International Aviation

ICAO is a UN agency, which governments created in 1944 with the aim of promoting the safe and efficient development of civil aviation (ICAO, 2020c). Environmental protection has historically not been an ICAO focus point.

In 1983, the ICAO Council established the technical Committee on Aviation Environmental Protection (CAEP) to assist the agency in developing new policies and Standards and Recommended Practices (SARPs) related to environmental protection. In 1997, countries decided that Annex I Parties to the UNFCCC “shall pursue limitation or reduction of emissions of greenhouse gases […] from aviation […] bunker fuels, working through the International Civil Aviation Organization” (Kyoto Protocol to the United Nations Framework Convention on Climate Change, 1997). However, it took the ICAO Assembly—composed of 193 Member States—until 2013 to agree on the goal of ‘carbon neutral growth from 2020’ (ICAO, 2013). In 2016, the ICAO Assembly established the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), which started its pilot phase in January 2021 (Resolution A39-3: Consolidated Statement of Continuing ICAO Policies and Practices Related to Environmental Protection – Global Market-Based Measure (MBM) Scheme, 2016).

The COVID-19 pandemic resulted in an unprecedented, sharp decline in international air traffic volumes and emissions in 2020. However, traffic volumes and emissions have since recovered: in 2024, international aviation’s level of activity is estimated to reach 2019 levels, aligning with ICAO’s CAEP high recovery scenario, in which pre-pandemic air traffic levels return by 2024 (ICAO, 2022c).

In its latest Environmental Report, ICAO projects that emissions from international aviation will reach 1,100 to 1,500 MtCO₂ by 2050, where the upper bound (called “CAEP-12 baseline”) assumes fleet renewal but no additional technology and operational improvements. The lower bound assumes technological and operational improvements, but no drastic changes compared to BAU (ICAO, 2022c). ICAO further notes that uncertainties around demand for international aviation will influence future CO₂ emissions and provides three demand forecasts; the ‘medium demand’ forecast is the basis for ICAO’s CAEP-12 baseline (ICAO, 2022c).

We base our projection of future emissions (beginning in 2025) on ICAO’s projections. We determined the upper bound of our current policy projections as follows:

• We calculated CO₂ emissions under ICAO’s ‘medium demand’ and ‘high demand’ forecasts (Figure 1-5 in ICAO (ICAO, 2022c). We used ICAO’s assumption that combusting 1 kg of jet fuel generates 3.16 kg of CO₂;
• We determined the percentual difference in annual CO₂ emissions under the ‘medium demand’ and ‘high demand’ forecasts;
• We increased the CAEP-12 baseline by this percentual difference.

As we do not know the assumptions behind ICAO’s lower bound projections, and how operational and technological improvements would be influenced by a change in demand, we used ICAO’s lowest projection for CO₂ emissions from international aviation (i.e. the lower bound of the orange-shaded area in Figure 1-5 in ICAO (ICAO, 2022c)). Given that international aviation’s activity level and, therefore, the emissions level, are estimated to fully recover to 2019 levels in 2024, we use 2024 as the base year and apply the growth rate for ICAO’s lowest emissions projections to project emissions levels from 2025 onwards.

CORSIA is a market-based measure set up by ICAO in 2016 that allows airlines to offset their increase in emissions through carbon offset credits (i.e. emissions units), or through the use of alternative fuels. The scheme is currently planned to run across three stages: a pilot phase from 2021 to 2023, the first phase from 2024 to 2026, and the second phase from 2027 to 2035. ICAO’s Member States expect CORSIA to play an important role in achieving carbon neutral growth from 2020 (ICAO, 2019c). However, the scheme has significant shortcomings, which make it unlikely international aviation will achieve its carbon neutral growth target. Further, CORSIA is planned to end in 2035, and ICAO has currently no plans for the period after that.

CORSIA’s coverage is limited. The scheme is designed to compensate for any increase in aggregate CO₂ emissions on routes that are covered by the scheme through the purchase and retirement of emissions units, which represent emission reductions or removals elsewhere.

The scheme applies only to flights between two states that participate in CORSIA. As a result, for the scheme to cover a majority of international aviation emissions, it is crucial to have participation from the country pairs that represent the majority of international air traffic. The following example makes this clear: if all countries participate, CORSIA would cover 100% of international aviation emissions in the period 2021-2035. If three of the countries responsible for a major share of international air traffic—for example China, the United Kingdom and the United States—do not participate, coverage would drop to merely 57% across CORSIA’s three phases (EDF, 2019).

