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. Abatement measures can broadly be categorised into four categories: demand management, operational measures, energy efficiency improvements, and decarbonisation technologies.

Demand management must play a crucial role in reducing aviation’s climate impact, particularly in the short 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 & Humpe, 2020). Just 1% of the global population was responsible for 50% of CO₂ emissions from air travel in 2018 (Gössling & 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 encouraging a modal shift to high-speed trains or a reduction in travel, which can be incentivised, for example, through increased costs for airlines and passengers. The COVID-19 pandemic brought with it an increased reliance on video conferences to replace face-to-face meetings, providing an alternative to the need for air travel. The IPCC’s Sixth Assessment Report showed that avoidance of long-haul flights and shifting short-haul flights 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 by making substantial financial investments in rail infrastructure and, in doing so, enabling a modal shift from short-haul flights to high-speed rail (Energy Transitions Commission, 2019). Governments could implement regulations to reduce the number of airport time slots during which an aircraft from a specific operator may land and take off from an airport.

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

Levies & taxes

Governments can reduce passenger demand for aviation by increasing the costs of flight tickets through taxes and fees, such as fuel and ticket taxes. Such taxes could provide governments with revenues to support the buildout of less-emitting modes of transport, such as high-speed rail, or to address the highly unequal distribution of aviation emissions (Teusch & Ribansky, 2021). 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 through 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 could 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 & 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 2021, the European Commission proposed introducing a tax on kerosene on intra-EU commercial and private flights (European Commission, 2021). However, the proposal has yet to be adopted and progress towards adoption seems unlikely before 2025 (see National policies section for more information).

Operational measures include improved air traffic management (ATM), such as optimising routing, managing air traffic flow, 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 the Single European Sky framework, could save approximately 5% of CO₂ by 2050 below business as usual scenarios (Energy Transitions Commission, 2019). The EU Council and Parliament agreed in March 2024 to revise the Single European Sky framework, which could unlock further emissions reductions (Soone, 2024). In addition to reducing CO₂ emissions, operational measures also have the potential of reducing the non-CO₂ emissions from aviation. For instance, optimised routing can lead to relatively large reductions in contrail formation and NO_X emissions (Grewe et al., 2017).

Improved energy efficiency can be achieved by, for instance, improving the thermodynamic efficiency of new engines and the design of new aircrafts. 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). Particularly promising are further advances in geared turbofan engines (IATA, 2024a). Further energy efficiency improvements can be achieved through novel, more aerodynamic designs like canard, truss-braced, and 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 support R&D focussing on improvements of airframe and engine efficiency (Energy Transitions Commission, 2019). Analysis from the International Council on Clean Transportation estimates that governments need to spend USD 0.8–1.4 trillion on supporting the development of emerging aviation technologies between 2020 and 2050. Further, governments should set high efficiency standards for existing aircrafts, which would accelerate fleet turnover (Rutherford, 2020).

Decarbonisation technologies include sustainable aviation fuels (SAF), hydrogen, and electric batteries. Generally, decarbonisation technologies for aviation are not yet commercially viable at scale and, therefore, require continued research and development from both public and private sectors.

Sustainable aviation fuels (SAF)

SAF are aviation fuels with a lower carbon content than standard jet fuels. Less than 1% of all jet fuel used today are SAF, but sectoral benchmarks for the aviation sector show that this share must increase to 10–15% by 2030 and 100% by 2050 (Boehm et al., 2023; IEA, 2023b; UNFCCC, 2021). SAF are currently much more expensive than standard jet fuels, which represents a challenge to wide-scale deployment (Scheelhaase et al., 2019; Searle et al., 2019). Government action, for instance in the form of SAF quotas and taxes on kerosene, are necessary to incentivise airlines to scale up SAF use.

Most SAF produced today are biofuels, which are made from waste cooking oil or forestry and agricultural residues. Synthetic power-to-liquid fuels, made from captured carbon, water, and (renewable) electricity, are expected to make up a significant share of jet fuels over the coming decades (Energy Transitions Commission, 2019; Searle et al., 2019). SAF have the advantage that the aviation sector could continue to use the existing infrastructure, vehicles, and engines, which allows a more gradual transition to alternative fuels (Scheelhaase et al., 2019).

