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Aviation

Not on track
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In this report

After increasing rapidly over the past two decades, CO2 emissions from aviation fell by one-third from the 2019 level in the wake of the pandemic to just over 600 Mt in 2020 – the lowest level since 1997. Despite this unprecedented collapse, passenger numbers and cargo volumes are set to rise in the coming decades. In the past, energy intensity reductions have been insufficient to counterbalance such activity growth. A range of operational, technical and behavioural solutions will be required to cut emissions from 2025 onwards, to reduce them to just over 780 MtCO2 by 2030 and around 470 MtCO2 by 2040 in line with the Net Zero Emissions pathway. Near- to medium-term priorities include implementing fiscal and regulatory measures to promote efficiency; managing the investment risks for scaling up sustainable fuels; and developing alternatives to jet kerosene, such as battery-electric and hydrogen-powered aircraft.

Direct CO2 emissions from fossil jet kerosene combustion in the Net Zero Scenario, 2000-2030

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Tracking progress

Global air passenger numbers tripled in just over 20 years, until the Covid-19 crisis resulted in an unprecedented collapse in global air travel. Air traffic contracted more than 75% between April and May 2020, and global passenger volumes were nearly two-thirds lower in 2020 than in 2019.

While expected to remain nearly 50% below pre-pandemic levels in 2021 and to return to the 2019 level only in 2023, passenger numbers are anticipated to grow over the next decades. In developing and emerging economies, where passenger air travel had already increased sixfold in the 20 years prior to the Covid-19 pandemic, a steady expansion of the middle class will enable many people to fly for the first time. In Europe and North America, however, some travellers may change their flying habits in response to growing concerns of the climate impacts of air travel. 

In contrast, the fall in demand for air cargo was less pronounced, with an 11% year-on-year decline in 2020. To compensate for reductions in freight capacity on passenger aircraft due to cancelled flights, airlines expanded their air freighter fleets and increased daily utilisation. By May 2021, air cargo traffic had surpassed the pre-crisis level by nearly 10%.

Air freight, comprising both dedicated cargo flights and cargo stored on passenger flights, accounted for around 15% of global aviation CO2 emissions in 2019, although this share has been significantly higher since the beginning of the Covid-19 pandemic. As e-commerce surges, more and more old passenger aircraft are being turned into freighters with possibly 100 airliners being converted in 2021, a 50% increase from 2019.

In the last 60 years, the fleetwide carbon intensity of commercial passenger aircraft has dropped more than 70% per available seat-km. Continuous engine and airframe improvements have made today’s new aircraft ~85% more efficient than the early jet planes that entered into service in the 1960s, and ~20% more efficient than the models they are replacing.

The fuel efficiency of international aviation improved 1.9% per year between 2010 and 2019, after improving by 2.4% annually in the previous decade. While the more recent rate of improvement surpassed the industry’s own target, it is just below the aspirational goal of 2% annual improvements adopted by the International Civil Aviation Organization (ICAO) in 2010. The short- and medium-term outlook for fuel efficiency remains somewhat uncertain due to impacts of the Covid-19 crisis. What is certain, however, is that further technical and operational efficiency improvements beyond the ICAO target will be needed to offer any prospect of substituting fossil-based jet kerosene with more sustainable fuels in the long term.

Energy intensity of passenger aviation in the Net Zero Scenario, 2000-2030

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Over the next decade, a range of technical, operational and behavioural solutions can be applied to reduce aviation emissions. The main technology option is to transition from fossil-derived jet kerosene to sustainable aviation fuels (SAFs), i.e. biofuels and synthetic jet kerosene. Operational and technical efficiency opportunities, and even modal shifts and demand restraint, are also promising prospects for reining in emissions that are otherwise set to increase through to mid-century.

Sustainable aviation fuels 

SAFs are a promising solution to decarbonise aviation, as their use requires only limited infrastructure and equipment modifications. However, they currently account for less than 0.1% of jet kerosene consumption, and because production levels are low, they still cost more than twice as much as conventional fuels. Although the International Air Transport Industry targets an SAF share of 2% by 2025, existing and announced SAF plants as of September 2020 could provide only half the amount needed.

