circulation for a long time, often taken to mean a century or more.
Most farmed tropical seaweed is harvested, dried and processed into food ingredients such as carrageenan. Those products are eaten or break down, and their carbon returns to the atmosphere relatively quickly. Harvested seaweed is best thought of as part of a short carbon cycle, much like a food crop on land.
What the science does say
- Seaweeds are highly productive and take up substantial carbon while they grow.
- Some of the material that breaks off from wild and farmed seaweed, and some dissolved organic carbon it releases, can reach the deep sea or be buried in sediments, where it may remain for long periods.
- Seaweed farming needs no feed, fertiliser or freshwater, so its production footprint compares favourably with many land crops; see our article on seaweed as a low-input crop.
- Seaweed farms can have local effects on water chemistry, including taking up nutrients, which researchers are studying as a possible benefit around farms.
What the science does not (yet) say
- How much of the carbon taken up by a given farm is actually stored for the long term. Estimates vary widely and are difficult to measure.
- That farmed seaweed qualifies as “blue carbon” in the same way as mangroves, seagrass meadows and salt marshes. Those ecosystems have established methods for carbon accounting; seaweed does not, at least not in widely accepted standard guidance.
- That sinking seaweed on purpose is a proven, safe removal method. Proposals exist, but the ecological effects and the verification are debated.
- That seaweed ingredients are “carbon negative” simply because the seaweed absorbed carbon while growing.
Climate change also affects seaweed farming itself, through warmer water, heat stress and changes in weather. Our article on seasons, disease and climate risk looks at that side of the relationship.
How to talk about it responsibly
For buyers and brands, a few principles keep sustainability communications on safe ground:
- Separate footprint from removal. “Grown without feed, fertiliser or freshwater” is defensible. “Removes carbon from the atmosphere” generally is not, for a harvested ingredient.
- Avoid numbers you cannot trace. Carbon figures for seaweed vary so widely that a single headline number is likely to mislead.
- Credit the social value directly. Livelihoods, fair prices and organised farmer groups are real, documentable benefits that do not need a carbon claim to support them.
- Watch the research. Measurement methods are improving, and what can be claimed may change. Revisit claims as evidence develops.
For the social side of seaweed sourcing, see our article on social impact and ESG, and for the investment picture, investing in Indonesia’s seaweed blue economy.
Frequently asked questions
Is seaweed farming good for the climate?
It is a low-input form of food production, which is a genuine advantage. Whether it removes carbon in a lasting way is uncertain and, for harvested seaweed, generally not demonstrated.
Can seaweed farms earn carbon credits?
Methods for crediting seaweed carbon are still under development and debate. Buyers should treat any seaweed carbon credit with caution and ask exactly how storage was measured and verified.
Why are mangroves and seagrass treated differently?
They build up carbon in their own sediments, in place, which can be measured. Seaweed mostly grows on rock or ropes, and any carbon it stores ends up elsewhere, which is much harder to track.
Working with Kospermindo
Kospermindo has supported seaweed farmer groups in South and Central Sulawesi since 2002. Development partners interested in seaweed livelihoods can learn about our partnership approach.