The steep descent to net-zero aviation

Aviation generates 12% of global CO₂ emissions from transport and 2.4% of CO₂ emissions from all human activity. The sector has shown robust growth over the past decade—global demand for air travel increased by 50% between 2013 and 2019—although the COVID pandemic slowed this growth and dampened estimates of future demand. Still, aircraft manufacturer Boeing projects that revenue passenger-kilometers could grow by 3.6% annually between 2018 and 2050, as increased prosperity in many countries opens air travel to people who have never had the means to fly.

While demand for air travel grows rapidly, decarbonization of aviation is advancing slowly. The two trends combined could enlarge aviation’s greenhouse gas (GHG) footprint substantially in the decades ahead. In fact, if traffic grows at 5% per year (pre-COVID traffic growth rate) and fuel efficiency improves at only its recent historical rate of about 1.5% per year, aviation would likely lose ground in the struggle to decarbonize. In such a case, it could consume 12.5% of the remaining carbon budget available to economies worldwide.

ICCT’s 2022 report Vision 2050: Aligning aviation with the Paris Agreement estimated that the aviation sector could cut GHG emissions over the next three decades by an amount consistent with a 1.75°C global warming scenario. Five basic strategies could bring that goal within reach: improve the technical efficiency of aircraft, improve the operational efficiency of commercial aviation, develop truly low-carbon sustainable aviation fuels (SAFs), develop zero-emission planes, and reduce demand. These strategies can and should be pursued simultaneously and where possible strengthened using economic incentives. The following chart illustrates the contribution the Vision 2050 report calculated each could make to cutting CO₂ emissions from aviation under three scenarios defined by varying levels of effort.

Decarbonization will require that aircraft manufacturers build on their experience in improving the fuel efficiency of aircraft and the operational efficiency of airlines.

Since 1990, fuel efficiency in commercial aviation has improved by 52% as airlines adopted more efficient aircraft and used them to greater capacity. In fact, each new generation of aircraft in recent decades has burned 15% less fuel per passenger-kilometer than the generation it replaced. This efficiency improvement of roughly 1.5% per year—driven more by economic than by climate concerns—is a concrete achievement but is insufficient for meeting aviation’s decarbonization goals.

Carbon reductions are also possible through operational improvements, for example by consistently achieving high rates of payload efficiency and traffic efficiency. ICCT estimates that improving payload efficiency can provide up to a 0.5% reduction in energy per passenger carried (measured in megajoules per revenue-passenger kilometer, or MJ/RPK). Traffic efficiency refers to operational changes that reduce fuel use, such as single-engine taxiing, reduced fuel loading, smart air-traffic management, continuous climb and descent, electric tows to gates, and formation flying. Taken together, these improvements could yield a decrease in energy use of 0.2% MJ/RPK in 2030, 0.7% MJ/RPK in 2040, and 1.9% MJ/RPK in 2050.

Importantly, efficiency advances to date have reduced only the growth of emissions, not their absolute levels, because efficiency gains were overtaken by increased demand for air travel. Thus, technical and operational efficiency improvements need to be complemented by other decarbonization strategies, such as alternative fuels, zero-emission planes, and shifts away from flying.

Cleaner fuels, more than efficiency gains, are expected to drive aviation decarbonization in the decades ahead. Sustainable aviation fuel, or SAF, is an umbrella term used to denote “drop-in” alternatives to conventional jet fuel that can be used in today’s aircraft and engines. In the Vision 2050 analysis, SAFs could account for 62% percent of emission reductions under the most ambitious Breakthrough scenario. Similarly, a 2022 ICAO analysis estimated that SAFs could deliver 55% of the CO₂ reductions needed. Such findings, while encouraging, come with caveats.

SAFs can be produced from a variety of biological feedstocks and renewable energy (also termed “power to liquid” or e-kerosene), with varying degrees of environmental integrity. Public policies intended to support the development of SAFs include a performance requirement: for example, the U.S. Inflation Reduction Act stipulates that only SAFs that produce a 50% reduction in lifecycle emissions compared to conventional jet fuel qualify for funding support under the law. This chart breaks down the direct and indirect emissions of potential SAFs and shows those fuels that meet a 50% reduction threshold.

GHG emissions from feedstocks that claim SAF status, and fossil jet fuel.

