01 — The distinction that matters
If you've seen a headline claiming seaweed "captures X tonnes of CO₂", it's worth pausing before you trust the number. How much carbon seaweed sequesters depends entirely on a distinction most figures gloss over: the difference between carbon uptake and carbon sequestration. They are not the same thing.
Uptake is not the same as sequestration
Carbon uptake is how much CO₂ seaweed absorbs from the water as it grows. Carbon sequestration is how much of that carbon is durably locked away over the long term, out of the atmosphere. Most headline "seaweed captures X" claims describe uptake — the fast, visible part — not sequestration, the part that actually matters for the climate. The two can be very different numbers, and conflating them is the single biggest source of confusion around seaweed carbon sequestration.
The reason this matters: carbon that is taken up but then breaks down and returns to the atmosphere within months does little for the climate. Only the share that reaches durable storage counts.
What we do know: seaweed takes up carbon fast
Start with the part that's well established. Seaweed — and kelp in particular — is among the most productive primary producers on Earth. It grows extraordinarily quickly, and as it grows it draws down large amounts of carbon from the surrounding water. On a per-area basis, healthy kelp forests rank with the most productive ecosystems anywhere. That much is not seriously disputed.
So if the question is "does seaweed absorb a lot of carbon as it grows?", the answer is a confident yes. The harder question — and the one that determines kelp carbon storage in any climate-relevant sense — is what happens to that carbon next.
- Uptake
- How much CO₂ seaweed absorbs as it grows. Large, fast, well established.
- Export
- The share of that carbon that sinks to the deep sea as detached fronds and organic matter.
- Sequestration
- The fraction that stays locked away long-term. Genuinely uncertain and still debated.
The hard question: how much is actually sequestered?
Here's the complication. Seagrass and mangroves bury carbon directly in the sediment beneath them, where it can stay for a long time and be measured with some confidence. Seaweed doesn't do that — it grows attached to rock, not soft sediment. For seaweed's carbon to count as sequestered, a meaningful share has to be exported away from the reef: detached fronds sinking into deep water, plus particulate and dissolved organic carbon carried below depths where it won't quickly cycle back to the atmosphere.
How much actually reaches durable storage — and how reliably we can measure it — is genuinely debated among scientists. Some of the carbon is exported and sequestered; some is consumed, respired, or returns to the surface relatively quickly. Pinning down the durable fraction across different coastlines, depths and currents is hard, and the science is still developing.
One influential estimate gives a sense of the scale researchers are discussing. Krause-Jensen and Duarte (2016) estimated that wild macroalgae could sequester on the order of ~170 million tonnes of carbon per year globally. That figure is widely cited — but it's important to treat it as an estimate with very large uncertainty, not a settled number. Different methods and assumptions can move the answer substantially, which is exactly why no single agreed figure exists.
No single agreed number — and crediting is still emerging
Putting it plainly: there is no single agreed answer to how much carbon seaweed sequesters. Measurement methods are still being developed, estimates carry wide error bars, and crediting frameworks that would let macroalgae carbon be formally counted and traded are not yet established. If you want the broader context for how ocean carbon is accounted for in this country, see our explainer on blue carbon in Australia, and on the state of blue carbon credits in Australia.
Any product or claim offering a precise, verified seaweed sequestration tonnage today should be read with caution. The honest position is that the climate value of seaweed carbon is real but not yet precisely quantified.
Where Ocean Greens fits
We restore kelp because its value is broad and well-evidenced: biodiversity, fisheries, coastal protection, and carbon drawdown as it grows. We also believe its long-term carbon role is genuinely promising. But we're upfront that macroalgae carbon accounting is still emerging, and we won't put a specific per-hectare sequestration number on our work, because the science doesn't yet support one. You can read how the restoration itself works on our seaweed farming page, and how kelp's carbon role compares to forests in giant kelp vs trees.
Common questions
Does seaweed really store carbon?
Seaweed takes up large amounts of carbon as it grows — that's well established. Whether it's durably stored depends on how much is exported to the deep sea, below depths where it won't quickly return to the atmosphere. Some is, but how much reaches long-term storage is still debated.
What's the difference between carbon uptake and carbon sequestration?
Uptake is how much CO₂ seaweed absorbs as it grows. Sequestration is how much of that carbon is durably locked away over the long term. Most "seaweed captures X" figures describe uptake, not sequestration — and the two can be very different numbers.
Can you measure how much carbon seaweed sequesters?
Not yet precisely. Because seaweed grows on rock rather than being buried in sediment, measuring how much of its carbon reaches durable deep-sea storage is difficult. Methods are still developing, and there's no single agreed global figure.
Can you buy seaweed carbon credits in Australia?
Not in any established way. Crediting frameworks for macroalgae carbon are still emerging and haven't been formalised here. Be cautious of any product claiming a precise, verified seaweed sequestration figure today.