Aviation

Notable progress towards getting aviation on track includes the following: 

• In 2024 Brazil adopted the Fuel of the Future law, which requires fuel operators to reduce GHG emissions from domestic flights by 1% in 2027, increasing to 10% in 2037, through use of SAFs.
• In 2023 the European Union adopted ReFuelEU Aviation, which mandates minimum sustainable aviation fuel (SAF) blend-in shares through 2050, with sub-targets for synthetic fuels.
• In 2022 the United States announced important tax credits and a competitive grant programme under the Inflation Reduction Act (IRA), granting up to USD 1.75 per gallon of SAF produced, with the aim of meeting the milestones of 3 and 35 billion gallons per year by 2030 and 2050, respectively. By 2024, USD 245 million in grants had been announced through the IRA, and the Department of Energy issued nearly USD 3 billion of loan guarantees for scaling up SAF production
• The 193 member states of the International Civil Aviation Organization (ICAO) adopted a long-term aspirational goal (LTAG) in 2022 of net zero carbon emissions from international aviation by 2050.
  

Aviation emissions rose in 2023 to reach more than 90% of their 2019 pre-pandemic peak level. After increasing at an average of 2.2% per year from 1990 to 2019, direct CO₂ emissions from fossil fuel combustion plummeted from more than 1 000 Mt CO₂ in 2019 to less than 600 Mt CO₂ in 2020, in the context of the pandemic. As demand from air passengers recovered in 2022 and 2023, emissions increased in all regions, with the exception of Russia (due to Russia's full-scale invasion of Ukraine and consequent international sanctions), reaching almost 950 Mt CO₂. CO₂ emissions are expected to surpass their 2019 level in 2025.

A comprehensive set of measures aiming to promote innovative technologies, scale up SAFs, and implement demand-side management will be needed to bring the currently rising emissions level below 1 000 Mt CO₂ by 2030, in line with the NZE Scenario. Policy and fiscal support can drive improvements in energy efficiency, stimulate investment in pre-commercial and low-emissions SAFs, and accelerate the development of alternatives to jet kerosene-powered aircraft, such as electric or hydrogen-powered aircraft. The use of SAF or flight altitude optimisation could reduce the effect of other important contributors to the aviation industry’s overall climate impact, such as contrails.

From 2010 to 2019, average fuel efficiency per revenue passenger kilometre (RPK) improved by over 2.5% per year. On a revenue tonne kilometre (RTK) basis, which includes passengers, their luggage, and freight, fuel efficiency improved by 1.7%, nearly reaching ICAO’s aspirational goal of 2% per annum (measured per RTK) through 2050. However, efficiency improvements have not kept up with demand growth to date, with RPK growing at an average rate of more than 6% annually between 2010 and 2019. To get on track with the NZE Scenario, efficiency will need to improve at a rate of 2.6% per year through 2030 on an RPK basis, in line with the historical average improvements over the past two decades.

In addition to technical efficiency improvements in engine and airframe designs, improvements in payload and traffic efficiency (i.e. the weight of cargo and number of passengers carried per aircraft) have also contributed to reducing the energy intensity of aircraft operation. Payload energy efficiency deteriorated in 2020, as planes were flown with fewer passengers, but recovered to about the same level as 2019 by the end of 2023.

Total commercial air passenger activity (domestic and international combined) rebounded to nearly 95% of pre-pandemic levels in 2023, with domestic aviation growing by 30% and international aviation by more than 40% year-on-year. This was primarily driven by growth in the Asia Pacific region, with demand for international flights that increased by more than 120% compared to 2022 due to the re-opening of international travel from China.

Air cargo volumes contracted by 8% in 2022 and 2% in 2023 after a historical peak in 2021, as the share of freight transported on passenger flights has slowly approached pre-pandemic levels.

Currently, demand for aviation fuel is dominated by jet kerosene, while SAFs account for less than 0.1% of all aviation fuels consumed. Manufacturers and operators are testing flights that are entirely fuelled by SAFs, which can be deployed in current infrastructure, engines and aircraft. The maximum SAF share allowed under current regulation is 50%, but dedicated task groups within fuel standard committees are assessing options to facilitate the use of 100% SAF and to have approved fuels ready by 2030. However, existing and planned SAF projects in advanced stages will meet just 2-4% of jet fuel demand by 2030. Increasing SAF use in aviation to over 10% by 2030, in line with the NZE Scenario, will require a significant ramp-up of investment in capacity to produce SAFs, and supportive policies such as fuel taxes and low-carbon fuels standards.

Commercial adoption of SAFs will be driven by policies such as the incentives for SAF production in the United States and the United Kingdom, and mandates for minimum GHG emissions through SAFs as per the Fuel of the Future law in Brazil or minimum SAF share as per the European Union’s ReFuelEU regulation. ReFuelEU excludes SAFs produced through food and feed crops due to sustainability concerns, and the potential to scale up biojet kerosene supply based on animal fat and waste oil feedstocks (which are included in the regulation) is limited by their supply. Supporting R&D on advanced biofuel technologies can unlock more abundant and cheaper feedstocks such as agricultural residues, dedicated energy crops and municipal solid waste. In the longer term, synthetic fuels based on hydrogen produced using electrolysers (and running on low-emissions electricity), combined with CO₂ from biogenic, concentrated waste streams or atmospheric sources can provide an alternative, although commercialisation may be challenging.

Incremental improvements to engines, materials, aerodynamics and mild hybridisation can and should be implemented. However, “revolutionary” designs, such as new airframe configurations and alternative propulsion technologies such as electric or hydrogen-powered aircraft, are needed in order to make a leap towards significant CO₂ emissions reduction.

