Primary sources · 4
- [1] ICAO — CORSIA programme page — Authoritative description of CORSIA phases, eligibility, and baseline rules · International Civil Aviation Organization · Current https://www.icao.int/environmental-protection/CORSIA/Pages/default.aspx
- [2] ICAO CORSIA FAQs — Official Q&A document — April 2024 update covers post-COVID baseline change · ICAO · April 2024 https://www.icao.int/sites/default/files/environmental-protection/CORSIA/CORSIA_FAQs_Apr2024.pdf
- [3] IATA — CORSIA fact sheet — Airline-industry perspective on CORSIA implementation and eligible unit availability · IATA · December 2025 https://www.iata.org/en/iata-repository/pressroom/fact-sheets/fact-sheet-corsia/
- [4] EASA — CORSIA implementation — European Union Aviation Safety Agency overview of CORSIA in EU context · EASA · Current https://www.easa.europa.eu/en/domains/environment/icao-carbon-offsetting-and-reduction-scheme-international-aviation-corsia
CORSIA is aviation's global carbon market for international flights — airlines from participating states must offset their CO₂ growth above a defined baseline by buying eligible emission units or using sustainable aviation fuel. It enters its first mandatory phase in 2027.
What CORSIA is
The Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) was adopted by ICAO in 2016 as the world's first global market-based measure for any industrial sector. It applies only to international flights (purely domestic flights remain under each state's own climate policy) and requires airlines to offset CO₂ emissions above a defined baseline. The scheme covers more than 80 % of the post-2020 growth in international aviation emissions.
The three phases
| Phase | Years | Participation | Compliance cycle length |
|---|---|---|---|
| Pilot | 2021–2023 | Voluntary, ~88 states | 3 years (one cycle) |
| First phase | 2024–2026 | Voluntary, ~126 states | 3 years (one cycle) |
| Second phase | 2027–2035 | Mandatory (most ICAO states) | 3 years × 3 cycles |
The voluntary phases cover roughly 88 to 126 states; the mandatory second phase will cover all ICAO contracting states except those with specific exemptions (least-developed countries, small-island developing states, and landlocked developing countries that do not opt in). The second phase runs three rolling three-year compliance cycles, with final reckoning in 2035.
The 2019 baseline — why it changed
The original CORSIA design used the average of 2019 and 2020 CO₂ emissions as the baseline. COVID collapsed 2020 international aviation to a fraction of normal volumes, which would have set an artificially low baseline and forced enormous offset purchases as flying recovered. The ICAO Council met in June 2020 and revised the baseline to 2019 only for the pilot phase, then to 85 % of 2019 for the first and second phases — softening the post-COVID growth offset requirement while still constraining long-term growth.
What airlines actually buy
Eligible emission units fall into a handful of categories. Some are forest-conservation credits (REDD+ projects in Brazil and Indonesia dominate the available supply). Others are renewable-energy projects that pre-date the Paris Agreement clean-energy windfall. A growing share is CORSIA-eligible sustainable aviation fuel — airlines may claim CORSIA credits for SAF use, which is the main reason European carriers have pushed for tighter SAF mandates.
How big are the obligations?
ICAO estimates CORSIA will cover the offsetting of about 2.5 to 4 gigatonnes of CO₂ across the 2021-2035 period — a number that depends heavily on actual aviation growth and SAF deployment. Per-airline obligations in 2024 are still modest because emissions have only just returned to baseline; obligations are expected to ramp quickly through the second phase.