International Aviation

Although emissions in the aviation sector are more challenging to abate than in many other sectors, studies suggest it is possible for the sector to decarbonise by mid-century (Energy Transitions Commission, 2019) or in the second half of this century (Napp et al., 2019).

There is no one silver bullet to reduce aviation’s climate impact. To successfully reduce GHG emissions and other climate forcers, governments should impose a combination of abatement measures. These can broadly be categorised into four categories: demand management, operational measures, improving energy efficiency and the adoption of decarbonisation technologies.

Demand management is to play a crucial role in reducing aviation’s climate impact, particularly on the shorter term, as operational and technological measures have either limited abatement potential or are not yet available at scale.

In 2018, only 11% of the global population flew, and only 4% internationally (Gössling and Humpe, 2020). Just 1% of the global population was responsible for 50% of CO₂ emissions from air travel in 2018 (Gössling and Humpe, 2020). This suggests that emissions from aviation can decrease substantially if demand from frequent fliers reduces. However, it also implies that emissions from aviation will increase significantly if a larger share of the global population gets access to air travel.

Demand management includes a modal shift to high-speed trains or reduced travel through, incentivised for example, by increased costs for airlines and passengers. The COVID-19 pandemic brought with it an increased reliance on the internet for meetings to replace face-to-face meetings, and this, too, provides an alternative to the need for air travel. The IPCC’s Sixth Assessment Report showed that avoidance of long-haul flights and shifting to trains, amongst other socio-cultural factors, may reduce aviation emissions by 10-40% by 2050 (Creutzig et al., 2022).

Modal Shift

Governments can introduce policies to reduce the need for aviation, such as supporting a modal shift from short-haul flights to high-speed rail by making substantial financial investments in rail infrastructure (Energy Transitions Commission, 2019). They could implement regulations to reduce the number of airport slots, which are time slots during which an aircraft from a specific operator may land and take off from an airport.

It must be noted, however, short-haul flights are responsible for a small share of total aviation emissions, so a modal shift should be seen as a complementary rather than a crucial mitigation measure (Graver, Zhang and Rutherford, 2019).

Levies & taxes

Governments can reduce passenger demand for aviation by increasing the costs of flight tickets through taxes and fees, such as fuel taxes and ticket taxes. Although various countries tax jet fuel on domestic flights (Larsson et al., 2019), there are few examples of a jet fuel tax for international flights. The US has a federal tax rate of USD 0.01 per litre, but international air carriers may be exempted if agreed in bilateral Air Service Agreements (CE Delft, 2019). Although it is unlikely that countries will agree on a global tax on kerosene (Larsson et al., 2019), states may enter into bilateral agreements to tax kerosene on flights between their airports.

A number of countries, including France, Germany, South Africa and the United Kingdom, have implemented a ticket tax (Faber and Huigen, 2018). A study by CE Delft (2019) found that in most EU Member States, a 10% increase in ticket prices would result in a 9-11% reduction in demand and a similar reduction in the number of flights. In July 2021, the European Commission proposed introducing a tax on kerosene on intra-EU commercial and private flights, which would start at a rate of zero in 2023 and gradually be increased (European Commission, 2021a).

Operational measures include improved air traffic management (ATM), such as optimising routing, air traffic flow management and minimising flight distances. National governments should collaborate with airports and air navigation service providers to improve existing ATM infrastructure.

Research indicates that regional air traffic management, such as under the Single European Sky framework, could save approximately 5% of CO₂ by 2050 below business as usual (Energy Transitions Commission, 2019). In addition to reducing carbon emissions, operational measures also have the potential of reducing the non-CO₂ emissions from aviation. For instance, optimised routing can lead to a relatively large reduction in contrail formation and NOX emissions (Grewe et al., 2017).

According to the Energy Transitions Commission (2019), a combination of operational and demand management measures could reduce 2050 CO₂ emissions by 15% compared to a (pre-COVID) business as usual scenario.

Improved energy efficiency, includes, for instance, improving thermodynamic efficiency of new engines and improved aircraft design. Engines of the current fleet have an average motor thermodynamic efficiency of approximately 50%. Estimates suggest this could be improved to 65-70%, depending on the development of new materials, design and component technologies (National Academies of Sciences Engineering and Medicine, 2016). Further energy efficiency improvements can be achieved through technologies like blended wing bodies and revolutionary changes in the positioning of fuel tanks, which would enable hydrogen-fuelled planes (Energy Transitions Commission, 2019).

National governments can invest public expenditure to support R&D focusing on improvements of airframe and engine efficiency (Energy Transitions Commission, 2019). Further, governments should set high efficiency standards for existing aircrafts, which would accelerate fleet turnover (Rutherford, 2020).

Decarbonisation technologies include synthetic power-to-liquid fuels, hydrogen, electric batteries and biofuels. Of these technologies, power-to-liquid synthetic fuels offer the largest abatement potential (Energy Transitions Commission, 2019; Searle et al., 2019). These have the advantage that the sector could continue to use the existing infrastructure, vehicles and engines, which allows a more gradual transition to alternative fuels (Scheelhaase, Maertens and Grimme, 2019). Power-to-liquid fuels are, however, currently extremely expensive, which represents a challenge to wide scale deployment (Scheelhaase, Maertens and Grimme, 2019; Searle et al., 2019).

Electric batteries are not expected to be feasible at scale by mid-century, except for short-haul flights (Hall, Pavlenko and Lutsey, 2018). Researchers expect biofuels to play an important role in decarbonising aviation from 2020 onwards (Napp et al., 2019) and biofuels will likely be approved as ‘CORSIA eligible fuels’. However, supply is limited and other sectors also expect to replace fossil fuels with bioenergy (Searle et al., 2019).

Decarbonisation technologies could reduce 2050 CO₂ emissions by 50% compared to 2005 to 100% compared to a current practice scenario (Energy Transitions Commission, 2019; Napp et al., 2019). They also can help reduce non-CO₂ emissions and impacts from aviation. For example, researchers found that soot emissions – and consequently contrail cirrus formation – can be reduced by switching to alternative fuels (Burkhardt, Bock and Bier, 2018).

However, airlines had a small profit margin prior to the COVID-19 pandemic and their economic situation has worsened since. This likely slows down the shift to a more sustainable aviation sector (Peeters and Melkert, 2021). While governments around the world spent billions of dollars on airline bailouts, they generally did not attach binding environmental conditions (Bogaisky, 2020; Climate Action Tracker, 2020; T&E, CMW and Greenpeace, 2021).

To realise the mitigation potential from alternative fuels and electric batteries, it is crucial that national governments play an active role and, among others, set sustainable fuel quota for aviation (Larsson et al., 2019), as well as establish clear sustainability criteria that such fuels should meet. Governments could also provide financial support, such as grants and loan guarantees, to lower financial risks for biofuel refinery project investments and use public expenditure to support R&D of alternative fuels (Energy Transitions Commission, 2019; Feuvre, 2019).