Stabilizing global mean temperature at 1.5°C above pre-industrial times means reaching net-zero CO2 emissions (i.e., balancing any ongoing emissions with removals) by 2050-2060, and net-zero greenhouse gas (GHG) emissions by 2070-21001. Large—and increasingly affordable—emissions reductions are available by improving energy efficiency, electrifying energy end uses, and switching to non-emitting sources of electricity1, and many countries, sub-national jurisdictions, and companies have announced net-zero emissions targets2. However, flying will be particularly challenging to decarbonize because modern aircrafts rely on energy-dense liquid hydrocarbons3–7.
The climate impacts of global aviation are substantial, with one-third of radiative forcing related to CO2, and two-thirds related mainly to nitrous oxides (NOx) and water vapor in the form of contrail cirrus clouds8–11. In 2019, aviation accounted for 1.03 GtCO2, or 3.1% of total global CO2 emissions from fossil fuel combustion12. Although emissions from air travel dropped 40% in 2020 due to the COVID-19 pandemic, aviation demand is expected to recover and grow in the future13,14, with emissions projected to reach as high as 1.9 GtCO2 in 205015,16 (2.6 times 2021 values). Demand for air travel across countries and population groups is closely associated with affluence and lifestyle17–21 (Supplementary Fig. 1), and flying has become a lightning rod for climate activists who criticize the hypocrisy of climate scientists and climate-concerned policymakers who fly22–24.
Many aircraft manufacturers, industry groups, and business consultants aim to meet rising demand while also reducing emissions by improving operational efficiencies25,26, offsetting carbon emissions27, and switching to net-zero emissions fuels28–32. In 2016, under the International Civil Aviation Organization (ICAO) 192 countries signed the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) to make pos-2020 growth of international aviation carbon neutral, either by fuel switching or by offsetting emissions. Most prominently, the International Air Transport Association (IATA) committed in 2021 that emissions from global aviation would be net-zero by 205033. Recent analyses have evaluated the technological potential of powering aircraft with sustainable aviation fuels (SAFs)4,7,34–37, hydrogen, or electricity27,33,38,39, as well as offsetting aviation emissions by removing equivalent quantities of CO2 from the atmosphere40,41 (Supplementary Fig. 2). SAFs include biofuels and synthetic fuels that are “drop-in” replacements for jet fuel (i.e., they would require little or no changes to existing aircraft and fueling infrastructure16) that meet ICAO’s sustainability criteria42 of a net GHG emissions reduction on a life cycle basis of at least 10% compared to fossil jet fuel, respecting biodiversity, and contributing to local social and economic development.
Here, we assess nine possible pathways to achieve net-zero direct emissions from aviation, including changes and trade-offs in demand, energy efficiency, propulsion systems, and alternative fuels for both passenger and freight transport. Details of our analytic approach are in the Methods (Supplementary Figs. 3-4). Using emissions, energy, and air travel demand data from the International Energy Agency (IEA)12,13,43–46, the Carbon Monitor47, the World Bank48–50, ICAO14,51–53 and IATA33 (Supplementary Table 1), we develop and analyze a range of midcentury decarbonization scenarios for the aviation industry, decomposing historical and future aviation emissions using a sector-specific variant of the Kaya identity:
where F represents fossil fuel CO2 emissions from global aviation (neglecting life cycle emissions of the aircraft and the supply chain of fuel), D is demand or distance flown, and E is the energy consumed by flying aircraft, such that e is energy intensity of air transport, and f is the carbon intensity of energy used for air transport.
Demand for aviation
Figure 1 shows historical and projected aviation emissions decomposed by the terms in Equation 1. Total aviation demand in 2019 was about 995 billion tons per kilometer equivalent (tkme), with 78% representing passenger flights and 22% freight (Fig. 1a, black line). Travel advisories and border restrictions during the global pandemic led to a sharp decline in the air transport of passengers54, driving global demand down to about 592 billion tkme in 2020 (18% and 47% decreases in freight and passenger transport, respectively). Freight demand fully recovered in 202155, but passenger demand in 2021 was still 27% less than in 2019 as estimated by its emissions to demand ratio13,47. Indeed, the ICAO estimates that it may be several more years before passenger demand recovers to 2019 levels, and that growth trajectories may be permanently altered by shifts in travel behavior56. Conversely, IATA’s most recent forecast projects a recovery to 2019 levels of air travel demand in 202357.
Despite such short-term uncertainty, industry projections consistently anticipate continued growth in demand of air transport in the coming decades14, whereas other researchers have argued that substantial reductions in future demand are possible via behavioral changes and shifts to high-speed trains6,13,46,58–62. The demand scenarios in Figure 1a thus span a wide range of trajectories: “business-as-usual” (BAU) increases of 4% per year (to 2890 billion tkme in 2050; red curve), “industry” projections of an average of 2.8% increase per year (2130 billion tkme; blue curve), and “ambitious” demand shifts that keep growth to an average of 1% per year (1115 billion tkme; green curve). It should be noted that the ambitious scenario implies a sudden and drastic divergence in the historical relationship between aviation demand and expected population and economic growth (Supplementary Fig. 1).
