CORSIA: aviation carbon offsetting explained
CORSIA is the global aviation carbon offsetting scheme run through ICAO. It is designed to address growth in international aviation CO2 emissions above a baseline, using monitoring, reporting, verification and eligible carbon credits. This guide explains what CORSIA is, how it works, which airlines
CORSIA is the global aviation carbon offsetting scheme run through ICAO. It is designed to address growth in international aviation CO2 emissions above a baseline, using monitoring, reporting, verification and eligible carbon credits. This guide explains what CORSIA is, how it works, which airlines and routes are covered, and why it matters for the carbon market.
What is CORSIA?
CORSIA stands for the Carbon Offsetting and Reduction Scheme for International Aviation. It is a global market-based measure agreed through the International Civil Aviation Organization, the United Nations agency responsible for international civil aviation standards.
The basic idea is simple: international aviation emissions are hard to reduce quickly because aircraft rely heavily on liquid fuels, long asset lives and global route networks. CORSIA requires aircraft operators on covered international routes to monitor their CO2 emissions and cancel eligible emissions units to meet offsetting requirements for emissions growth above a baseline.
CORSIA does not cover every aviation emission and it does not make flying carbon neutral. It is one policy tool used alongside aircraft efficiency, operational improvements, sustainable aviation fuel, new propulsion technologies and national or regional carbon pricing schemes.
Why aviation needs a carbon scheme
Aviation is a difficult sector to decarbonise. Electric and hydrogen aircraft may eventually help on short routes, but most medium and long-haul flights still depend on energy-dense liquid fuels. Sustainable aviation fuel can reduce lifecycle emissions, but supply is limited and prices remain high. That leaves a gap between aviation demand and near-term decarbonisation capacity.
CORSIA was created to address part of that gap by putting a market-based obligation on international aviation emissions growth. It does not replace in-sector reductions. It sits alongside them.
Who runs CORSIA?
CORSIA is administered through ICAO. The detailed requirements sit in ICAO Annex 16, Volume IV, with implementation elements covering monitoring, reporting, verification, state pairs, emissions unit eligibility and related technical rules.
States implement CORSIA through their own aviation authorities. Aircraft operators report emissions to the state to which they are attributed. That state calculates offsetting requirements and oversees compliance.
Which flights are covered?
CORSIA applies to international flights between participating states, not domestic flights. For the pilot phase and first phase, participation is voluntary at state level. ICAO says CORSIA has three phases: a pilot phase from 2021 to 2023, a first phase from 2024 to 2026, and a second phase from 2027 to 2035.
As of 2026, ICAO lists 130 states participating in CORSIA. Flights between two participating states are generally within the offsetting scope for that year, subject to the detailed rules and exemptions.
| Phase | Years | Participation | Practical meaning |
|---|---|---|---|
| Pilot phase | 2021 to 2023 | Voluntary | Early implementation and first compliance period |
| First phase | 2024 to 2026 | Voluntary | Current phase, with expanded participation and eligible units for 2024 to 2026 |
| Second phase | 2027 to 2035 | Participation determined by ICAO criteria, with exemptions | Broader mandatory phase for qualifying states and routes |
How the baseline works
CORSIA is designed around emissions growth above a baseline. The original framework used an average of 2019 and 2020 emissions, but the COVID-19 collapse in aviation traffic made 2020 highly unrepresentative. ICAO subsequently moved to use 2019 emissions for the pilot phase baseline, and Assembly decisions reshaped the baseline approach for later phases.
The practical point for readers is that CORSIA is not a cap on all aviation emissions. It is an offsetting scheme for emissions growth above the relevant baseline. That distinction matters when comparing CORSIA with cap-and-trade systems such as the EU ETS.
What airlines have to monitor and report
Aircraft operators covered by CORSIA must monitor fuel use and calculate CO2 emissions from covered international flights. They submit annual emissions reports, which are verified by accredited verification bodies and then reported through the relevant state authority.
The monitoring, reporting and verification process matters because offset obligations are calculated from reported emissions. Weak data would weaken the whole scheme. For airlines, CORSIA is therefore not only a credit-purchasing issue. It is a carbon accounting and compliance issue.
