Double counting decides whether a carbon credit means anything. Here's how the same tonne gets claimed twice — and how markets try to stop it.
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This article is stop 8 of 10 on Blue Carbon for Buyers — From first question to a defensible contract.
When a seafood company in Rotterdam retires a blue carbon credit generated by a seagrass project in Portugal, a quiet question travels with that tonne of carbon: whose climate goal does it count toward? The company's net-zero target? Portugal's national inventory under the Paris Agreement? Both? If the answer is both, the atmosphere only improves once while the paperwork improves twice — and that is double counting, arguably the single most corrosive failure mode in carbon markets.
We wrote a plain-language introduction to double counting elsewhere in the Journal. This piece goes a level deeper: where double counting actually occurs, why blue carbon projects are especially exposed to it, and how corresponding adjustments — the wiring installed by Article 6 of the Paris Agreement — are meant to make a tonne belong to exactly one claimant. At Tidewright, our Canopy Ledger platform is designed around this ownership question from the first drone flight onward, because a credit with unclear ownership is not an asset; it is a liability waiting to be cancelled.
The atmosphere is an accounting system with no auditor. When a company claims to be carbon neutral using credits, it is asserting a zero-sum ledger: its emissions went up in one column, and its purchased removals went down in another. Double counting breaks this in two ways.
First, two buyers (or a buyer and a host country) can claim the same reduction, so the world records twice as much climate progress as physically occurred. Second — and just as damaging — market actors lose confidence that any tonne is real, which deflates prices, strangles project finance, and punishes the honest projects alongside the sloppy ones. Trust is the market's only collateral.
The project developer sells the same credit to two buyers, sometimes through different registries. This is straightforward fraud or sloppiness, and it is exactly what unique serial numbers and cross-registry retirement checks exist to prevent. (We cover the plumbing in our piece on carbon registries.)
A credit is split across overlapping claims — for example, a buyer claims the "neutrality" of a flight while the project's host community separately claims the restored forest toward a community climate pledge. Both can be well-intentioned; the atmosphere only registers one.
This is the form that matters most at scale. Under the Paris Agreement, every country maintains a national inventory of emissions and removals. If a mangrove restoration in Indonesia sequesters 100 tonnes, Indonesia's inventory will record those tonnes when it reports progress toward its nationally determined contribution (NDC). If a foreign company also buys those 100 tonnes as offsets without any adjustment, the reduction is counted twice globally.
Even without a formal Article 6 trade, a voluntary credit claim can overlap with a national inventory number. This is the grey zone where most voluntary market activity currently lives, and where the phrase corresponding adjustment becomes decisive.
A corresponding adjustment (CA) is a bookkeeping entry a host country makes when it authorises a credit for use toward another party's target. The country adds the transferred amount back to its own tally, in effect saying: this progress is yours to claim, not mine.
Mechanically it is elegant. Politically it is thorny, for reasons buyers don't always appreciate:
Blue carbon projects are unusually exposed to all three. Their assets sit in intertidal and subtidal zones where legal regimes overlap, their carbon travels across jurisdictions via currents and fisheries, and many of the world's best blue carbon sites are in countries whose carbon-rights statutes are works in progress. We explore the Article 6 machinery itself in Article 6 and the Ocean.
The market's compromise answer has been to fork the word "offset" in two:
Authorised credits (with a corresponding adjustment) can be used toward a compliance target or a hard net-zero claim. They are scarce and carry price premiums.
Contribution claims (no CA) let a company say: we funded removal of 100 tonnes in a verified project — without asserting that those tonnes neutralise its own emissions for target purposes. The host country keeps the inventory credit. This avoids double claiming, at the cost of a softer marketing claim buyers once found irresistible.
Honest labelling between these two is where integrity now lives. A company claiming "carbon neutral" on the strength of non-adjusted credits is making a national-inventory-shaped promise it does not own.
Ownership of blue carbon is not answered by the registry alone. Before purchasing, trace the claim through five layers:
If sellers cannot answer all five crisply, the credit may be perfectly real biologically and still be a legal and reputational minefield. Our blue carbon project lifecycle piece shows where these questions slot into development. And if your project sits on Indigenous sea country, note that tenure and benefit-sharing questions precede all registry questions — see Blue Carbon on Sea Country.
As a fictional but earnest developer, Tidewright's approach on Canopy Ledger is to treat double counting as a design constraint, not a disclosure afterthought. In our illustrative project documents, each restored hectare's carbon and biodiversity outcomes are logged against a clear claim-type register: what has been retained for host-community goals, what has been authorised, and what is sold only as contribution. It is the kind of discipline the whole market is converging on — and the kind of question a buyer owes a project before money moves.
No. Some forms are outright fraud (double selling), but buyer–host overlap is often the default outcome of markets that predate Article 6. Whether it is culpable depends on disclosure: the problem is claiming rights you do not clearly hold, not that inventories behave like inventories.
Not under current rules, and the debate is genuine. Some argue CAs should apply only to credits used toward compliance-style offset claims; others want universal application to stop all double claiming. The direction of travel in integrity frameworks is toward clearer claim labelling either way.
Not necessarily — it funds real, verified restoration. What changes is what you may say about it: you financed a tonne of removal; you may not also book it against your net-zero target. Many buyers now regard the honest contribution claim as more defensible than a stretchy offset claim.
Because coastal carbon sits in overlapping legal jurisdictions (seabed, foreshore, EEZ), moves physically across boundaries, and is concentrated in jurisdictions where carbon-rights law is still being written. Every ambiguity is a possible second claim.
Only if the host country's monitoring program detects and reports them. Good projects share data so that inventories and registries can be reconciled — reconciliation is precisely how double counting gets caught.
If two parties claim the same tonne of carbon, the atmosphere loses. Here is what double counting is, why Article 6 exists, and what buyers should check.
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How Article 6 actually works for ocean carbon — ITMO trades, corresponding adjustments, the 6.4 mechanism — and what it changes for blue carbon buyers and coastal nations.
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Follow a blue carbon credit through its full project cycle — feasibility, design, validation, monitoring, verification, issuance and retirement.
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