Indirect emissions comprise two categories. Displaced emissions occur when a large industry’s securing of limited supplies of a low-carbon fuel is achieved at the expense of other industries, which then must resort to carbon-intensive substitutes. For example, if the palm fatty acid distillates (PFADs) – a fuel worse than conventional jet fuel (see above chart) – currently used in livestock feed are diverted to make aviation fuel, feed producers would likely turn to palm oil, whose total emissions are very high—higher than those of conventional jet fuel. Indirect land-use change (ILUC) refers to land use changes that occur when existing cropland is used to produce feedstock for biofuels. It results in the displacement of other agricultural activities, for example: in Brazil, as soy oil production increased, farmer’s pasture raising lands decreased. As a consequence, farmers moved their pastures to unclaimed forest land –creating a cycle of land displacement.

In sum, full life-cycle accounting of emissions from SAFs is necessary. Only SAFs that can substantially reduce GHGs after accounting for both direct and indirect emissions are likely to help decarbonize aviation.

SAFs will likely be rolled out over the near, medium, and long term as different types of SAFs mature.

Near term. Fuels produced from used cooking oil and beef tallow, known as hydro-processed esters and fatty acids (HEFA) fuels, are already available for use in aircraft. The HEFA process converts virgin vegetable oils, or waste fats, oils, and greases, into hydrocarbons through deoxygenation. It creates a hydro-treated vegetable oil that is chemically similar to kerosene and therefore requires little modification for use in an aircraft. As the chart shows, the GHG life cycle (direct and indirect) emissions of most HEFA fuels are half the levels of conventional jet fuel or less, and therefore qualify as SAFs.

The chief obstacle to the use of HEFA fuels is their limited supply. With competition from other transportation sectors that can also use HEFA fuels, ensuring a sufficient volume of HEFA fuels for aviation may be difficult.

Medium term. Advanced biofuels derived from agriculture and forestry wastes are expected to become more common in the medium term. These are not the biofuels made from food crops like corn and soybean that are still being used today but provide little emissions reduction benefits. Instead, advanced biofuels are made from waste residues such as corn stalks and forest branches that offer substantial GHG savings if harvested sustainably, i.e., in ways that avoid erosion and soil carbon loss. Because advanced biofuels are made from inedible (by humans) plants and parts of plants consisting mainly of cellulose, they are also referred to as cellulosic biofuels. The greatest opportunity in cellulosic biofuels may be to use “energy crops” such as willows and elephant grass. Fuels derived from these sources have much lower emissions from land use than food-based biofuels have; note the low emissions values of agricultural residues and forestry residues shown in the chart above. Food-based biofuels, made from crops such as corn, soy, rapeseed, and palm, do not meet the emissions criteria to qualify as SAFs (see chart).

Long-term. SAFs in the long-run are likely to be e-fuels and liquid hydrogen, which can offer large reductions in GHGs if renewable energy is used to make them. Both e-fuels and liquid hydrogen (LH₂) fuels are made by using electricity to split water into hydrogen and oxygen, then combining the hydrogen with carbon dioxide to produce liquid synthetic hydrocarbon fuels that are chemically indistinguishable from the fossil fuels (diesel, methane, jet fuel) they are intended to replace; hence the term “drop-in” fuels. Producing these fuels requires substantial amounts of power, because the process is inherently energy inefficient. Around half of the input energy of the electricity used (56-63% for LH₂ and 51-58% for e-kerosene) is lost in the production process. And while e-kerosene (as aviation e-fuel is known) and liquid hydrogen can be very low-carbon replacements for conventional fuel, that is true only if they are produced using truly renewable electricity – i.e., generated with zero carbon emissions. But alternative fuels produced using power from the U.S. grid (with current state policies) could be as much as twice as carbon intensive as fossil jet fuel. Further, additional amounts of e-kerosene and LH₂ produced for aviation would be low carbon only if they’re produced using additional renewable power. Poaching renewable electricity from another use, if it must be replaced by non-renewable energy, is merely an accounting trick.

The chart below depicts the transition from conventional jet fuel to lower-carbon alternatives under a high-effort scenario that leads to 100% use of SAFs by 2050. The orange line shows a decline in use of conventional jet fuel from 2025 through 2050. Biofuels start their growth around 2023 (blue line), e-kerosene begins to take off around 2030 (brown line), and hydrogen fuels begin in 2035. Electricity (yellow line) is hardly visible, largely because the demand for electric aircraft will, we estimate, account for only 0.01% of total energy demand.

Fuel projections through 2050 in the Vision 2050 Breakthrough scenario

Zero-emission planes (ZEPs) refer to aircraft that use either electricity or liquid hydrogen as fuel. These emerging technologies could contribute up to 5% of cumulative emission reductions in aviation through 2050, with potentially larger gains later from hydrogen aircraft. While they have inherent performance limitations, the likelihood of achieving the decarbonization of aviation by 2050 is higher with ZEPs than without.