However, notable announcements were made in 2021 in major aviation markets. Under the Fit for 55 initiative, the European Commission proposed a ReFuelEU Aviation regulation mandating minimum SAF blending volumes in aviation fuel, rising from 2% in 2025 to 5% in 2030 and 63% in 2050. In the United States, the Sustainable Aviation Fuel Grand Challenge aims to scale up SAF production to 11 billion litres annually by 2030 and to eventually meet the country’s entire aviation fuel demand by 2050.

Blending SAFs with jet fuel is likely to raise ticket prices only moderately. For example, a flight from Shanghai to Istanbul using a 15% SAF blend from technology available today would cost each passenger an extra USD 20 only.

Seven biojet fuels are currently approved for international flights. Of these, only HEFA (produced by treating vegetable oils, waste oils and fats) is commercially available, but it is competing with road transport biofuels for feedstocks. Alternatively, power-to-liquid fuels are showing potential as drop-in SAFs. If produced using renewable electricity and carbon from sustainable biomass or direct air capture, they can eliminate nearly all CO2 emissions from fuel combustion.

The first passenger flight partly fuelled by sustainable synthetic kerosene took off in February 2021, though synthetic hydrogen-rich fuels are currently three to six times more expensive than conventional jet kerosene. Although their shares in aviation fuel supply will remain low initially, the European Union is targeting a minimum share of 0.7% by 2030 and 28% by 2050 for European aviation.

Importantly, even if methods for producing SAFs at acceptable costs could be developed and integrated at scale into aircraft fuel supplies, at best they would reduce – but not eliminate – the climate impacts of flying due to upstream emissions in the production cycle and non-CO2 climate-forcing impacts. 

Operational and technical improvements 

In the near term, various operational and technological solutions are available to reduce fuel burn and maximise service efficiency, though their potential to reduce emissions is limited. For example, “big data” and analytics can help airlines optimise engine cleaning schedules, while airports can exploit information technologies to improve air traffic congestion. New current and next-generation aircraft can achieve reductions in cruising weight through increased reliance on composites and optimised cabin space use. 

Modal shifts and behavioural change 

In some cases, high-speed rail connections are a convenient low-carbon alternative to short-haul flights, and video conferencing can help avoid flights in the first place by substituting for business travel. There are also options for passengers to reduce emissions from leisure trips, by opting for airlines with more efficient fleets and routes with fewer stopovers, travelling to closer destinations and switching to alternative transport modes, when feasible.

In the longer term, solutions range from more efficient jet engines over a blended wing body design to battery-electric and hydrogen-powered aircraft.

Battery-electric propulsion 

While current all-electric aircraft prototypes have proven able to carry up to 20 passengers for more than 100 km, electric propulsion systems will be restricted to smaller aircraft and distances of up to 1 000 km at best until mid-century.

To accelerate to speeds that generate the lift needed to take off, planes require fuels that have high energy densities. Breakthroughs in battery chemistry will therefore be required, as lithium-air batteries that could reach the same energy density as jet fuel are at very early stages of development. Another challenge is that batteries, unlike fuel tanks, do not get lighter during the flight.

Since all-electric flights are limited to short-haul journeys, which account for a small share of aviation emissions, emission reductions may initially be limited. In the coming decades, however, there is potential to develop hybrid or turboelectric designs for flights of up to 3 000 km.

Hydrogen-powered aircraft 

Hydrogen, either combusted in jet turbines or used in fuel cells for electric propulsion, also presents potential. Airbus recently announced plans to develop three short-haul hydrogen-powered planes that could enter into service by 2035, fly as far as 3 500 km and carry up to 200 passengers.

However, hydrogen-based air travel would require transformations of airport infrastructure and vast amounts of low-carbon power to produce hydrogen, which cannot yet compete with conventional jet kerosene in terms of costs. Plus, as some non-CO2 climate-forcing effects will persist, hydrogen jet engines might reduce the total climate impact of planes by only 50-75% compared with conventional jet engines, though flight path optimisation could reduce some of these non-CO2 effects.

