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. International aviation currently contributes 1.2% of global GHG emissions - or about 600 MtCO₂ annually (IEA, 2019). Total aviation emissions – that is emissions from both international and domestic flights - contributed approximately 2.4% of total global CO₂ emissions in 2018 (Graver, Zhang and Rutherford, 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 NOx and water vapour (H2O), which, when emitted at altitude, forms contrail cirrus (Owen, Lee and Lim, 2010). Contrail cirrus traps heat that is radiated from the Earth’s surface back to the atmosphere.

Historical CO₂ emissions from 1990 to 2017 were taken from the IEA and emissions for 2018 to 2020 from ICAO’s CAEP (CAEP, 2021).In our 2020 assessment, we extrapolated emissions from 2017, the last year for which the IEA provided data, using an average of the 2014-2017 growth rate. In this assessment, we used data from ICAO’s CAEP for 2018, 2019 and 2020. CAEP estimates that annual CO₂ emissions are around 30 Mt lower in this period than what the IEA provided.

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

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 this level.

In June 2020, the ICAO Council decided that 2019 emissions will form CORSIA’s baseline in the pilot phase (2021-2023) (ICAO, 2020d). In 2022, the ICAO Assembly will decide on the baseline for CORSIA’s first and second phase (ICAO, 2020d). As the baseline after 2023 is currently uncertain, this CAT assessment rates the target based on the 2019 baseline, but also provides a baseline composed of average emissions in 2019 and 2020 as a reference point.

IATA committed to net zero carbon by 2050 and shows this may mean a reduction of 81% through SAF and technological 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 1800Mt 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 213-339 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, 2022). As ICAO does not provide exact data points for future years, we extracted data points from the ‘CAEP-12 Baseline’, which is the basis for the upper bound of our current policy projections, and the lower end of the area ‘Additional Contribution of Improved ATM and Infrastructure’, which is the basis for 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 a Figure showing fuel consumption under high, mid and low demand scenarios. It is, however, not clear what assumptions went into these demand scenarios. We derived the difference in fuel consumption (in Mt) between the mid and high forecasts and multiplied this with a factor 3.16 to come to the upper bound of our projections. ICAO provides that 1 kg of fuel burned generates 3.16 kg of CO₂ (ICAO, 2022).

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.

For the years 2015–2017, the historical emissions used by ICAO are very close to historical emissions as provided by the IEA, so we did not need to harmonise the data.