How carbon credits work comes down to one tradable unit: each credit stands for one tonne of greenhouse gas emissions reduced, avoided or removed by a certified project, recorded in a registry and retired once used.
This article covers the unit and its life cycle. For the wider picture, start with the hub guide on what is carbon offsetting.
What a single credit represents
A carbon credit is a measured claim about the atmosphere. Gold Standard defines it as one tonne of greenhouse gas emissions that has been reduced, avoided or removed from the atmosphere through a certified climate project. The same idea appears in its description of voluntary markets, where participants buy credits that represent one tonne of carbon dioxide equivalent, reduced or removed through verified climate projects.
Two points follow from that wording. First, the unit is a mass of gas, not a payment, a certificate of good intent or a share in a project. Second, the phrase carbon dioxide equivalent matters, because many projects deal with gases other than carbon dioxide. Those gases are translated into a common unit using global warming potential. The US Environmental Protection Agency describes that measure as a comparison of the warming from 1 ton of a gas relative to 1 ton of carbon dioxide, and notes that the period usually used is 100 years.
A credit therefore does not describe how many trees were planted or how many stoves were handed out. It describes the amount of emissions the project is judged to have kept out of the air, or taken out of it, when compared with a stated baseline.
How a project earns credits
Credits are not created by buying them. They are earned by a project that follows a methodology set by a crediting programme. The project developer writes up the activity, estimates what emissions would have been without it, and sets out how the result will be measured. An independent body then reviews the design and, later, the measured outcome.
That second review is a core quality requirement. The Integrity Council for the Voluntary Carbon Market, in its Core Carbon Principles, says a programme must have requirements for independent third-party validation and verification of mitigation activities. Only after the verified result is accepted does the programme issue credits for the period in question.
Two further tests decide whether the credits mean what they say. The first is additionality: the same principles say the benefit must be of a kind that would not have occurred without the incentive created by credit revenues. The second is permanence: the benefit shall be permanent or, where there is a risk of reversal, measures must be in place to address those risks and compensate reversals. Both are explored in more depth elsewhere on this site, and both are why two credits that look identical on paper can differ in real climate value.
Issuance, registries and serial numbers
Once credits are issued they are entered in a registry, which works like a ledger. Each credit is identified so that it can be followed from issue to final use. The Core Carbon Principles state that a programme shall operate or make use of a registry to uniquely identify, record and track mitigation activities and credits issued, so that credits can be identified securely and unambiguously.
In practice this means that every credit can be given a serial number. Gold Standard says that in its system users can track credits using unique serial numbers, and that its credits are issued, transferred and retired through a public registry. A buyer who wants to confirm a claim can look up the entry rather than rely on a seller’s statement.
The registry also guards against a basic accounting error. Another of the ten Core Carbon Principles is headed no double-counting, which is the rule that the same reduction or removal must not be claimed twice. A single, public record of each credit is the practical tool for upholding it. The Council describes the principles as ten fundamental, science-based principles for identifying high-quality credits.
Trading, retirement and what happens next
After issuance a credit can change hands one or more times, directly or through intermediaries. Ownership transfers are recorded, so the registry keeps showing who currently holds each unit. Prices vary widely between credit types, and that variation reflects differences in project type, standard, vintage and perceived quality, so no single figure describes the market.
The end of a credit’s life is retirement. A retired credit is permanently taken out of circulation, which stops it from being resold or counted by someone else. This is the moment that turns a credit into an offset: the holder retires it against a stated amount of its own emissions. Gold Standard notes that, once purchased and retired, credits give organisations a transparent and verifiable way to take responsibility for emissions that remain while they continue to decarbonise, and that anyone can verify retired credits in the registry.
Retirement is also the point at which the buyer’s claim begins. What may fairly be said depends on the credit’s quality and on the buyer’s wider climate plan. A business that publishes climate claims should check the advertising and consumer rules where it operates; in the United Kingdom, for instance, the Competition and Markets Authority has issued guidance to help businesses meet their obligations under consumer protection law when making environmental claims.
Where credits are bought and sold
Credits trade in two settings. In regulated schemes, governments set the rules and demand arises from legal limits. In the voluntary setting, buyers choose to purchase and the rules come from private standards. The structure of the second setting, including who takes part and how it relates to regulated trading, is covered in the article on the voluntary carbon market.
Everyday examples show the unit in use. A traveller might retire credits against the emissions of a trip, and the arithmetic behind that estimate is set out in the article on how to offset flight emissions. A company might retire credits against emissions it has measured but not yet eliminated. In both cases the credit is only as good as the project behind it.
Limits and cautions
Credits are not a perfect instrument. The Oxford Offsetting Principles, published by the University of Oxford’s Smith School, state that most offsetting that occurs today is still not net zero aligned, and note that analyses of common project types have found evidence of over-crediting. They also say that well thought-out offsetting can contribute to net zero strategies, particularly in hard-to-decarbonise sectors, and that their first element is to cut emissions.
The practical lesson is that the existence of a serial number does not prove a good outcome. It proves only that a credit can be tracked. Quality still depends on the baseline, the monitoring and the durability of the benefit. This site is independent and informational: it does not issue, sell, verify or certify credits, it is not affiliated with any programme or registry, and nothing here is financial, investment or legal advice.
Frequently asked questions
How much does one carbon credit represent?
One tonne of carbon dioxide equivalent that has been reduced, avoided or removed through a certified project, according to Gold Standard.
What does retiring a credit mean?
It means the credit is permanently taken out of circulation in the registry so that it cannot be sold again or claimed by another party.
Who checks that a project really delivered?
An independent third-party auditor validates the design and verifies the results, which is a core requirement set out by the Integrity Council for the Voluntary Carbon Market.
Why do credits from different projects differ in value?
Quality varies with additionality, permanence, measurement accuracy and the type of project, so identical-looking units can carry different climate value.
