Carbon Registries and Serial Numbers: Checking a Credit Yourself

What a carbon registry records, how issuance, transfer and retirement work, and how a serial number lets you confirm that a listed credit is real and unused.

Every listing on our marketplace names a registry and gives a registry ID. Those two fields matter more than the photographs or the project story, because they point to the one place where the credits can be checked independently of us. This article explains what that place is and how to use it before you commit to a purchase.

What a registry is for

A carbon registry is an electronic ledger run by a standard or programme. Its job is simple: keep a single, authoritative record of how many credits exist for a project, who holds them, and which ones have already been used.

A credit is not a document you can photocopy. It is an entry in that ledger. If the ledger says a unit is retired, it is finished, whatever any certificate, invoice or email might suggest. That is why we treat the registry, and not our own desk, as the official record of issuance and retirement.

The main programmes each run their own registry. Verra keeps one for VCS projects, Gold Standard runs its impact registry, and the Global Carbon Council operates a registry for its programme. They differ in layout but record the same basic events.

The life of a credit, in registry terms

Think of a credit as moving through four states.

Project registration. The project is listed on the registry with its own ID, methodology, location and documents. At this point no credits exist yet.
Issuance. After an independent verifier confirms a quantity of reductions or removals for a monitoring period, the registry creates that many units and places them in the developer's account. Each unit represents one tonne of CO₂ equivalent (tCO₂e).
Transfer. Units move from one account holder to another. A developer may sell to a trader, the trader to a buyer. Every move is logged.
Retirement or cancellation. The holder takes units out of circulation permanently, usually naming a beneficiary and a purpose. Some registries use "retirement" for voluntary claims and "cancellation" for compliance use such as CORSIA, but the effect is the same: the units can never be sold or claimed again.

Some units also leave circulation without being claimed by anyone, for example when a share of a forestry project's issuance is set aside in a buffer pool to cover future losses.

How serial numbers stop a tonne being used twice

When units are issued, each batch receives a serial number range. The string usually encodes the registry, the project ID, the vintage (the year the reduction happened) and a start and end number for the block.

Because every tonne sits inside exactly one numbered block, and every block can sit in exactly one account and one status at a time, the same tonne cannot be retired by two different buyers. If a seller transfers part of a block, the registry splits it into two new ranges so ownership stays clear.

For a buyer, the serial range is the most useful piece of evidence you will receive. A retirement certificate without it is a nice piece of paper. With it, anyone can look up the record.

Checking a listing before you buy

You do not need an account to search most public registries. Here is the routine we suggest when a listing interests you.

Check	Where to look	What should match
Project exists	Search the registry by the ID on our listing	Name, country, methodology and developer broadly agree with the listing
Status	Project page on the registry	Registered, not withdrawn, rejected or on hold
Credits issued	Issuance history tab	Vintages on the listing appear as issued, unless the delivery type is Forward
Volume plausibility	Issuance and retirement totals	The volume offered is not larger than what remains unretired for that vintage
Documents	Documents tab	The PDD, validation and verification reports match the versions you receive

A Forward lot will not show issued units for that vintage yet, which is expected. For Spot and Issued lots, the units should already be on the registry. If the numbers do not line up, ask us before going further. There is often an innocent reason, such as a recent issuance not yet reflected on a summary page, but it should be explained in writing.

After the purchase

Once you buy, one of two things happens. Either the units are transferred into your own registry account, or they are retired on your behalf in the name of the beneficiary you choose. Our page on registry accounts for buyers sets out the trade-offs.

If they are retired for you, you should receive the serial numbers and the retirement certificate. Search the registry's public retirement list for those serials and confirm three things: the beneficiary name is spelled as agreed, the quantity is right, and the retirement reason fits your purpose. Our guide to retirement certificates covers what the certificate itself should contain.

For CORSIA, cancellation is followed by a report to the national authority, which in India is the DGCA. Airlines should keep the registry record alongside the cancellation evidence, because that is what a verifier and the authority will ask for. Our CORSIA eligible units checklist covers the extra conditions.

Warning signs

A registry ID that returns no result, or returns a different project.
A seller who cannot say which account the units sit in.
Serial numbers offered only as a screenshot, with no way to search them.
A "certificate" issued by a broker rather than generated by the registry.
Pressure to pay before availability and registry details are confirmed in writing.

None of these automatically means fraud, but each deserves a direct question.

When a listing catches your eye, note its registry ID and look it up before you send us a request. We are happy to walk through the record with you on a call through our contact page.