CORSIA Explained: ICAO's Carbon Offsetting and Reduction Scheme for Aviation

What Is CORSIA?

CORSIA — the Carbon Offsetting and Reduction Scheme for International Aviation — is the global carbon offsetting mechanism for international commercial aviation, developed under the auspices of the International Civil Aviation Organization (ICAO) and adopted by ICAO member states in 2016. It is the first global market-based measure for any single industry sector and represents a historic multilateral agreement, though its effectiveness in addressing aviation's climate contribution remains the subject of intense debate.

CORSIA was developed in response to aviation's growing climate impact and the absence of a comprehensive international regulatory framework for the sector. Aviation's greenhouse gas emissions from international flights were explicitly excluded from national inventory accounting under the Kyoto Protocol framework — because international flight emissions cannot be straightforwardly attributed to a single country, they fell through the gap between national accounting frameworks. The Paris Agreement similarly does not include international aviation within national Nationally Determined Contributions (NDCs), leaving ICAO as the primary international governance body for aviation climate policy.

The political context of CORSIA's development is important for understanding its structure. Aviation's climate governance faces a fundamental tension between developed countries — primarily the US, EU member states, and other high-income nations — that account for the majority of current aviation emissions and have committed to ambitious domestic climate targets, and developing countries — particularly China, India, Brazil, and others — that have rapidly growing aviation sectors and argue that constraining aviation growth would impede development. CORSIA's design reflects careful compromise between these positions: it addresses net emission growth above a baseline rather than absolute emission levels, includes phase-in provisions and exemptions for smaller states, and relies on offsetting rather than absolute reduction obligations on airlines.

ICAO sets CORSIA standards through the Standards and Recommended Practices (SARPs) framework in Annex 16 (Environmental Protection), Volume IV, which was adopted in 2018. The SARPs define monitoring, reporting, and verification (MRV) requirements for airlines, eligibility criteria for offset units, and the institutional framework through which ICAO oversees implementation. Member states that have volunteered for the pilot and first phases implement CORSIA through national legislation that incorporates the ICAO standards into domestic aviation law, creating legal obligations on airlines registered in those states.

How CORSIA Works: The Mechanism Explained

CORSIA operates through three integrated components: monitoring, reporting, and verification (MRV) of airline fuel use and CO2 emissions; calculation of each airline's offsetting obligation relative to a growth baseline; and surrender of eligible carbon offset units to discharge that obligation. Each component has detailed technical specifications that airlines must follow.

The MRV framework requires airlines operating routes that fall under CORSIA to measure fuel consumption and calculate CO2 emissions for each flight. Airlines may use one of three approved methodologies: fuel uplift measurement (measuring fuel added to the aircraft before each flight, which requires meters at every refueling point globally), block-hour methodology (using type-specific fuel burn rates per flight hour), or fuel bill methodology (using fuel purchase records). Most large airlines use fuel uplift or fuel bill methodology; smaller carriers with simpler operations may use block-hour rates. Emissions must be reported annually to the competent national authority, which in turn reports to ICAO. An independent verifier must certify the accuracy of each airline's emissions report.

The offsetting obligation is calculated by comparing each airline's CO2 emissions on covered routes in a given year to that airline's share of the 2019–2024 sector-level baseline. If an airline's covered route emissions grow above its pro-rata share of the baseline, it must purchase and surrender carbon offset credits equal to that growth. The calculation involves a "sector growth factor" that distributes the industry-level growth above baseline proportionally across all participating airlines, and an "individual growth factor" that reflects each airline's own growth rate relative to the sector. The exact formula creates a blended obligation that partially individualizes the offsetting requirement — airlines that grow faster than the sector average have higher obligations — while keeping it connected to the aggregate industry performance.

Offset credit surrender occurs at the end of each compliance period. Airlines submit eligible offset units to their national competent authority, which cancels them in the relevant carbon registry and confirms compliance to ICAO. An airline that fails to surrender sufficient offsets faces sanctions determined by the national authority — in most implementing states, this involves financial penalties equivalent to the market cost of the unsurrendered offsets plus additional penalties. ICAO does not itself enforce compliance; it relies on member state national authorities to impose and collect penalties, creating potential variability in enforcement rigor across jurisdictions.

Baseline, Growth, and Phase Structure

CORSIA's baseline — the level against which "growth" in emissions is measured, triggering offsetting obligations — was originally set as the average of 2019 and 2020 international aviation CO2 emissions. However, 2020 was profoundly abnormal due to COVID-19: international aviation traffic fell approximately 75% from 2019 levels, meaning the 2019-2020 average was dramatically lower than 2019 alone. ICAO's Council voted in 2020 to use 2019 as the sole baseline year rather than the 2019-2020 average, preventing CORSIA from requiring massive offsetting simply because traffic recovered from COVID levels. This decision — while arguably technically correct — drew criticism from environmental groups who pointed out that using the higher 2019 baseline makes the growth offsetting requirement less stringent than it would otherwise be.

CORSIA operates in three phases. The Pilot Phase covers 2021–2023 and applies only to routes between states that have voluntarily opted into the scheme. More than 100 countries covering approximately 80% of international aviation activity have opted in during this phase. The First Phase covers 2024–2026 and also applies only to voluntary participant states. The Second Phase, beginning in 2027, applies to all states except those that qualify for an exemption based on a combination of low share of international aviation activity (under 0.5% of the global total) and being a Least Developed Country, Small Island Developing State, or Landlocked Developing Country.