Hydrogen can be used via direct combustion in jet engines, or in fuel cells to generate electricity for electric motors, or in a combination of the two. However, using hydrogen in aircraft poses significant challenges, including the need for innovative fuel storage and delivery methods, low-cost and lightweight cryogenic tanks, and redesigned airframes to accommodate them. In addition, questions on the impact of hydrogen-powered aircrafts on contrails still need to be answered and dedicated studies are still underway.

Test flights for aircraft equipped with fuel cell-powered electric propulsion systems took place in early 2023, with ZeroAvia flying a 19-seat aircraft equipped with a hydrogen-electric engine on its left wing, and Universal Hydrogen flying a 40-seat regional aircraft, with one of its engines powered by fuel cell. This followed its demonstration at the Blagnac airport in Toulouse, France, of prototype modular capsules that can store liquid hydrogen.

Demonstrations of alternative hydrogen propulsion systems have also progressed. In early 2024, Airbus's ZEROe engines were tested successfully. In 2022, Rolls-Royce and easyJet tested combusting hydrogen to run a regional jet engine with hydrogen produced from wind and tidal power. In the same year, Avio Aero launched a demonstration programme for megawatt-level hybrid electric propulsion technologies, coupling a propulsion engine with a fuel cell-powered electric motor. H2FLY has also begun the integration of a liquid hydrogen storage system tank in its four-seat aircraft with hydrogen-electric propulsion.

Battery electric aircraft have no direct emissions, potentially much lower operational and maintenance costs (dependent on battery durability) and high efficiency, as well as creating far less noise pollution. However, current battery energy density and weight severely restrict the range of battery electric flights and the size of the aircraft. In September 2022 Eviation performed a test flight of its nine-seat electric aircraft, with a maximum flight range of around 450 km. In 2024, in collaboration with the battery company CATL, a 4-tonne civil electric aircraft was successfully tested.

The success of deploying electric aircraft technologies will largely depend on the evolution of battery technologies. The energy density of today’s state-of-the-art lithium-ion batteries ranges between 180 and 300 Wh/kg at the pack level. In 2023, CATL released a condensed state battery (500 Wh/kg) targeting passenger aircraft applications. However, further breakthrough innovations are still required in this sector for passenger flights of 1 000 km or beyond, as a battery pack energy density of at least 800 Wh/kg would be needed.

Airbus and ArianeGroup are working towards building the first liquid hydrogen refuelling facility for ZEROe aircraft at Blagnac airport in Toulouse, France.

SAF production gained substantial support through the passage in 2022 of the IRA in the United States, which provides up to USD 1.75 per gallon of SAF produced.

In addition to amending the EU Emissions Trading System to phase out allowances given to the aviation industry by the beginning of 2027, the ReFuelEU Aviation mandates minimum SAF blend-in shares, with sub-targets for synthetic fuels, through 2050. Individual countries such as France and Norway have already had SAF blending mandates in place since early 2022. Sweden’s GHG emissions intensity reduction target will also drive SAF adoption, and in 2023 the Swedish government announced its aim to invest SEK 15 million (Swedish Kronor) annually to support R&D of electric aircraft. The United Kingdom, following its 2022 Jet Zero pledge, has dedicated GBP 180 million to support SAF projects, with funding allocation running to 2025. In 2024 the United Kingdom legislated the sustainable aviation fuel initiatives, mandating minimum targets of 2% in 2025, 10% in 2030, and 22% in 2040, with sub-targets for synthetic fuels.

In the Asia Pacific region, Japan proposed legislation in late 2022 mandating that SAFs must account for 10% of aviation fuel by 2030. In the same period the Civil Aviation Administration of China also set ambitions to increase SAF use and lower GHG emissions intensity.

In South America, Brazil adopted the Fuel of the Future law, which requires airlines to reduce domestic flight greenhouse gas emissions by 1% in 2027 and increasing to 10% in 2037 through SAFs.

In late 2022, ICAO member states adopted a long-term aspirational goal (LTAG) to achieve net zero carbon emissions from international aviation by 2050. The agreement aims to reduce emissions within the sector itself (i.e. directly from aviation activity, as opposed to by offsetting emissions through the purchase of credits). Although it remains non-binding and lacks intermediate goals, governments are expected to produce national plans within a set timeframe. Similarly, in 2021 the world’s largest airline industry association, the International Air Transport Association (IATA), agreed on its Net Zero Initiative setting targets for net zero emissions from aviation by 2050.

In addition, in late 2022 countries agreed on a new baseline for the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), at 85% of the 2019 emissions level of international aviation from 2024 until the end of the scheme in 2035. Emissions beyond this would need to be reduced by using SAFs or by purchasing certified emissions reductions to offset CO₂ emissions. In 2024, ICAO published updated CORSIA eligible emission rules, many of which rely on offsets outside the energy sector.

Increasing announcements of SAF offtake agreements between fuel suppliers and airlines marked a stark increase in contracted volume from 9 billion litres in 2021 to 22 billion litres in 2022. Almost 12 billion litres were contracted in 2023, meaning that cumulatively, offtake agreements have reached over 40 billion litres. The largest offtake volume per year purchase to date was agreed between Alder Fuels and United Airlines, accounting for over 5.6 billion litres, followed by Gevo and OneWorld Alliance, accounting for over 3.7 billion litres to be sourced from ethanol from inedible corn products. Many of the contracted volumes have planned delivery after 2025, and new SAF plants take around 3 years to build after a final investment decision has been taken.

• Lynnette Dray, University College London
• Haldane Dodd, Air Transport Action Group