Double counting in carbon markets and how registries try to stop it

21 September 2026

Double counting is the carbon market's equivalent of spending the same dollar twice. It happens when a single emissions reduction or removal is claimed by more than one entity, or counted toward more than one target. Without robust safeguards, double counting undermines the entire purpose of carbon credits - making climate claims meaningless. Registries exist to prevent this, but their systems are not foolproof.

What double counting actually looks like

Double counting takes several forms. The most obvious is when a carbon credit is issued by one registry, then tokenised and sold on a blockchain platform without retiring the underlying credit - so the same tonne of CO₂ is represented by both a registry credit and a token. Less obvious but equally damaging is when a credit is counted toward a company's voluntary net-zero goal while also being counted toward a country's nationally determined contribution under the Paris Agreement. Another form occurs when two buyers purchase the same tokenised credit from different secondary market sellers, each believing they own the exclusive rights to that tonne.

How registries prevent double counting: serial numbers and retirement

Registries like Verra and Gold Standard assign each carbon credit a unique serial number when it is issued. That number travels with the credit through every transfer. When a credit is retired - meaning it is permanently taken out of circulation to offset an emission - the registry marks that serial number as used. No one can transfer or claim that credit again.

This system works because the registry maintains a single, authoritative ledger. A credit cannot exist in two wallets or accounts simultaneously. When a tokenised credit is created, the project developer must first retire the underlying credit in the registry and then issue a corresponding token. The retired serial number proves the credit is no longer available for any other use.

The problem with tokenised credits: registry gaps

Tokenised carbon credits introduce a new vulnerability. If a project developer issues tokens on a blockchain without first retiring the credits in the official registry, the same tonne exists in two places. The registry still shows the credit as active, while the blockchain shows a token that someone can buy, sell, or retire again.

Reputable tokenisation platforms avoid this by integrating directly with registries. They retire the credit in the registry before minting the token, and often include the retirement serial number in the token's metadata. But not all platforms do this. Some issue tokens based on credits they claim to hold in a separate account, without any public proof of retirement. Buyers of those tokens have no guarantee that the underlying credit has not been sold to someone else.

How to check for double counting

You can verify a credit's status by looking up its serial number in the issuing registry's public database. Verra's registry, for example, shows whether a credit is active, transferred, or retired. For a tokenised credit, the token's metadata should include the serial number and ideally a link to the retirement entry. If the token claims to be backed by a credit but the serial number is missing or the registry shows that credit as still active, double counting is likely.

Some blockchain platforms also use "proof of retirement" systems, where the retirement transaction is recorded on-chain and linked to the registry entry. This creates a verifiable chain from the registry to the token. But even this is only as reliable as the platform's honesty - if the platform never actually retired the credit, the on-chain record is meaningless.

Limits of registry controls

Registries can only track credits within their own systems. They cannot prevent a company from counting the same credit toward both a corporate target and a national target unless the credit is explicitly labelled for one use. The Paris Agreement's Article 6 attempts to address this by requiring "corresponding adjustments" when credits are traded between countries, but voluntary carbon markets largely operate outside that framework.

Another limit: registries rely on project developers and third-party auditors to report accurately. If a project inflates its emission reductions, the credits issued are based on false data. The registry's serial number system prevents double spending of those credits, but it does not correct the underlying error. The credit is real in the registry's ledger, but the claimed emission reduction never happened.

What to watch for

When evaluating a carbon credit token or a company's offset claim, ask three questions: Is the credit retired in the official registry? Is the retirement serial number publicly available? And does the claim match the credit's scope - for example, is it labelled as a voluntary offset or a compliance instrument? If any answer is unclear, the risk of double counting is real.

Registries provide the essential infrastructure to prevent double counting, but they cannot enforce honesty outside their own ledgers. The buyer's own verification is the final safeguard.

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