Participation in CORSIA is voluntary at first and will be phased in for countries above certain economic and travel thresholds over time. However, countries may file a “difference” between their own regulations and the Standards and Recommended Practices (SARPs) that outline the CORSIA rules. By filing a difference, countries can justify non-compliance with CORSIA (ICAO, 2018; Mendes de Leon et al., 2015).

Eighty-three states participated in CORSIA from its inception in January 2021 (ICAO, 2020a). Although these 83 states account for more than 75% of international aviation traffic (ICAO, 2020b), flights between them cover less than 50% of international aviation emissions (EDF, 2019). Notably, Brazil, China, India and Russia did not participate in CORSIA’s inception and, as of August 2024, still do not participate. These four countries filed reservations on an ICAO Assembly Resolution that specified important aspects of CORSIA (Brazil, 2019; China, 2019; India, 2019; Russia, 2019). It is not possible to determine whether these countries also filed differences with the CORSIA SARPs, because differences are not publicly available. However, based on the reservations, we consider it likely these four countries did file a difference between the CORSIA SARPs and domestic regulations.

By 2024, the number of participating states increased to 126. Out of the total, 54 states are classified as Small Island Developing States (SIDS), least developed countries (LDCs), or landlocked developing countries (LLDCs) (ICAO, 2024b). Given that—even jointly—developing states are responsible for an insignificant share of international aviation emissions, their participation does not significantly expand CORSIA’s coverage of emissions. The number of participating countries is set to increase to 128 in 2025 (ICAO, 2020b).

Emission offsets are likely to deliver insufficient reductions due to heterogenous quality. Under CORSIA, aircraft operators can purchase emissions units to offset any growth in emissions. The ICAO Council issued a set of eligibility criteria for emissions units and for the programmes supplying emissions units (ICAO, 2020e). The set of criteria includes eight principles for assessing emissions unit integrity and outlines 11 specific design elements for emissions unit programmes (ICAO, 2019a). Based on the eligibility criteria, the ICAO Council approved unit programmes to supply emissions units during CORSIA’s pilot phase (ICAO, 2024a). The approved offset programmes primarily specialise in nature-based emissions reductions projects, such as reforestation.

However, independent analysis of the programmes approved for the pilot phase show that ICAO is not following its own eligibility criteria (Schneider et al., 2019). Chief among the concerns is that many of these programmes cannot guarantee that the emissions units generated are additional (i.e. would not have occurred in the absence of CORSIA) and will only be used to meet one climate target. Another independent review of CORSIA’s pilot phase found that 67% of eligible projects from approved programmes posed a significant risk of additionality (Christie-Miller & Howse, 2023).

For CORSIA’s first phase, ICAO requires a corresponding adjustment to emissions units. This means that governments hosting offset projects need to pledge to not double count emissions reductions sold under CORSIA towards their own emissions reductions targets (Gourlay, 2024a). As a result, most programmes that were previously approved during the pilot phase were denied full approval for the first phase. As of March 2024, only two programmes have been deemed eligible for the 2024–2026 compliance period: American Carbon Registry (ACR) and Architecture for REDD+ Transaction (ART) (Forest Trends’ Ecosystem Marketplace, 2024a). Both programmes focus on biological carbon dioxide removals (CDR): in August 2024, 88% of registered projects across the two programmes focus on biological CDR (Climate Focus, 2024).

It is important to note that biological carbon dioxide removals are not equivalent to emission reductions and, therefore, not suitable to make a neutralisation claim. Biological CDR is not permanent: it is very likely that the CO₂ stored in trees will be released back into the atmosphere within decades. This negates any benefits of sequestration (NewClimate Institute, 2024).

Expected emissions unit prices are unlikely to trigger investments in operational and technical measures to reduce CO₂ emissions from international aviation. Before the COVID-19 pandemic, multiple assessments predicted the supply of emissions units would likely exceed demand during CORSIA’s pilot phase (Ecosystem Marketplace, 2020; Fearnehough et al., 2019). The pandemic exacerbated this oversupply, as airlines only needed to purchase emissions units for routes that exceeded 2019 emissions levels, but emissions levels during the pandemic remained well below 2019 emissions. As a result, prices for emissions units were low: from 2021 through 2023, the average price of an eligible unit was approximately USD 6.60/tCO₂ (Forest Trends’ Ecosystem Marketplace, 2023, 2024b).

