What Are Carbon Offsets?

Carbon offsets are tradable units, each representing one tonne of carbon dioxide equivalent reduced or removed from the atmosphere by a specific project, that a buyer retires to compensate for emissions occurring elsewhere. They rest on an economic argument that the atmosphere responds to global totals rather than to the location of a reduction, so abatement should be purchased where it costs least. Whether a given unit delivers on that argument is an empirical question about the project behind it, which is why the technical literature on offsets concerns measurement and verification far more than trading mechanics.

An offset is defined against a counterfactual baseline: the emissions that would have occurred without the project. Because that baseline is unobservable, it must be modeled, and every serious critique of offsetting traces back to how baselines are constructed and who has an incentive to set them high.

Crediting Methodologies and Registries

Projects are developed under a published methodology that specifies eligibility, baseline construction, monitoring parameters, and calculation of net reductions. Categories include forest conservation and restoration, improved forest management, cookstove distribution, landfill and coal mine methane capture, industrial gas destruction, renewable energy, soil carbon, biochar, enhanced rock weathering, and direct air capture with geological storage. An accredited third party validates the project design and later verifies reported performance, after which the registry issues serialized credits carrying a project identifier and a vintage year. Retirement removes a credit from circulation permanently and is the act that supports a compensation claim. Under the Paris Agreement, the Article 6.4 supervisory body performs the equivalent function at intergovernmental level, developing methodologies and overseeing issuance for the mechanism succeeding the Clean Development Mechanism.

Additionality, Permanence, and Leakage

Four integrity conditions govern whether a credit is real. Additionality requires that the reduction would not have happened anyway, tested through financial, barrier, and common practice analysis. Permanence requires that stored carbon stays stored, a serious constraint for forest and soil projects exposed to fire, disease, harvest, and policy reversal, and it is managed through buffer pools and reversal insurance rather than eliminated. Leakage requires accounting for activity displaced outside the project boundary, such as logging that simply relocates. Accurate quantification requires monitoring, reporting, and verification adequate to the claimed effect, which is easier for a metered methane flare than for hectares of standing forest. Empirical evaluations have repeatedly found these conditions unmet at scale: a study in Nature Communications on demand for low-quality offsets found that most credits bought by large corporate purchasers came from project types at high risk of overstating their climate effect, and the Berkeley Carbon Trading Project's collection of research on offset quality assembles the wider evidence base.

Market Structure and Integrity Reform

Offsets trade in two arenas. Compliance markets accept a limited quantity of qualifying credits toward regulatory obligations, with the accepted types and volumes set by the regulator. Voluntary markets serve corporate buyers making neutrality or compensation claims, and prices there span more than two orders of magnitude between avoidance credits and engineered removals. Aggregate issuance, retirement, and price data are tracked in the World Bank's State and Trends of Carbon Pricing report. Reform efforts have concentrated on standardizing integrity criteria for supply, disciplining how buyers describe their claims, tightening baselines in the most criticized methodologies, and shifting demand toward durable removals with measurable storage.

Applications

Carbon offsets are used in a range of fields, including:

• Corporate carbon neutrality and residual emissions compensation programs
• Compliance flexibility mechanisms within emissions trading systems
• Forestry, agriculture, and land management project finance
• Aviation sector obligations under international offsetting schemes
• Climate finance transfers between countries under Article 6
• Development of carbon dioxide removal technologies through advance purchase agreements