The Carbon Offsetting and Reduction Scheme for International Aviation (“CORSIA”) is the first sector-wide, globally-agreed market-based measure for a greenhouse-gas–emitting industry.^[1] Established by the International Civil Aviation Organization (ICAO) — the United Nations specialised agency for civil aviation — CORSIA operates within a body of law that predates the Paris Agreement, yet its interaction with post-Paris mechanisms (particularly Article 6 corresponding adjustments) is what makes it consequential for voluntary carbon market practitioners in 2026 and beyond.

This briefing sets out the anatomy of the scheme for readers whose responsibility is not to run an airline but to buy, structure, verify, or supply carbon credits that airlines (or airline-adjacent obligated buyers) may retire against it. The lens is procurement- and portfolio-oriented; the treatment is technical.

Regulatory architecture and objective

CORSIA was adopted by the ICAO Assembly in October 2016 (Resolution A39-3) and refined at successive Assemblies, notably A41 in October 2022.^[2] Its stated objective is to complement a broader basket of measures — technological, operational, and sustainable aviation fuels — to stabilise international aviation’s net CO₂ emissions relative to a defined baseline. Domestic aviation is out of scope; international aviation is where CORSIA acts.

The scheme operates in three phases. The Pilot Phase ran from 2021 to 2023 on a voluntary basis. Phase 1 covers 2024 to 2026, still voluntary but with broader state participation and a materially tightened baseline. Phase 2, running from 2027 to 2035, is mandatory for the great majority of ICAO member states, with narrow exemptions for Least Developed Countries, Small Island Developing States, and Landlocked Developing Countries whose share of international RTKs is below a specified threshold.^[3]

The 41st ICAO Assembly’s decision to change the offsetting baseline from “average of 2019 and 2020 CO₂ emissions” to 85% of 2019 emissions (effective from 2024) was one of the most consequential regulatory adjustments in the scheme’s history.^[4] Because 2020 emissions collapsed during the pandemic, the original baseline would have made almost every year post-2020 exceed baseline immediately, generating large offset obligations. The 2019-referenced 85% figure is both more scientifically defensible (it aligns with the industry’s own long-term ambition trajectory) and creates a tighter obligation structure than the pre-A41 rules implied.

What airlines actually owe

Under CORSIA, each operator’s annual offsetting requirement is calculated at the individual operator level, but distributed as a share of the total sector obligation using a formula defined in Assembly Resolution A41-22, Appendix A.^[5] Airlines report annual CO₂ emissions and receive an offsetting requirement in tonnes CO₂-equivalent, to be satisfied by retiring CORSIA-eligible emissions units and/or by using CORSIA-eligible sustainable aviation fuels.

Two features matter for buyers:

First, the offsetting obligation is cumulative for a compliance period: Phase 1 runs 2024–2026 as one triennium, with compliance settled after each period. This means airlines can plan procurement over a multi-year horizon rather than settling annually — a structural feature that makes forward-contracting attractive.

Second, the compliance requirement is denominated in CO₂ tonnes, not credits of a specific type; the credit unit chosen must simply be CORSIA-eligible. This gives obligated buyers meaningful flexibility across the approved programme list — and creates competition among approved carbon programmes for airline demand.

The eligibility architecture

Eligibility is a two-tier system: ICAO approves crediting programmes, and eligibility is then modulated at the level of individual credit vintages, project types, and host-country authorisations.

The ICAO Council, acting on recommendations from the CORSIA Technical Advisory Body (TAB), maintains a public list of Eligible Emissions Units programmes.^[6] Historic approvals include the American Carbon Registry (ACR), the Architecture for REDD+ Transactions (ART), the Climate Action Reserve (CAR), the Gold Standard for Global Goals, and the Verified Carbon Standard (Verra VCS), among others. Each approval is time-bounded and specifies the vintage windows and project categories permissible under that programme.

Programme-level approval, however, is only the first hurdle. A credit issued under an approved programme is not automatically CORSIA-eligible. It must additionally satisfy the following:

1. Vintage window. Only credits from specified vintage years (i.e. the year the underlying emissions reduction occurred) are eligible for each compliance phase. ICAO has progressively narrowed permitted vintages, generally excluding older credits.
2. Project type. Some project types are excluded within an otherwise-approved programme. Historically, certain industrial-gas destruction projects and some land-use activities have been outside the eligible envelope.
3. Host-country authorisation. For credits arising from projects in Non-Annex I countries, host-country consent is required — and for Phase 2 (with limited exceptions for Phase 1), this must take the form of a corresponding adjustment authorised under Article 6 of the Paris Agreement.^[7]

The last of these is where the machinery gets consequential for developers and buyers alike, and it warrants dedicated treatment.

