What Are Carbon Credits?

Carbon credits are tradable certificates representing the right to emit, or the verified reduction of, one metric tonne of carbon dioxide equivalent (tCO2e) of greenhouse gas. They function as the currency of carbon markets, providing an economic mechanism for distributing the cost and responsibility of emissions reduction across governments, industries, and project developers. The underlying logic is that a tonne of CO2 avoided or removed anywhere in the world has the same atmospheric effect, so the cheapest available reductions should be pursued first regardless of location, and a market price for carbon creates the incentive to find them.

Carbon credits appear in two distinct market structures: compliance markets created by government regulation and voluntary markets where participation is self-directed. Both types depend on rigorous accounting standards to ensure that each credit represents a genuine, additional, and permanent emissions reduction.

Cap-and-Trade Compliance Markets

In a compliance carbon market, a regulatory authority sets a cap on total emissions from covered sectors and issues or auctions allowances equal to that cap. Each allowance permits the emission of one tonne of CO2e. Regulated entities must surrender allowances equal to their actual emissions at the end of each compliance period; those who reduce emissions below their allocation can sell surplus allowances, while those who exceed it must buy from others. The European Union Emissions Trading System, launched in 2005, was the world's first major cap-and-trade scheme and covers electricity generation and heavy industry across EU member states. China launched the world's largest ETS by covered emissions in 2021. The UNFCCC Emissions Trading mechanism established the international legal framework for cross-border trading of emissions units under the Kyoto Protocol, a structure updated and extended under Article 6 of the Paris Agreement.

Voluntary Carbon Markets

Outside mandatory compliance regimes, organizations and individuals purchase carbon credits to offset their remaining emissions or to support climate action beyond regulatory requirements. These credits are generated by projects that reduce or remove greenhouse gases: reforestation, avoided deforestation, methane capture from landfills, cookstove distribution, and direct air capture are common project types. Credits are verified against standards such as the Verified Carbon Standard administered by Verra, which requires independent third-party auditing to confirm that emissions reductions are additional (they would not have occurred without credit revenue), quantified accurately, and permanent. The voluntary market generated billions of dollars in transactions annually in recent years, though scrutiny over credit quality and additionality claims has led to tightening methodological requirements.

Credit Quality and Integrity

The integrity of a carbon credit depends on four properties: additionality, measurability, permanence, and independent verification. A reduction is additional if it goes beyond what would have happened in the absence of carbon market incentives. Measurability requires that emission reductions be quantified using approved methodologies with defined uncertainty ranges. Permanence matters most for biological sinks like forests, where fire or disease can reverse stored carbon decades after the credit was issued; buffer pools of extra credits are typically maintained to compensate for such reversals. UNDP analysis of carbon market mechanisms describes how both compliance and voluntary markets approach these quality attributes and the governance structures that enforce them.

Applications

Carbon credits have applications in a range of fields, including:

• Corporate net-zero and science-based target strategies, where residual emissions are offset
• Project finance for renewable energy, reforestation, and methane abatement in developing economies
• Supply chain sustainability programs requiring Scope 3 emissions accountability
• National policy compliance under the Paris Agreement's Article 6 cooperative mechanisms
• Consumer-facing carbon neutrality claims on products and services