Commentary: Capping Aviation Emissions – A Pressing Necessity with a Potential Solution

by Chris Lyle ( Air Transport Economics/GreenAir) For the “difficult to decarbonise” air transport sector, the advancement of sustainable aviation fuels (SAF) and alternative propulsion sources are well under way, along with technological and operational emissions mitigation measures. But even collectively they are likely to fall significantly short of the achievement of emissions reduction goals stemming from the Paris Agreement on climate change. Hence demand management will be necessary. Fiscal instruments such as taxes and frequent flyer levies fall foul of privacy and competitive issues, and notably of the economic regulatory framework unique to international air transport services. But capping emissions directly is feasible and would have definitive effect. Chris Lyle elaborates on the issues and puts on the table an emissions capping concept with airports as the nub in the context of the flights they enable.

Irrespective of the ‘ownership’ of flight emissions – generally attributed to passengers/shippers and particularly air carriers, to whom mitigation action is today predominantly addressed – the best origin for capping the emissions is the airport. And the prevalence of self-interest and privacy issues for passengers/shippers, airlines and other market players means that government regulation is necessary. Emissions capping action with airports as nexus is feasible within the existing regulatory framework and could be most effective.

Blending mandates for SAF are effectively airport-oriented and amongst recent examples one by the Singapore government is of particular interest. In February 2024, it announced SAF blending targets for departing flights to be applied from 2026, in this case to be partly funded by a levy on passengers which will vary according to both class of travel and distance (in bands). This initiative falls within both the environmental and the economic regulatory frameworks for international air services – as could emissions capping action based on airport departures. Different circumstances apply from the 2023 Amsterdam Schiphol flight capping proposal, which was aimed essentially at noise and local air quality rather than emissions reduction and came up against the globally agreed ‘balanced approach’ to aircraft noise management.

At present, emissions assessed for airport accountability are limited in compass. Emissions ascribed to airports can be restricted to the Scope 1 airport operations, although increasingly they include a component of Scope 2 emissions. However, and crucially, they do not include emissions from the flights that the airports empower. Also, emissions from international flights are presently treated separately from those generated by domestic flights and from the not insubstantial emissions generated by local ground transportation and business created around airports by all flights.

Airports could become key enablers in aviation emissions reduction if their accountability were to move to a form of Scope 3 which had the extent of including emissions from the flights departing their runways to their first destination, whether domestic or international.

A climate-based approach could be to cap not simply the number of flights but rather the volume of emissions from the first leg of all departing flights, whether by passenger, combi or freight aircraft. Data regarding CO2 from domestic operations should be readily accessible at the national level and data from international operations, in total and for individual routes, are available through various sources, notably including the Monitoring, Reporting and Verification system of ICAO. While not government instituted, the necessary information is also accessible from entities such as Google Flights and Travalyst. For the 1,300 largest global airports it has already been packaged since 2019 by Airport Tracker, a project attempting to visualise the climate impact of airports worldwide. In cases where data may be incomplete, emissions evaluated from fuel uplifted by flight could be used as a surrogate.

The capping approach – for an airport or for a group of airports serving the same city – could follow the UNFCCC direction by starting with the 2025 emissions peaking and being reduced annually through to half the 2019 levels in 2030. Beyond that time frame is presently aspirational and somewhat speculative. Parameters to apply beyond 2030 could be made at a later date; similarly, targets for non-CO2 emissions could be incorporated once a more definitive impact for them is accepted – although applying limits on CO2 emissions would automatically cap non-CO2 pro rata.

The table below, with data developed from Airport Tracker and flight search engines, illustrates the stark reality and enormity of such a challenge. The data are representative of the first leg of outbound flights for a typical large European airport (adjusted for illustrative purposes to a round total of 10 MtCO2 emissions for 2019 and hence half that, 5 MtCO2, for 2030). There is an assumed overall percentage increase of 3% in emissions for 2025 over 2019. A single capping reduction is here applied across the board, irrespective of distance. As shown, for the 2019 base year, 19% of departing flights were over 4,000 km but they produced 66% of the emissions. The contribution of long-haul is actually significantly higher than this at major hubs such as Paris Charles de Gaulle, Frankfurt or London Heathrow.

In pursuance of climate justice and the UNFCCC principle of Common but Differentiated Responsibilities, exemptions could be given for flights to and from Least Developed Countries, Landlocked Developing Countries and Small Island Developing States, with the cap(s) for other flights being reduced pro rata.

The procedure for application of the caps would, as today, be through the airport slot allocation process. While allocation based on emissions generated by a flight, rather than simply the flight itself, would add a dimension of complexity to the process, this would be well within the bounds of practicability, with likely fewer flights and little effect on timings.

Action could be taken nationally, with the level of capping or reduction determined in the context of other emissions mitigation measures in place for the country concerned. Such a national approach, if carefully designed and applicable to all carriers at an airport, would not breach the Chicago Convention or air services agreements. Even action by a handful of countries would help, preferably in a co-ordinated framework. READ MORE