Energy intensity of aviation
The energy intensity of both freight and passenger aircraft has declined by an average 1% per year for since 197063, for example falling from 31.6 MJ/tkme in 1990 to about 13.3 MJ/tkme in 2021 (Fig. 1b, black line). Improvements since 2010 reflect the release of fuel-efficient aircraft such as the Airbus A320neo and A350, and the Boeing 737 MAX and 787, but because there are no major new aircraft models expected soon, the International Council on Clean Transportation does not expect significant decreases in energy intensity in the next few years63. IATA established a 1.5% improvement in fuel efficiency goal up to 202064 and expects that efficiency improvements would reduce about 3% of 2050 aviation emissions40. Yet despite the half century of 1% per year reductions, the ICAO’s A40-18 resolution in 2019 set a goal of improving the fuel efficiency of international flights by 2% per year until 205051. Even more ambitiously, a mid-century net-zero scenario developed by the IEA includes reductions in the energy intensity of international flights of an average 7% from 2019-2025, followed by a subsequent 2% yearly reduction to 203013.
The scenarios shown in Figure 1b again span the full range of these future energy intensities, from “BAU” reductions of 1% per year (to 9.9 MJ/tkme in 2050; red curve), “industry” reduction commitments of 2% per year (7.4 MJ/tkme; blue curve), and “ambitious” reductions of an average of 4% per year (extrapolating the rapid decreases in the IEA net-zero scenario to reach 3.7 MJ/tkme in 2050; green curve). Here again, it is not clear that the energy intensities in the most ambitious scenario are physically possible, but some studies have theorized that revolutionary improvements such as open rotors65, blended wing-body airframes66, and hybridization67, as well as more efficient air traffic management40, could bring significant efficiency gains46.
Carbon intensity of energy for aviation
Historically, jet fuel (i.e., fossil kerosene-based Jet A/A-1) has been the energy source for almost all commercial aircraft, resulting in a near-constant carbon intensity of 73.5 gCO2/MJ (including combustion emissions only; Fig. 1c, black curve). In recent years, some airlines have begun using bio-based jet fuel—which could decrease carbon intensity of aviation energy—but uptake has been slow: bio-based jet fuel production was about 140 million liters in 2019. This represented less than 1% of aviation fuel use in that year16,68 and was mostly blended with fossil fuels based on standard D7566 from the American Society for Testing Materials (ASTM), which allows a maximum 50% blend16,69–72. The first commercial demonstration plane using 100% biofuels flew on December 2021, and few have done it since73,74. Looking forward, industry groups nonetheless project rapid decreases in the carbon intensity of aviation energy. The International Renewable Energy Agency’s (IRENA) 1.5°C scenario assumes that by mid-century 70% of aviation’s energy demand is met by SAFs, while 14% comes from electricity and hydrogen75. Similarly, IATA’s net-zero commitment expects that 65% of 1.8 GtCO2 (their estimated 2050 emissions) will be abated by using SAFs, with hydrogen and electricity-powered aircraft abating 13%, and the remainder being abated with efficiency improvements (3%) and offsets (19%)33. The IEA’s net-zero scenario includes 75% of all aviation energy demand being SAF by 2050, but with more modest deployment of electric planes (accounting for less than 2% of 2050 aviation energy demand)46.
The scenarios of carbon intensity shown in Figure 1c include continued reliance on fossil jet fuel (a “carbon intensive” option which maintains 73.5 gCO2/MJ; red curve), a “reduced fossil” pathway in which 65% of fuel demand in 2050 is met by SAFs (with the rest still fossil jet fuel) and 13% of projected short-haul transport is met by non-emitting propulsion systems like hydrogen or electric planes (reaching 23.9 gCO2/MJ in 2050; blue curve), and a “net-zero” pathway in which 100% of aviation energy in 2050 is supplied by SAFs and/or other non-emitting propulsion systems (i.e., 0 gCO2/MJ; green curve). Note that these scenarios assume that the combustion emissions from SAFs are net-zero with respect to atmospheric carbon, an assumption we discuss in more detail below.
Aviation emissions
Aviation emissions were 1.03 GtCO2 in 201913, 64% of which were related to international flights and 36% from domestic flights47. Emissions plunged to 0.61 GtCO2 in 2020 amidst COVID-19 lockdowns12 and rebounded somewhat to 0.73 GtCO2 in 202112,47 (Fig. 1d, black curve). Future emissions will reflect the combination of changes in demand, energy intensity of aviation, and the carbon intensity of aviation energy.
Combining our scenarios of demand and intensities in different ways thus gives ranges of emissions trajectories, as shown in Figure 1d. On the upper end, BAU growth in demand (i.e., +4% per year) and improvements in energy intensity (i.e., -1% per year), with continued use of fossil jet fuel leads to annual aviation emissions of 2.11 GtCO2 in 2050 (top of red shading in Fig. 1d). At the other extreme, phasing out fossil jet fuel entirely would eliminate aviation emissions by 2050 (green shading in Fig. 1d)—but might entail large cost increases (as discussed below). Notably, replacing 65% of fossil jet fuel with SAFs could still result in annual emissions of 0.69 GtCO2 in 2050 (more than emissions in 2020) under BAU changes in demand and energy intensity (top of blue shading in Fig. 1d; Fig. 2d).