Hydrogen aircraft, or planes fueled by liquid hydrogen, could provide service on short-haul and many medium-haul flights (up to 3400 kilometers), which will constitute roughly a third of the aviation market by 2050 based on projected traffic demand. “Green” hydrogen, produced from water using renewable power, is entirely carbon-free. But liquid hydrogen has low energy density relative to fossil jet fuel, and hydrogen aircraft will require larger fuel tanks than current designs, reducing carrying capacity and necessitating configuration changes. Hydrogen aircraft will also need new fuel production and storage facilities, and new re-fueling equipment. These investments would be significant; for example, developing hydrogen storage and refueling infrastructure for one airport would cost approximately as much as opening a new airport terminal. Current hydrogen production is nowhere near adequate, and nowhere near green. The International Energy Agency estimates that of the 69 million tonnes of hydrogen produced worldwide in 2019 for industrial or energy uses, less than 0.1% was produced using renewable energy. In other words, essentially, no “green” hydrogen is currently being produced.

Electric aircraft are battery-powered planes that may be suitable for the shortest regional flights. Electric aircraft can provide a 49% to 88% reduction in CO₂-equivalent (CO₂e) emissions per passenger kilometer relative to fossil-fueled aircraft. Several designs are in development, with the first electric aircraft for commercial use potentially in service in 2027. Their chief drawback is the weight of their batteries, which limits their range and payload, confining them to servicing only a small portion of the aviation market. An ICCT study modeled three potential 2050 electric aircraft designs carrying 9 to 90 passengers, the smallest having a range of 500 km and the largest 280 km. If the electricity used to charge their batteries were generated by renewable sources, such aircraft could mitigate 0.2% of the projected passenger emissions in 2050. On the other hand, the benefits of electric aircraft extend beyond their contributions to climate stabilization. Because regional flights constitute a high share of take-offs and landings, electric aircraft can help to reduce air pollution around airports, bringing health benefits to nearby communities.

The International Civil Aviation Organization (ICAO) estimates that up to $2.8 trillion will be needed to achieve even a low-effort decarbonization scenario and $4 trillion for a high-effort scenario. Financial support from the public sector will be needed to unlock that much investment, and likely still other economic policies, like mandates, will be needed to incentivize (and fund) decarbonization.

Sustainable aviation fuels currently cost two to five times more than conventional jet fuel; given that fuel accounts for about 25% of an aircraft’s operational expenses, introducing large volumes of qualified SAFs today could raise ticket prices and adversely affect the airline industry. Vision 2050 outlines scenarios for SAF introduction that are consistent with the long-term decarbonization of aviation, notably a 2050 net-zero CO₂ goal (the “Breakthrough” scenario). Policies to support SAF introduction are modelled as a global carbon price that subsequently triggers other changes across the aviation sector, notably an increase in ticket prices and a demand response wherein consumers fly somewhat less.

The implied carbon price by scenario (in USD) is shown in the next graph.

In the high-effort scenario (blue line), carbon prices peak before 2040 because early, aggressive government policies lead to economies of scale for SAF production. The mid-effort scenario (yellow line) shows a delayed carbon price peak around 2045, after which time prices drop steeply as SAF volumes expand. In the low-effort scenario (red line), lower SAF production volumes in early years mean that SAF costs do not peak until sometime after 2050.

Another potential tool for generating funding for clean fuels would be to tax tickets and earmark the revenue for technology demonstration and deployment. Taxes, like a ticket tax, could be designed to shift some of the cost of technology from those who fly infrequently to relatively prosperous frequent fliers. Indeed, almost two-thirds of aviation CO₂ is emitted by upper-income country residents, while countries representing the poorer half of the world emit only 10%. Any discussion of the prospects for decarbonizing air travel should recognize that air travel’s benefits skew toward prosperous people, while its climate impact affects everyone.

Technological and efficiency gains alone cannot achieve the carbon reductions required in aviation. Changes in travel habits, from allowing consumers to choose lower-emission flights to encouraging modal shift for short journeys, will make the path to decarbonizing aviation easier.