Article 6 and corresponding adjustments

The Paris Agreement’s Article 6 provides the international legal architecture through which climate outcomes may transfer between parties (states) without double counting.^[8] Two operational mechanisms sit inside Article 6:

• Article 6.2: cooperative approaches between parties, with mitigation outcomes (“Internationally Transferred Mitigation Outcomes”, ITMOs) authorised for use elsewhere. Requires corresponding adjustments by the host country’s greenhouse gas inventory.
• Article 6.4: a UN-supervised centralised mechanism under the Conference of the Parties (CMA), with a Supervisory Body accrediting activities and issuing 6.4 emission reductions (“A6.4ERs”). Corresponding adjustments apply.

The critical function of the corresponding adjustment is bookkeeping: when a credit is transferred internationally and retired by an operator (or a state) outside the host country, the host country must subtract the transferred reduction from its own NDC accounting. Without a corresponding adjustment, the same tonne of avoided CO₂ would be claimed simultaneously by the host country (as part of its NDC progress) and by the retiring operator (as part of its CORSIA compliance), producing environmentally meaningless “double counting” — the failure mode that Article 6’s rules exist to prevent.

For CORSIA specifically, corresponding adjustments are:

• Required for all eligible units in Phase 2 (from 2027 onwards).
• Required for many but not all units in Phase 1, depending on programme rules and vintages.
• The single most binding constraint for developers of projects hosted in Non-Annex I countries seeking CORSIA eligibility.

The mechanics of obtaining a corresponding adjustment are non-trivial. The host country’s Designated National Authority (or equivalent focal point for Article 6) must engage in a multi-quarter process: reviewing the project, ensuring alignment with NDC accounting, signing a Letter of Authorization, and formally recording the adjustment on international tracking registries. In practice, a project developer speaking about “the CORSIA pathway” without documented, active engagement with the host-country focal point is speaking aspirationally rather than operationally.

Five questions procurement should ask

For any organisation evaluating a project developer’s claim of CORSIA eligibility (or “pathway to eligibility”), five questions distinguish serious projects from marketing.

1. Which programme are your credits issued under, and is that programme currently approved by ICAO for the phase I need to meet? Programme approval is published; the developer should be able to name the programme and cite the current ICAO CORSIA Eligible Emissions Units list.

2. Is the project type within the permitted CORSIA scope for that programme? Programme-wide approval does not extend to every project type under that programme. This is where several projects that assumed eligibility have been surprised.

3. What is the vintage window, and what fraction of your pipeline sits within it? Vintage constraints are sharp. A developer expecting to issue credits in 2027 for a Phase 2 compliance need must be clear about what “vintage 2027” means in terms of when the underlying reductions actually occurred.

4. What is the status of Article 6 corresponding adjustment? For Non-Annex I projects, this is the binding constraint. Answers on a scale from “we have not started” through “we hold a Letter of Approbation and are in active engagement with the DNA” through “we hold a Letter of Authorization on named vintages” are materially different.

5. What is your MRV and audit stack for satisfying CORSIA’s registry transparency and no-double-counting requirements? CORSIA expects an audit trail that satisfies ICAO’s rules on registry disclosure, retirement handling, and chain-of-custody. Programmes handle this differently.

A developer able to give crisp, specific answers to all five is a materially different counterparty from one who can only answer the last.

Where SaniTap sits

SaniTap’s clean cooking and safe water projects, developed under the Gold Standard for Global Goals — which is on the current ICAO CORSIA Eligible Emissions Units list^[9] — are progressing toward CORSIA eligibility along three parallel workstreams:

• Article 6 engagement. SaniTap holds a Letter of Approbation from the Government of Madagascar formalising host-country support for the project pipeline. Work continues toward the full Article 6 authorisation for international transfer that Phase 2 requires.
• Programme alignment. Monitoring, documentation, and registry integration are being aligned with CORSIA-approved programme requirements, anticipating the tighter expectations Phase 2 will bring.
• Integrity stacking. Credits are developed to Gold Standard’s higher-level gender-responsive certification, aligned with the ICVCM Core Carbon Principles, and supported by third-party sensor monitoring on a randomised sample of installations — features that exceed the CORSIA minimum bar.

For an obligated buyer, this is the shape of a credible counterparty on the eligibility pathway rather than a settled claim of eligibility itself. Both matter, and it is worth being able to tell them apart.

Further reading

• ICAO, CORSIA Frequently Asked Questions.^[10]
• Öko-Institut, CORSIA and the Voluntary Carbon Market: Overlaps, Divergences, and Buyer Implications, 2023.^[11]
• Warnecke, C. et al., “Robust eligibility criteria essential for a credible CORSIA Phase II”, Climate Policy, 2019.^[12]