Airlines, airports and aircraft manufacturers can prove their commitment to reducing aviation emissions by establishing goals for efficiency improvements, clear schedules for SAF adoption, and absolute emissions reduction targets. The aviation industry as a whole aims to achieve net-zero carbon emissions by 2050, and has set increasing SAF production targets. Europe’s aviation sector had previously outlined a plan to achieve net zero CO2 emissions for all flights within and departing from Europe by 2050, while the largest US airlines and 14 major airlines from the Asia Pacific region had made similar announcements in 2021.

To be effective, it is important that such commitments are based on a concrete roadmap and that actions address not only direct CO2 emissions but all climate-forcing emissions on a lifecycle basis. Using offsets could be a cost-effective way to eliminate emissions from the hardest-to-abate parts of value chains in upcoming decades, but reliance on offsets should be limited. It is also critical that schemes generating emissions credits result in permanent, additional, real and verifiable emission reductions.

This period in which the aviation sector is recovering from the impact of the Covid-19 pandemic is important, as it is a policy window offering an opportunity to shift the aviation industry onto a more sustainable pathway.

The ICAO Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) was established to help international aviation achieve carbon-neutral growth from 2020 onwards. Airlines operating on routes between CORSIA states will need to purchase offset credits based on their CO2 emissions relative to the sector’s baseline emissions. These credits are generated through emissions reductions and removals from mitigation projects in other sectors, though some approved offsetting programmes have been criticised for not meeting all required criteria for environmental integrity.

Originally, the scheme’s baseline emissions level was set at the average total CO2 emissions for 2019 and 2020. However, ICAO decided to exclude 2020 from the calculation due to that year’s exceptionally low passenger numbers, which could reduce the offsetting and mitigation requirements by 25-75%. This decision is likely to nullify any offset obligations until 2024, and even afterwards the scheme will not reduce net emissions from current levels, as it is designed to only limit further growth in emissions.

Furthermore, CORSIA is expected to cover less than 10% of aviation CO2 emissions during its lifetime, as it exempts major aviation markets such as China, India and Brazil until its second phase (starting in 2027) as well as any domestic flights. Therefore, domestic aviation policies are also needed in addition to a stronger CORSIA mechanism.

Policies are needed to support SAF consumption and boost demand growth, which is required to enlarge production to the level needed to realise economies of scale. Fuel offtake agreements committing airports or airlines to purchase biojet fuels at a given price – typically at very close to the pre-Covid-19 price of jet kerosene – are an important tool to provide market certainty.

Both low-carbon fuel standards and blending mandates with minimum GHG emissions reduction thresholds are promising policy instruments that can provide clear long-term demand signals. Regardless of the policy approach, however, sustainability guardrails must be established and enforced to avoid other environmental or social impacts while increasing supply.

On the supply side, financial de-risking measures to enable suppliers to deliver capital-intensive commercial-scale SAF production facilities are key to mobilise investment. Funding will be needed to promote continued innovation on novel, low-carbon and sustainable production processes based on feedstocks such as agricultural waste, forestry or municipal waste, and residues.

Action from leading airlines and airports that serve as key international and domestic hubs can generate the market pull that is needed to catalyse SAF adoption. Over 20 airlines have begun to use SAFs and are announcing goals for blending shares into their overall fuel consumption. At the same time, nearly 20 airports regularly distribute biofuels and others have announced plans to do so in the future. 

Carbon pricing beyond the CORSIA scheme is critical to reflect and internalise the negative externalities of air travel. By passing on costs to passengers, carbon pricing can help curb demand growth, while revenues generated could be used to foster low-carbon innovation and address potential economic hardships faced by airlines.

Since frequent flyers likely account for around half of all aviation emissions, progressive tax rates that increase with flight frequency as well as higher taxes on business and first class tickets could discourage excessive flying.

Domestic aviation is already covered under emissions trading schemes in the European Union, the United Kingdom, Switzerland, New Zealand and Korea. While some countries have levied taxes on jet fuel or airline tickets, others are still considering implementing such taxes. In general, however, the aviation sector has a long history of tax exemptions. 

Acknowledgements

The authors extend their thanks to Ron van Manen and Brandon Graver for reviewing and providing valuable feedback on an earlier draft of this section.