CLEARING THE AIR
Unpacking the role of
sustainable aviation fuels
Fueled by a surge in demand, emissions from aviation have been growing faster than those from other transportation sectors. Authorities such as the International Civil Aviation Organization have set ambitious goals to rapidly reduce emissions from international aviation by 2050. As governments and industry work toward this target, sustainable aviation fuels (SAFs) have emerged as a primary strategy for reducing emissions from air travel.
What is sustainable aviation fuel (SAF)?
SAF refers to aviation fuels derived from non-fossil sources. Chemically similar to conventional jet fuel, SAF can be produced from biomass (biofuels) or by combining green hydrogen with carbon dioxide (e-kerosene). The term “sustainable” refers to their potential for reducing greenhouse gas emissions. However, emissions reductions from SAF vary widely depending on the feedstock and conversion pathway. That’s why certain governments such as the UK and EU have restricted the types of SAF that can qualify for mandates and incentives.
SAF is expected to contribute the majority of aviation sector emission reductions in most government and industry roadmaps. In 2024, SAF made up of jet fuel used globally use. By 2050, SAF is expected to grow to more than half of global jet fuel use.
Not all SAF is created equal.
Emissions reductions vary by feedstock and conversion pathway.
SAFs vary significantly in terms of feedstock, production pathways, and their potential to reduce greenhouse gas (GHG) emissions. They fall into two main categories: biofuels, derived from biomass, and e-fuels, or e-kerosene, produced from renewable electricity and carbon dioxide.
Among biofuels, the feedstock—the raw material used—is the most critical factor for assessing sustainability. We distinguish between:
- First-generation bio-SAF: Made from food-based feedstocks such as vegetable oils, sugar, or starch crops. These feedstocks are already used to produce fuel at commercial scale for the road sector, but their availability is limited, and they carry significant sustainability risks.
- Second-generation bio-SAF: Produced from non-food, cellulosic materials such as agricultural residues, woody biomass, or municipal solid waste. These materials are harder to process and require newer, emerging technologies to convert into fuel.
The wide variation in climate impacts across different SAF feedstocks and conversion technologies means that simply displacing petroleum jet fuel with any alternative jet fuel will be insufficient to drive deep decarbonization in aviation.
Instead, meeting aviation’s climate targets will require widespread deployment of “advanced” SAF, meaning second-generation bio-SAF (relying on emerging technologies and scalable, non-food feedstocks) and e-kerosene.
| First-generation biofuel: oilseeds | First-generation biofuel: starch crops | First-generation biofuel: wastes, fats, and greases | Second-generation biofuel: cellulosic | E-kerosene | |
|
Technology readiness level |
Mature | Developing | Mature | Emerging | Emerging |
|
Feedstocks |
 |
 |
 |
 |
 |
| Rapeseed, soybeans, palm | Corn, sugarcane | Used cooking oil, tallow | Municipal solid waste, agricultural & forestry residues | Renewable energy combined with CO2 captured from point sources or the atmosphere | |
|
GHG savings |
⬤ | ⬤ | ⬤ | ⬤ | ⬤ |
|
Sufficient raw material |
⬤ | ⬤ | ⬤ | ⬤ | ⬤ |
|
Low conflict of use |
⬤ | ⬤ | ⬤ | ⬤ | ⬤ |
Key: ⬤ High | ⬤ Moderate | ⬤ Low
Source: Table adapted from SkyNRG.
Most of today’s SAF is produced using HEFA technology. Meeting future demand requires scaling up SAF from scalable, non-food sources and new technologies.
When made from waste materials like used cooking oil or tallow, SAF can cut life-cycle emissions by up to 80% compared to fossil jet fuel, but these materials are limited. To meet growing demand, it is possible that SAF could instead be produced from crops—like corn and palm oil— which can lead to emissions similar to or higher than fossil fuels.
Achieving meaningful long-term decarbonization will require scaling up advanced pathways—such as e-fuels from renewable electricity and second-generation bio-SAF from cellulosic biomass. These SAFs could offer deeper reductions and greater scalability, if successfully developed.
SAF combustion is often called carbon-neutral because the CO2 it releases was recently absorbed from the air, unlike fossil fuels.
The source of that carbon can have a large impact on net CO2 emissions from fuel consumption – emissions that occur after fuel is produced and delivered to aircraft. Fuels made from fast-growing plants that absorb carbon quickly emit CO2 that is assumed to be reabsorbed quickly on an annual cycle, while fuels made from slow-growing trees may create a “carbon debt,” with the CO2 released from forest clearance and fuel consumption taking many decades of growth to counteract. Capturing CO2 and combusting it as e-kerosene fits carbon-neutral accounting best, but it’s important that the CO2 comes from waste sources. Overall, most SAF pathways generate lower emissions over their life cycle compared to fossil jet fuel, but their combustion can contribute to a near-term increase of CO2 in the atmosphere, particularly if sourced from materials like whole trees or roundwood.
Figure. How “carbon debt” affects the concept of carbon neutrality at the tailpipe.
SAF is expected to play a larger role in near- and medium-term aviation decarbonization than zero-emission aircraft.
SAFs are “drop-in” fuels, meaning they can replace fossil jet fuel with minimal changes to aircraft and infrastructure. Most engines today are certified to use a 50% blend of SAF and fossil jet fuel without modification. Meanwhile, zero-emission planes powered by hydrogen or electricity face technical challenges such as limited range, heavier energy storage, and costly new airport infrastructure. However, they are more energy efficient to produce, are less land-intensive, and emit no carbon dioxide or air pollutants during operation.
Figure. Specific energy and energy density of commonly used energy sources
SAF costs between 2 and 5 times more than fossil jet fuel.
Because of high production costs, limited availability, costly feedstocks, and complex processes, SAF currently costs 2–5 times more than conventional jet fuel.
Although SAF can be made from many materials and processes, all pathways are expected to remain more expensive than fossil jet fuel due to costly feedstocks and complex production. Production costs are particularly high for advanced SAF, produced from non-food feedstocks and novel technologies, which will be critical to scaling supply and meeting long-term climate goals. Moreover, many advanced SAF projects stall before construction due to financing gaps, policy uncertainty, and technical setbacks. Lowering costs will require a mix of measures, including public and private investment, demand side strategies such as mandates and long-term offtake agreements, and cost-sharing mechanisms to distribute added costs fairly.
Unlike CO2 emissions, non-CO2 impacts—such as PM emissions and contrail formation—can be partially attributable to fuel composition. SAFs’ higher hydrogen content, lower aromatics, and minimal sulfur can reduce PM emissions and contrail radiative forcing, though the magnitude of these benefits depends on the SAF blend on a given flight.
Figure. The role of SAF in reducing contrail formation and soot emissions
Airlines receive credit for SAF use through sustainability certificates, which can be retired on behalf of passengers to account for emissions reductions.
To track SAF use and related emissions reductions, airlines rely on accounting methods and sustainability certificates that document SAF production and consumption. These certificates help prevent double counting and ensure emissions benefits are assigned correctly. When passengers pay for SAF upgrades, airlines can retire certificates on their behalf, allowing individuals to claim a lower carbon footprint. However, outside of regions with SAF blending mandates, most SAF credits are sold to corporations, meaning the business’ carbon footprint, and not that of individual passengers, is reduced.
Figure. System to track SAF use and assign GHG savings. Source: Adapted from IATA
Additional resources