The coverage expansion in 2027 is significant because it will bring China, India, Brazil, and other major and growing aviation markets under the scheme for the first time on a mandatory basis (some are voluntary participants in earlier phases). China in particular has resisted mandatory participation, citing principles of common but differentiated responsibilities — the same principle used to justify developing country exemptions in the Kyoto Protocol. How China, India, and other non-voluntary participants respond to mandatory phase-two requirements will significantly affect CORSIA's practical climate impact.

CORSIA does not address aviation's total emissions — it addresses only the growth in emissions above the 2019 baseline on covered international routes. This is a critical limitation: if 2019 international aviation CO2 was approximately 600 million tonnes, and if airlines in the CORSIA period achieve total emissions of 700 million tonnes, CORSIA requires offsetting of approximately 100 million tonnes of growth but does not address the underlying 600 million tonnes. This design choice reflects the political compromise that made CORSIA achievable as a multilateral agreement, but it means CORSIA is structurally incapable of achieving absolute emission reductions even if perfectly implemented.

Eligible Offsets: Quality Standards and Approved Programs

CORSIA's environmental credibility depends substantially on the quality of the carbon offset units that airlines surrender to discharge their obligations. Low-quality offsets — those representing emissions reductions that would have occurred anyway, or that have been double-counted, or that are subject to permanence risk — provide no real climate benefit while allowing airlines to claim compliance. ICAO has established an eligibility review process to identify offset programs whose units meet CORSIA quality standards.

ICAO's Technical Advisory Body (TAB) assesses carbon offset programs against a set of criteria including additionality (reductions would not have occurred without the carbon finance), permanence (reductions are durable and not reversible), quantification accuracy, avoidance of double-counting with national NDC accounting, and third-party verification. Programs that pass the TAB assessment are recommended to the ICAO Council for approval as CORSIA-eligible programs.

As of 2024, the CORSIA-approved offset programs include the Gold Standard, Verified Carbon Standard (Verra VCS), American Carbon Registry (ACR), Climate Action Reserve (CAR), Architecture for REDD+ Transactions (ART TREES), and several others. Notably, some widely used offset programs initially submitted for review were found not to meet CORSIA standards or were approved with restrictions. The Clean Development Mechanism (CDM), which generated the majority of offset credits under the Kyoto Protocol, has CDM units eligible for CORSIA only under specific conditions reflecting concerns about the integrity of some CDM project types.

Independent research has raised significant questions about the quality of many REDD+ (Reducing Emissions from Deforestation and Forest Degradation) offset projects — the largest category of offsets by volume in several approved programs. A major investigation by The Guardian, Zeit Online, and Source Material in 2023 found that approximately 90% of Verra's REDD+ rainforest offset credits were likely "phantom credits" that didn't represent real carbon savings. This finding — disputed by Verra — struck at the heart of one of the most widely used offset types under CORSIA and prompted significant reassessment of offset quality within the aviation sustainability community. ICAO's response was to refine its offset quality criteria for the second phase, with new requirements expected to tighten additionality standards.

The Sufficiency Debate: Is CORSIA Enough?

CORSIA's most fundamental limitation is that it is designed to achieve "carbon neutral growth" — preventing aviation's CO2 from increasing above 2019 levels — rather than the absolute reductions needed to align aviation with Paris Agreement temperature targets. The IPCC's Special Report on 1.5°C found that achieving the Paris goals requires global greenhouse gas emissions to reach net zero by approximately 2050 and to decline by 45% from 2010 levels by 2030. Aviation's CORSIA mechanism allows emissions to remain at 2019 levels indefinitely (as long as growth is offset) rather than declining toward net zero.

IATA's own net-zero 2050 commitment, adopted in 2021, acknowledges that reaching net zero will require absolute emission reductions well beyond what CORSIA requires. The IATA pathway relies on SAF (65% of reductions), new aircraft technology and operations (13%), infrastructure and operational improvements (3%), and offsetting/carbon capture (19%) to achieve net zero by 2050. CORSIA plays a role in the near-term offsetting component but is explicitly not intended to be the mechanism that achieves net zero.

The scientific literature on aviation emissions pathways consistently finds that CORSIA, as designed, is insufficient for 1.5°C or even 2°C alignment. A 2021 study in Nature Climate Change concluded that aviation's contribution to warming would continue to grow significantly under CORSIA implementation alone, because the scheme does not address the large stock of pre-2019 emissions, does not cover domestic aviation, and covers only CO2 rather than aviation's total climate forcing (which includes non-CO2 effects estimated to be comparable in magnitude to CO2).

Environmental NGOs and some national governments have proposed supplementing CORSIA with additional measures: expanding the EU ETS to cover international aviation (currently the EU ETS covers only intra-EEA flights), introducing global kerosene taxes, setting mandatory SAF blending requirements, or integrating non-CO2 effects into climate frameworks. The EU has been most aggressive in this direction, maintaining the EU ETS for intra-EU flights despite ICAO pressure to defer entirely to CORSIA, and introducing a SAF blending mandate (ReFuelEU Aviation) that requires 2% SAF by 2025 scaling to 70% by 2050.

Proponents of CORSIA argue that it should be evaluated not against the theoretical ideal but against the alternative — no global aviation climate mechanism at all — and that the political achievement of multilateral agreement across 193 ICAO member states on even an imperfect scheme represents meaningful progress. They also note that CORSIA's MRV infrastructure, which will generate comprehensive, verified data on international aviation CO2, creates a foundation for future policy development. The debate between these positions reflects the fundamental tension in multilateral climate governance between the achievable and the necessary — a tension that CORSIA embodies rather than resolves.