International Aviation

Critically Insufficient4°C+
Highly insufficient< 4°C
Insufficient< 3°C
2°C Compatible< 2°C
1.5°C Paris Agreement Compatible< 1.5°C
Role model<< 1.5°C


Countries created ICAO 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. However, in 2013, the ICAO Assembly – composed of 193 Member States - agreed on the goal of ‘carbon neutral growth from 2020’ and in 2016 it established the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA).

Projected emissions in the COVID-19 era

It is highly uncertain what medium to longer-term impact COVID-19 will have on international aviation and associated emissions. The recovery trajectory depends on a number of factors, including when travel restrictions are lifted; how the world economy will recover and grow; whether demand will change (such as whether working from home and holding virtual meetings become the default option); and whether social distancing norms will prevent airlines from flying at full capacity.

ICAO estimates that passenger numbers could decrease by 45-62% in 2020 compared to 2019 (ICAO, 2020b). This is roughly in line with IATA’s prediction that passenger demand for both domestic and international aviation will decline by 48% this year compared to 2019 (IATA, 2020b). IATA further expects that aviation emissions will not return to pre-COVID-19 levels before 2023 (IATA, 2020c).

Based on these estimates, we developed two scenarios to show how international aviation emissions could develop up to 2050, covering the full uncertainty of reduced emissions in 2020 and fast or slow recovery.

Low emissions: assumes a 60% decrease in 2020 international aviation emissions, compared to 2019; and that emissions will be back to 2019 levels by 2024, one year later than IATA assumes. We assume that after 2024, aviation emissions will continue to grow at the same rate as emissions under the optimistic technology improvements scenario, as provided for by the ICAO in its Environmental Report (ICAO, 2019b). We used the optimistic improvements here to generate a possible lower bound for future aviation emissions, assuming that airlines will not fly at full capacity in the near future and leave their oldest, least efficient aircraft on the ground. In this low emissions scenario, international aviation emissions would amount to 715 Mt in 2030.

High emissions: assumes a 45% decrease in 2020 international aviation emissions, compared to 2019; and that aviation emissions will be back at 2019 levels by 2022, one year earlier than IATA assumes. We assume that after 2022 aviation emissions continue to grow at the same rate as in the low technology improvements scenario, as projected by the ICAO in its Environmental Report (ICAO, 2019b). We used the pessimistic improvements here to generate a possible upper bound for future aviation emissions, assuming that airlines will fly at full capacity again, and continue to use older, less efficient aircraft. In this high emissions scenario, international aviation emissions would amount to 835 Mt in 2030.

Pre-COVID projections suggested that international aviation emissions would amount to 750 Mt in 2030 under an optimistic technology improvements scenario, or to 880 Mt under a low technology improvement scenario.


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, or through the use of alternative fuels. The scheme is currently planned to run across three phases, from 2021 to 2035. ICAO’s Member States expect CORSIA to play an important role in achieving carbon neutral growth from 2020 (ICAO, 2019b). However, we find that the scheme has significant shortcomings, which make it unlikely international aviation will achieve its target.

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 emission units, which represent emission reductions or removals elsewhere.

The scheme applies only to flights between two states that participate in CORSIA, so 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 (Mendes de Leon et al., 2015; ICAO, 2018).

As of May 2020, the ICAO lists 83 States it expects will participate in CORSIA from its inception (ICAO, 2020a). Although these 83 states account for more than 75% of international aviation traffic (ICAO, 2020a), flights between them cover less than 50% of international aviation emissions (EDF, 2019). Notably, Brazil, China, India and Russia are missing from ICAO’s list. 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 and will not participate in CORSIA from its outset.

Emission offsets are likely to deliver insufficient reductions due to heterogenous quality. Under CORSIA, aircraft operators can purchase emission units to offset any growth in emissions. In order for emission units to neutralise emissions, it is imperative they:

  • Represent additional emission reductions that would not have occurred in the absence of CORSIA;
  • Do not result in an increase in emissions elsewhere in the world;
  • Are accurately measured, reported, and verified;
  • Are permanent; and
  • Are used and claimed only once towards any type of climate target.

The ICAO Council approved a set of emissions unit eligibility criteria. These include eight criteria for the emissions units and 11 criteria for the programmes supplying these units (ICAO, 2020d). However, independent analysis of the programmes approved for the pilot phase show that the 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.

Expected emissions unit prices are unlikely to trigger investments in operational and technical measures to reduce CO2 emissions from international aviation. Various assessments predict the supply of emissions units is likely to exceed CORSIA’s demand (Fearnehough et al., 2019; Ecosystem Marketplace, 2020). If the baseline for carbon neutral growth is adjusted to reflect only 2019 emissions, this oversupply will be exacerbated by COVID-19 due to a drop in near-term demand. To date, prices for these emissions units have remained fairly low – approximately €3 between 2016 and 2018 (Forest Trends’ Ecosystem Marketplace, 2019). Consequently, airlines have very limited incentive to reduce emissions though technical and operational measures, because purchasing offset credits is cheaper.

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, 2019b).

At the time of writing, national representatives working through the ICAO Council have approved two criteria for eligible fuels (ICAO, 2019a):

1) Fuels must achieve net GHG emission reductions of at least 10% compared to standard jet fuel, and;

2) Fuels must not be made from biomass sources from land with a high carbon stock.

The first criterion of 10% could, pending further rule making, allow for a wide range of lower carbon fuels to be used as ‘CORSIA eligible fuels’. Aircraft operators can claim the life cycle emissions benefits, which means that if an eligible fuel leads to a 10% reduction, the aircraft operator can claim 10% emissions reduction, if the fuel leads to a 30% reduction compared to standard fuel, aircraft operators can claim 30% (paragraph 3.3.1, ICAO, 2018a). While the use of lower carbon fuels is unlikely to lead to decarbonising international aviation, it may help achieve the goal of carbon neutral growth from 2020.

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.

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