SAF, in particular synthetic power-to-liquid fuels, are considered the decarbonisation technology with the largest emissions reduction potential for the aviation sector (Energy Transitions Commission, 2019; Searle et al., 2019). An assessment of 12 aviation decarbonisation roadmaps show that SAF can reduce CO₂ emissions in the aviation sector by 70% by mid-century, compared to scenarios with no or limited mitigation action (Becken et al., 2023). However, this estimate may be optimistic, as the roadmaps do not account for the decadal time lags between when biofuels from forest biomass are burned and when newly planted trees start to sequester CO₂.

While the production of SAF needs to rapidly increase to decarbonise the aviation industry, there will be economic and social limitations to the amount of SAF that can be produced. Producing SAF requires significant amounts of renewable electricity, which other sectors also need to reduce their emissions. The aforementioned assessment of aviation decarbonisation roadmaps found that the production of SAF could require 9% of global renewable energy and 30% of the world’s sustainable biomass by 2050 (Becken et al., 2023). This means that scaling SAF production presents a huge opportunity cost for decarbonising other sectors, including the power sector.

Scaling up SAF also requires more land dedicated to renewable energy installations and, possibly, the production of bioenergy crops (Becken et al., 2023). This land can then no longer be used to grow food crops or store carbon, which makes it more difficult to neutralise economy-wide residual emissions and bring global emissions to net zero. In addition, SAF do not necessarily reduce non-CO₂ climate effects of aviation.

For these reasons, SAF are not the silver bullet to decarbonising the aviation industry. Instead, scaling SAF should go hand-in-hand with demand management and other abatement measures.

Hydrogen

Hydrogen, in particular liquid green hydrogen, is receiving increasing interest as an alternative to SAF (Mukhopadhaya, J; Rutherford, 2022). Liquid green hydrogen would have zero CO₂ and non-CO₂ emissions. However, hydrogen combustion in fuel cells would produce water vapour and therefore result in contrail cirrus, which has a radiative forcing effect.

Neither liquid hydrogen nor hydrogen fuel cells are mature technologies. The rollout of hydrogen-power aircraft and necessary infrastructure is not expected to happen before 2035 (Grimme & Braun, 2022). In an ambitious scenario, hydrogen aircraft could reduce emissions from aviation by about 15% by 2050. Since hydrogen-power aircraft are heavier and less efficient than aircraft flying on fossil fuels or SAF (Mukhopadhaya, J; Rutherford, 2022), their uptake will likely be limited to short- and medium-haul flights; distances at which these aircrafts may also compete with high-speed rail.

Like SAF, liquid hydrogen and fuel cells are still substantially more expensive than jet kerosene, which presents a barrier to adoption (Hoelzen et al., 2022). At scale, the production of hydrogen also requires massive amounts of renewable energy (Grimme & Braun, 2022), which is currently not available. Scaling renewable energy production for the aviation industry presents an opportunity cost to decarbonising other economic sectors.

Electric batteries

Electric batteries charged with renewable energy have no in-flight emissions and non-CO₂ effects (Hoelzen et al., 2022). Battery-electric aircraft are also more energy-efficient than aircrafts fuelled by SAF, the production of which incurs large energy losses.

However, electric batteries are not expected to be operational at scale by mid-century, except for short-haul flights, where they may contend with high-speed rail (Hall et al., 2018; Mukhopadhaya & Graver, 2022). The International Council on Clean Transportation (ICCT) estimates that electric aircrafts can contribute to an annual reduction of 3.7MtCO₂ by 2050, if used on all possible routes (Mukhopadhaya & Graver, 2022). This represents 0.2% of expected emissions from the aviation industry (domestic and international) by mid-century.

Airlines had a small profit margin prior to the COVID-19 pandemic and their economic situation has worsened since. The level of profitability likely slows down the shift to a more sustainable aviation sector (Peeters & Melkert, 2021). While governments around the world spent billions on airline bailouts, they generally have not attach binding environmental conditions (Bogaisky, 2020; Climate Action Tracker, 2020; T&E et al., 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 quotas 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 the research and development of alternative fuels (Energy Transitions Commission, 2019; Feuvre, 2019).