Given that only two offset programmes have so far been approved for CORSIA’s first phase, the supply of eligible emissions units is more limited than during the pilot phase. The demand for emissions units is expected to range from 64–162 million for the first phase (Prasanna, 2024). However, if no additional offset programmes are approved for the first phase, only an estimated 34 million units will be provided during this phase (Gourlay, 2024b). The limited supply of eligible units is likely to put upwards pressure on unit prices: indicative data suggests that eligible units may be valued above USD 50/tCO₂ during the first phase (Gourlay & Szabo, 2024). However, prices would need to reach over USD 200/tCO₂ to provide airlines with an incentive to use SAF made from waste cooking oil and over USD 800/tCO₂ for synthetic SAF (Pavlenko et al., 2019).

CORSIA eligible fuels may not deliver sufficient reductions. Along with purchasing and cancelling emissions units, airlines can use lower carbon or sustainable aviation fuels that are eligible under CORSIA against their emissions targets (ICAO, 2019c).

The ICAO Council first outlined its criteria for CORSIA eligible fuels in 2019. In 2021 and 2022, the Council expanded the set of criteria. Sustainable aviation fuels (SAF) produced by a certified fuel producer after January 1, 2024, must satisfy 13 criteria in order to be CORSIA eligible (ICAO, 2022a):

1. Fuels must achieve net GHG emission reductions of at least 10% compared to conventional jet fuel;
2. Fuels must not be made from biomass sources from land with a high carbon stock;
3. Emissions reductions attributed to fuels must be permanent;
4. Fuel production must maintain or enhance water quality and availability;
5. Fuel production must maintain or enhance soil health;
6. Fuel production must minimise negative effects on air quality;
7. Fuel production must maintain biodiversity, conservation value, and ecosystem services;
8. Fuel production must promote responsible waste management and chemical use;
9. Fuel production must respect human and labour rights;
10. Fuel production must respect land and land use rights, including indigenous and customary rights;
11. Fuel production must respect existing formal and customary water use rights;
12. Fuel production must contribute to social and economic development in regions of poverty;
13. Fuel production must promote food security in food insecure regions.

SAF produced before January 1, 2024, only need to meet the first two requirements. The first batches of CORSIA eligible SAF were produced in 2023. In comparison to standard aviation fuels, the waste-derived batches reduced CO₂ emissions by 75–84% (Uniting Aviation, 2023).

To qualify as an eligible lower carbon aviation fuel—a type of aviation fuel that is distinct from SAF—the abovementioned 13 criteria must be met, as well as one additional criterion: lower carbon aviation fuels must also minimise seismic, acoustic, and vibrational impacts (ICAO, 2022b).

The first criterion of 10% allows for a wide range of lower carbon fuels, which are still based on fossil fuels, to be classified as ‘CORSIA eligible fuels.’ Aircraft operators can claim emissions reductions benefits from any reduction in the full lifecycle of the fuels' value chain (paragraph 3.3.1, ICAO, 2018a). While the use of lower carbon fuels is unlikely to contribute to decarbonising international aviation, it may help achieve the goal of carbon neutral growth from 2020 (see Mitigation options section for more information on aviation fuels).

The second criterion should ensure that the production of biofuels used under CORSIA does not result in a stark increase in emissions caused by indirect land use changes (ILUC). Therefore, the Committee on Aviation Environmental Protection (CAEP), which is a technical committee of the ICAO Council and which negotiated and prepared most of the technical rules for CORSIA, determined the Indirect Land Use Change (ILUC) emissions from various biofuels. However, the CAEP’s methodology to derive ILUC emissions is based on optimistic assumptions and has led to significant optimism bias in the default ILUC values (Malins, 2019). Accordingly, aircraft operators may claim higher emission reductions than those actually achieved.

The remaining criteria consider the broader social and environmental implications of sustainable and lower carbon aviation fuel production (IATA, 2024b). ICAO-approved Sustainability Certification Schemes (SCS) are responsible for determining fuel eligibility (ICAO, 2022b). It remains to be seen how stringently these criteria will be applied and how compliance will be certified.

It remains unclear how participating states will adopt and enforce CORSIA. Most participating countries have not formalised CORSIA in their national laws, even though ICAO provides clear guidelines under CORSIA’s Standards and Recommended Practices (SARPs) to transpose CORSIA into law. Missing from CORSIA’s SARPs is a well-defined penalty system for noncompliance. Therefore, the responsibility for penalising airlines is shifted to participating countries, which will likely result in heterogeneity in the severity and enforcement of penalties (Gualandi, 2024a).