Glossary

Carbon glossary

The words you'll meet when you calculate a footprint or buy carbon credits, explained in plain language. Each term has its own link you can share.

Additionality: A project is additional if it would not happen without the money from carbon credits. If it is already profitable, or required by law, its credits don't stand for extra climate action.; How we check projects
Avoided emissions and emission reductions: Emissions prevented compared with what would otherwise have happened, for example by capturing landfill gas or protecting a forest from being cleared. They differ from removals, which take CO₂ out of the atmosphere.
Baseline: The scenario a project is measured against: what emissions would have been without it. An inflated baseline leads to over-crediting.
Buffer pool: A reserve of credits that forest and other nature-based projects set aside under their standard. If stored carbon is lost, for example to fire, credits from the reserve replace it.; What happens if something goes wrong
Carbon credit: A certificate for one tonne of CO₂e reduced or removed by a verified project. It has a unique serial number on a public registry and counts only once it is retired.
Carbon footprint: The total greenhouse gas emissions caused by a person, product, event or organisation, usually measured in kilograms or tonnes of CO₂e.; Calculate your footprint
Carbon offsetting: Funding projects that reduce or remove emissions elsewhere, for emissions you can't avoid yet. It doesn't cancel out your own emissions, so it comes on top of cutting them, not instead.
Carbon removal: Taking CO₂ out of the atmosphere and storing it: in trees, soils and mangroves (nature-based) or underground and in minerals (technology-based). These are Oxford Types 4 and 5.; See the projects by type
Carbon standard: The organisation that sets the rules a project must follow and issues its credits, such as Verra (VCS), Gold Standard, Climate Action Reserve or Puro.earth.
Climate-neutral claims: Statements that a product or company is carbon neutral because of offsetting. Since 27 September 2026, EU rules (Directive 2024/825) ban claims that a product has a neutral, reduced or positive climate impact because of offsetting, so we say what a purchase does instead: it funds verified projects and retires credits.
Co-benefits: Positive effects beyond the carbon, such as jobs, clean water or protected wildlife. For nature-based projects, labels such as CCB (Climate, Community & Biodiversity) certify them.
CO₂e (CO₂ equivalent): A common unit for all greenhouse gases: the amount of CO₂ that would cause the same warming. Methane, for example, warms far more than CO₂, so one tonne of it counts as many tonnes of CO₂e.
Core Carbon Principles (CCP): A quality benchmark set by the Integrity Council for the Voluntary Carbon Market (ICVCM). Credits from approved methodologies can carry the CCP label, which our credit-quality policy prefers.; Our credit-quality rules
Double counting: When the same tonne is counted twice: issued twice, sold twice or claimed by two parties. Unique serial numbers and retirement on a single registry prevent it.; How we check projects
Durability (permanence): How long the carbon stays out of the atmosphere. Forests store carbon for decades but can burn; storage underground or in minerals can last for centuries or longer.; How we check projects
Greenhouse gases (GHG): Gases that trap heat in the atmosphere, mainly carbon dioxide, methane and nitrous oxide. Their emissions are added up in CO₂e.
Leakage: Emissions that move elsewhere because of a project, for example when protecting one forest pushes logging to another. Good methodologies estimate it and deduct it from the credits.
Net zero: A state in which the greenhouse gases emitted are matched by the same amount removed from the atmosphere. For companies it means cutting emissions deeply first and using removals only for what is left.
Over-crediting: Issuing more credits than the real climate impact, for example because of an inflated baseline or leakage that was not counted. Independent ratings and reviews look for it.; How we check projects
Oxford Principles: The Oxford Principles for Net Zero Aligned Carbon Offsetting (University of Oxford, revised in 2024). They ask buyers to cut emissions first, use high-quality credits and shift over time towards durable removals.
Oxford types: The five categories of the Oxford Principles: 1 technology-based reductions, 2 nature-based reductions, 3 capture and storage of fossil CO₂, 4 nature-based removals and 5 technology-based removals.; See the projects by type
Rating agencies: Independent firms that score carbon projects on their risks, such as BeZero, Calyx Global, Renoster and Sylvera. Every project we offer is rated highly by at least one of them.; How we check projects
REDD+: Reducing Emissions from Deforestation and forest Degradation: projects that protect forests at risk of being cleared. They are nature-based reductions (Oxford Type 2).
Registry: The public database where a standard records each project, issues its credits with serial numbers and marks them as retired, such as Verra's or Gold Standard's registry.
Retirement: Taking a credit out of circulation for good on its registry, on the buyer's behalf, so it can never be sold or used again. A credit only counts once it is retired.; See a sample certificate
Reversal: When carbon that a project stored is released again, for example by fire, drought or illegal logging. Buffer pools are there to cover it.; What happens if something goes wrong
Scope 1, 2 and 3: The three groups of a company's emissions in the GHG Protocol: 1 from its own fuel and processes, 2 from the energy it buys and 3 from its value chain, such as suppliers, business travel and the use of its products.; Business emissions calculator
Serial number: The unique code each credit gets on its registry. Your certificate links to the order that lists the serial numbers of the credits retired for you.; See a sample certificate
Validation and verification: Checks by an accredited independent body. Validation approves a project's design before it starts; verification confirms its reductions or removals before credits are issued.
Vintage: The year in which the reduction or removal behind a credit took place. The vintage of every retirement is shown on your order.; Our credit-quality rules
Voluntary carbon market: Where companies and people buy carbon credits by choice, unlike compliance schemes such as the EU Emissions Trading System, where emitters must hold allowances by law.

Last reviewed: 8 October 2026