International Aviation

International aviation, which refers to flights that take off in one country and land in another, is generally not covered by countries’ NDCs. Before the outbreak of the COVID-19 pandemic, international aviation contributed 1.2% of global GHG emissions, or about 600 MtCO₂ annually (IEA, 2019). Total aviation emissions—emissions from both international and domestic flights—contributed approximately 2.4% of total global CO₂ emissions in 2018 (Graver et al., 2019).

Our assessment is of international aviation only. GHG emissions from domestic aviation are considered as part of national totals and are included in the assessment of individual CAT countries.

Due to data availability, our analysis is limited to CO₂ emissions. However, international aviation is also a source of non-CO₂ emissions and impacts that contribute to global warming, including NO_X and water vapour (H₂O), which, when emitted at altitude, forms contrail cirrus (Owen et al., 2010). Contrail cirrus traps heat that is radiated from the Earth’s surface back to the atmosphere.

We took historical CO₂ emissions from 1990 to 2022 from the IEA (IEA, 2023a, 2024). We estimated emissions in 2023 and 2024 based on estimates from ICAO and IATA for these two years (IATA, 2024c; ICAO, 2024c).

We evaluate ICAO’s target of achieving carbon neutral growth relative to 2020 levels. In addition, we evaluate CO₂ emission levels under ICAO CAEP’s integrated scenarios, which were built to inform ICAO’s discussion on adopting a long-term visionary goal, and IATA’s target of net-zero aviation by 2050.

ICAO’s goal of carbon neutral growth implies that international aviation’s net CO₂ emissions should remain constant at those levels. ICAO expects CORSIA to be the main measure to achieve carbon neutral growth and CORSIA’s baseline represents the basis of this goal (ICAO, 2020f).

When ICAO set the goal of carbon neutral growth in 2013 and established CORSIA in 2016, the organisation and airlines anticipated 2020 CO₂ emissions to be approximately 682–775 Mt (ICAO, 2013b, ICAO Assembly, 2016b, paragraph 11). However, due to COVID-19 international aviation emissions were much lower than anticipated.

In June 2020, the ICAO Council decided that 2019 emissions will form CORSIA’s baseline in the pilot phase (2021-2023) (ICAO, 2020d). The baseline for CORSIA’s first and second phase is set at 85% of 2019 emissions(ICAO, 2022d; ICAO Assembly, 2022b).

IATA committed to net zero carbon by 2050 and shows this may mean a reduction of 81% through SAF, technological improvements, and operational measures, while the remaining 19% will be achieved through offsetting of emissions (IATA, 2021).

IATA projects that CO₂ emissions from all aviation (i.e. domestic and international) may reach 1,800 Mt by 2050. As we look at international aviation emissions only, we assumed the 81/19 split applies equally to domestic and international aviation. If international aviation emissions were to be reduced by 81% by 2050, compared to the higher and lower end of our current policy projection for that year, residual emissions would reach 249–339 MtCO₂ by 2050.

We based the current policy projections on the projections for CO₂ emissions from international aviation until 2050 in ICAO’s Environmental Report of 2022, Figure 1-6 (ICAO, 2022c). The ‘CAEP-12 Baseline’ in this graph provides input to the upper bound of our current policy projections, and the lower end of the area ‘Additional Contribution of Improved ATM and Infrastructure’ provides input to the lower bound of our projections. The CAEP-12 baseline is based on a ‘mid demand forecast’ and subject to uncertainties.

ICAO shows the range of uncertainties in Figure 1–5 of the same report, showing fuel consumption under high, mid, and low demand scenarios. It is important to note that it is not clear what assumptions went into these demand scenarios.

We used the mid- and high- fuel consumption values from this graph to scale the mid-demand CAEP-12 baseline to a high demand scenario, yielding the upper end of our current policy projections (ICAO, 2022c).

We made no changes to the lower end of ICAO’s ‘Additional Contribution of Improved ATM and Infrastructure’ for two reasons. First, we do not know how a change in demand would influence operational and technological improvements. It could be that with lower demand, airlines have fewer resources to implement such measures. Second, neither the aviation industry nor governments push for demand management measures, so we consider it unlikely that ICAO’s low demand scenario will materialise.

ICAO’s ‘Additional Contribution of Improved ATM and Infrastructure’ projects that international aviation emissions bounce back to 2019 levels by 2027. However, in reality the sector is projected to reach 2019 emission levels in 2024 already. We therefore shift the annual growth rates in ICAO’s scenario upwards by three years; in other words, we use ICAO’s projected growth rate for 2028 in 2025. For the years 2048 through 2050, we assumed the average growth rate of the last three years projected by ICAO’s ‘Additional Contribution of Improved ATM and Infrastructure’ scenario.