What are CORSIA eligible emissions units?
CORSIA eligible emissions units are carbon credits that ICAO has approved for use against CORSIA offsetting obligations. An aircraft operator cannot simply buy any voluntary carbon credit and assume it will count. ICAO maintains a document called CORSIA Eligible Emissions Units, which identifies approved emissions unit programmes and their scope of eligibility for specific compliance periods.
Eligibility is assessed at programme level by ICAO, with recommendations from the Technical Advisory Body. Programmes are assessed against CORSIA Emissions Unit Eligibility Criteria, including programme design and carbon credit integrity criteria.
How airlines use credits
An airline can buy emissions units before it needs them, but it can only use units that are eligible for the relevant compliance period and within the specific scope approved by ICAO. To meet CORSIA obligations, the operator must cancel eligible emissions units equal to its final offsetting requirement for that compliance period.
Cancellation is important. It is the carbon market equivalent of using the credit. The record should show which units were cancelled, by whom, for what purpose and for which compliance period.
CORSIA vs voluntary offsetting
Voluntary offsetting is discretionary. A company or individual chooses to buy credits to make a claim, fund climate projects or address emissions outside a legal requirement. CORSIA is different because covered aircraft operators have compliance obligations created through an international aviation framework.
That does not mean every CORSIA credit is automatically better than every voluntary market credit. CORSIA eligibility means the unit meets the scheme's eligibility rules. Credit quality still depends on project type, additionality, permanence, measurement, registry treatment, corresponding adjustments where required and the details of the approval scope.
CORSIA vs EU ETS
CORSIA is an offsetting scheme. The EU ETS is a cap-and-trade system. That is the biggest difference. Under the EU ETS, covered entities must surrender allowances, and the number of allowances is constrained by a cap. Under CORSIA, airlines use eligible emissions units to offset growth above the relevant baseline.
This is why airlines can face multiple carbon regimes at once. A flight's route, region, operator and regulatory treatment determine whether CORSIA, EU ETS, UK ETS or another scheme is relevant.
Criticisms and limitations
CORSIA is often criticised for being less stringent than a full cap on aviation emissions. Because it focuses on growth above a baseline, it does not price all emissions. It also depends on the quality and availability of eligible carbon credits, which is a live concern across the carbon market.
Another criticism is that offsetting can distract from in-sector reductions. A strong aviation climate strategy should prioritise operational efficiency, fleet renewal, sustainable aviation fuel and demand management where relevant. CORSIA should not be treated as a substitute for reducing aviation emissions directly.
Plain-English summary
CORSIA is a global aviation offsetting framework. It does not make flying emissions-free, and it does not cover all aviation emissions. It creates monitoring, reporting and offsetting obligations for covered international aviation emissions growth using ICAO-approved eligible emissions units.
CORSIA glossary
Aircraft operator: The airline or operator responsible for reporting emissions and meeting obligations under the scheme.
Eligible emissions unit: A carbon credit approved by ICAO for use in CORSIA within a defined compliance period and scope.
Cancellation: The act of permanently using an emissions unit for CORSIA compliance.
State pair: A route relationship between two states that determines whether flights between them are subject to CORSIA offsetting requirements for a given year.
Corresponding adjustment: An accounting adjustment under Article 6 of the Paris Agreement to avoid double counting of an emissions reduction between countries and buyers.
Read the full CORSIA cluster
- CORSIA eligible carbon credits: what airlines can use
- CORSIA vs EU ETS: how aviation carbon rules compare
- CORSIA Phase 1: what changes from 2024 to 2026
- Why CORSIA matters for the voluntary carbon market
Sources cited
- ICAO CORSIA overview
- ICAO CORSIA participating states and state pairs
- ICAO CORSIA eligible emissions units
- ICAO CORSIA FAQ
Key takeaway
CORSIA is a central carbon market topic because it turns international aviation compliance into demand for eligible carbon credits. Its importance is not just aviation. It affects credit eligibility, Article 6 accounting, registry approvals and the broader debate over what counts as a credible carbon offset.