Trains, trains, trains…

For short trips, trains can be cleaner and more convenient than air travel. Short flights are the most carbon intensive, because takeoff, with its high rate of fuel burn, accounts for a larger share of a short flight than of a long one. A flight of less than 500 kilometers emits roughly 199 g CO₂/passenger-kilometer, while a diesel train in the U.S. emits roughly 64 g CO₂/passenger-kilometer. By one estimate, train travel could reduce short-haul flight traffic by up to 28% and significantly reduce carbon emissions. Moreover, trains are often more spacious and comfortable than planes, while train stations can be more accessible and convenient than airports. And for short distances, trains are competitive with planes in terms of door-to-door trip duration. In sum, for short trips, train travel can be a better option, environmentally and practically.

Still, the modal shift from planes to trains relies on government initiative to invest in rail. For example, as part of their decarbonization path, France imposed a ban on short-haul flights where train journeys of two and a half hours or less are available, which is projected to eliminate 12% of French domestic flights. Other EU member states are reportedly considering similar bans. The map below highlights the routes impacted (in red) by the French ban compared to those unaffected by the ban. Policies such as this will help to shift travel away from short-haul flights, and limit aviation’s emissions into 2050.

Flights impacted by the French ban on short flights. Gray lines indicate flight paths that will remain unaffected, red lines indicate banned flight paths.

Many people assume that flying a given distance generates a predictable level of pollution. But ten years of ICCT research has shown that the carbon intensity of air travel can vary substantially. For example, one study concluded that providing consumers with information about the carbon intensity of specific flights could help them reduce emissions by 22%. Travel search engines such as Google Flights now provide emission estimates to consumers at the time of booking to help them choose cleaner flights. Those estimates will help environmentally conscious consumers consider emissions when selecting flights along with other factors like cost, travel time, convenience, etc.

If carbon-reporting travel sites are not available, a few guidelines can help reduce the carbon footprint from flying. (These rules will work better for consumers flying out of large airports. Someone leaving a small regional airport will have fewer options to apply them.) The key is to fly like a NERD:

• Book on a new plane—new planes tend to be more fuel efficient than the ones they replace. The Boeing 787, Airbus 320neo, and Airbus 220 are examples of efficient commercial aircraft.
• Fly economy—business- and first-class travelers account for four times more carbon per kilometer traveled than economy-class passengers do, simply because they take up a greater share of aircraft space.
• Fly regular—very large (quad) aircraft burn more fuel per passenger than mid-sized aircraft, as do smaller regional aircraft. So, stay in the middle of the aircraft size range, opting for twinjets like the A320 and Boeing 777 families.
• Fly direct—because shorter flights are more carbon intensive than longer ones, a single direct flight likely has a smaller carbon footprint than a set of shorter flights.

Revenue passenger-kilometers: The amount of revenue per paying passenger multiplied the total distance travelled.
Fuel efficiency measures: A unit measured by the margin from the International Civil Aviation Organization (ICAO) aircraft CO₂ standard, which established an internationally agreed method of assessing and comparing fuel efficiency in aircraft.
Fuel per passenger-kilometer: The amount of fuel required to move 1 passenger for 1 kilometer based on plane fuel efficiency.
Payload efficiency: A flight’s payload as a percentage of its maximum payload. The closer a flight is to full capacity, the better its payload efficiency.
Traffic efficiency: Rate in aircraft fuel burn during a mission.
Drop-in: Any fuel that can be used within current aviation infrastructure (no changes to craft or port).
CO₂ per passenger-kilometer: The (g) CO₂ emitted per 1 passenger for 1 kilometer.

Authored by Shraeya Mithal and Dan Rutherford
Edited by Gary Gardner and Joe Schultz

Selected sources

ICCT papers

Vision 2050: Aligning Aviation with the Paris Agreement
Performance Analysis of Evolutionary Hydrogen Aircraft
Assessing the sustainability implications of alternative aviation fuels
Performance Analysis of Regional Electric Aircraft
CO2 Emissions from Commercial Aviation, 2018
Planes, Trains, and Automobiles: Counting Carbon
Need to Fly? Fly like a NERD
Polishing my Crystal Ball: Airline Traffic in 2050
Airline Fuel Efficiency: ‘If You Can’t Measure It, You Can’t Improve it’
Size Matters for Aircraft Fuel Efficiency. Just Not The Way You Think.

Other sources

Boeing, Commercial Market Outlook 2022-2041, 2022
ICAO Committee on Aviation Environmental Protection, Report on the Feasibility of a Long-Term Aspirational Goal (LTAG), 2022
Air Transport Action Group, Fact Sheet 3: Tracking Aviation Efficiency, 2021
International Energy Agency, The Fute of Hydrogen, 2019
Kil Willsher, France to Ban Some Domestic Flights where Trains Available, 2021