Figure 2 reveals the relative contributions of different mitigation levers by comparing relative changes between 2021 and 2050. For example, annual emissions nearly triple assuming BAU changes (+190%), driven by surging demand for air transport (blue bar; Fig. 2a). In contrast, assuming somewhat lower increases in demand, an almost tripling of historical decreases in energy intensity, and that two-thirds of fuel are sustainable and net-zero, annual emissions in 2050 could be roughly half of what they were in 2021 (-48%; Fig. 2e). Finally, the required decreases in carbon intensity of aviation energy in net-zero scenarios are heavily dependent on projected changes in aviation demand and energy intensity (Figs. 2g-i).
Sustainable aviation fuels
The quantity of SAFs required to meet net-zero goals is inversely proportional to decreases in aviation demand and energy intensity (Fig. 3). Although this demand might also be reduced by using hydrogen or battery electric propulsion systems, the low energy density of such alternatives will probably limit their use to short-haul applications. For example, assuming a 60% fuel fraction (i.e., the share of maximum take-off weight allocated to fuel), 90% increases in energy efficiency, and 1500 kWh/tH276, larger body aircraft such as a Boeing 777-200 or Airbus 380-800 (whose fuel fraction is 50%) converted to hydrogen propulsion would not be anywhere near able to cover the distance of common long-haul routes such as New York to London (5500 km) or Los Angeles to Beijing (10000 km; Supplementary Fig. 5). Similar estimates show that the range of large battery electric planes would be 500 km (Supplementary Fig. 5). Nonetheless, our net-zero scenarios assume that half of short-haul flights might be serviced by hydrogen or battery electric planes.
Thus, Figure 3 shows that without extreme reductions in aviation demand and energy intensity (i.e., the green “ambitious” curves), by 2050 demand for SAFs in all of our scenarios is more than double the quantity of global production of biofuels in 2020 (3.6 EJ including ethanol, biodiesel, and hydrotreated vegetable oil)77. In addition to biofuels, SAFs might ultimately include hydrocarbons produced by Fischer-Tropsch (FT) or methanol synthesis using carbon captured from the atmosphere and hydrogen generated without fossil CO2 emissions (e.g., by electrolysis using renewable or nuclear electricity).
Whether biofuels or synthetic fuels, a major barrier to the penetration of SAFs is cost, which in turn depends on the cost of feedstocks and the costs and efficiency of conversion processes. In the case of synthetic fuels, the cost of hydrogen primarily reflects electrolyzer and electricity costs and the cost of captured carbon depends on the technology involved. For example, assuming current costs of electrolytic hydrogen and captured carbon are around $4.50/kgH278,79 and $0.25/kgCO280, respectively, synthetic jet fuel costs are about $2.25/L, more than three times higher than the global 2022 average cost of fossil jet fuel (as of 05/31/2022)81 (Fig. 4a). These calculations are broadly consistent with other recent studies that reported costs of synthetic fuel ranging from $1.30 to $4.72 per liter82,83. Economies of scale and learning-by-doing may substantially reduce electrolyzer and carbon capture costs in the future, making synthetic fuels more competitive84,85.
Even though there are several conversion pathways for biofuels, FT biofuels and Hydro-processed esters and fatty acids (HEFA) are among the few advanced biofuels with “near commercial” fuel readiness level, though FTs have more abundant feedstocks than HEFA. Near commercial readiness means the conversion pathway has been certified, and the technology is beyond the research and development stage70. Based on average feedstock costs of $0-1.10/kg of biomass and conversion efficiencies between 30-50% (2-4 kg biomass per kg fuel)86,87, current production costs for FT biofuels are between $1.00-2.29/L16 (Fig. 4b). The lower end uses a zero-cost waste feedstock with 67% and 33% of the production cost represented by capital and operating expenditures, respectively; the upper end uses a lignocellulose feedstock that is 33% of production cost, with the remainder 45% and 22% represented by capital and operating expenses, respectively16. Although the low end of this range approaches the current cost of fossil jet fuel, the additional expense may be limiting uptake in a cost-competitive industry where, at least in the near-term, emissions reductions remain mostly voluntary. Achieving cost parity could thus greatly increase use of FT biofuels and might entail a carbon price of as little as $78/tCO2. For HEFA biofuels, costs of feedstocks (e.g., from used cooking oil to jatropha oil) are routinely $0.70-2.60/kg16 and unlikely to decrease much in the future70. The HEFA conversion pathway has the highest efficiencies compared to other bio-based jet fuel routes, at around 76%88 (1-2 kg biomass per kg fuel), with production cost ranges between $0.78-2.29/L (Fig. 4c)16. Although the lower end costs are less than fossil jet fuel, feedstock availability is limited as it represents used cooking oil that is a byproduct of consumption, and 90% of this feedstock is already used for biodiesel production (at least in the EU)16,70.