Hope or hype: Can SAF really justify airport expansion?
4th September, 2025
“Sustainable” aviation fuels are often promoted as the answer to aviation’s decarbonisation problem, but they are not a magic bullet. Here we summarise 6 reasons why SAF won’t be sufficient to mitigate increased emissions from airport expansion.
What are SAFs?
Currently, the kerosene used to power most aircraft engines is made from fossil oils. “Sustainable” aviation fuels (or SAF) is a name given to fuels which use non-fossil based feedstocks. The fuels have been dubbed “sustainable” because although they emit the same amount of CO₂ as kerosene when they are combusted, the emissions avoided or sequestered during the ‘life cycle’ of the fuel feedstock should produce a net emissions saving. As volumes increase, they should displace the amount of fossil fuel used by the sector .
They can also be called alternative fuels to avoid the mistaken impression that they are low-carbon fuels – they are not, the carbon content of SAF is the same as for kerosene.
Some examples of feedstock include used cooking oil, black bin bag waste, ethanol, waste forestry or crop residues. E-fuels, which combine captured carbon with green hydrogen, are in the early stages of development. Some countries permit the use of biofuels (based on feedstocks such as palm oil), but these are not eligible under the UK’s SAF mandate due to real concerns about deforestation and competition for scarce farmland.
Government estimates suggest that SAFs can reduce emissions on a lifecycle basis by up to 70% on average. AEF is clear that these emissions savings must be constantly assessed and monitored – not merely assumed.
SAFs maybe good in theory, but there are many problems associated with their deployment.
- Scaling up
There are significant concerns that SAFs are not available in anything like the sufficient quantities needed. Even as the UK SAF mandate is spurring an increase in SAF production, growth in demand for flights threatens to wipe out any emissions savings, as this report from Carbon Brief shows. The UK SAF mandate requires that 22% of jet fuel should be SAF by 2040, but early signs are that this target is unlikely to be met. Based on figures from the Climate Change Committee, AEF analysis suggests that there will be a shortfall of 1.1 million tonnes – 38% less SAF than is needed. The main reason for this slow scaling is fierce global competition for certifiably sustainable feedstocks.
- More expensive
Estimates of the cost of various pathways for SAF vary widely, but one thing is clear – they are more expensive. This estimate puts the cost at between 3 and 7 times the cost of conventional jet fuel.
- Fraud
Strict environmental criteria in the UK and the EU mean that biofuels are not permitted, which means all fuel producers are on the hunt for waste. In 2025, the SAF mandate allows for Hefa (used oils) to make up to 100% of the feedstock, although that proportion declines over time. Waste cooking oil is in very short supply and already around 90% of the UK’s supply comes from abroad. Recent investigations have revealed that fraud is being committed in international supply chains, with virgin palm oil being labelled as “used”.
- “Best use” of potential feedstocks
What consideration is going into whether aviation should monopolise these scarce resources? Almost all potential SAF feedstocks are already being used in other industries. Should carefully collected compost food waste from local authorities be used to improve soil health, create “green” bio gas, or be used as a jet fuel feedstock? In every scenario modelled, the answer is that at the moment, aviation is not the ‘best-use’ of these feedstocks. This is also true for e-fuels – one estimate suggests that in order to power the Net Zero Carbon Road Map, aviation could use up to an additional 147TWh of renewable energy to achieve its contribution to net zero by 2050.
- Are e-fuels the answer?
Synthetic fuels (or e-fuels) are made from green hydrogen and captured carbon, and offer the potential, with surplus renewables, to be the most sustainable form of SAF while promising significant industrial opportunities for the British workforce. However, the high costs of this type of fuel (estimated to be around 9x kerosene) is one of the main barriers to scaling. E-fuels are also an inefficient use of currently scarce renewable power. This report from Concito suggests that making e-fuels is only half as beneficial to the climate compared to just taking the CO₂ out of the atmosphere and storing it underground. The CCC estimates that by 2030, around 500,000 tonnes of synthetic aviation fuel will be needed per year. Currently there are no domestic e-fuel plants at final investment decision (FID) in the UK; the picture is similar in Europe. It’s very tempting to ask if SAFs are the future, why is Big Oil not investing?
- Tracing emissions savings across global supply chains
And finally – how can we be sure the claimed emissions savings are occurring? We know despite an increasing amount of SAF being used in the UK in 2024, emissions from international aviation actually went up. How can we be sure that the methodologies and science behind the “closed loop” theory hold true in the real world? The regulatory landscape and the voluntary carbon markets compliance frameworks are fiendishly complex, running across international boundaries, multiple actors and supply chains. This report from AEF and the Sustainable Energy Systems Research Centre at the University of Bath shows that there are risks of double counting, and regulatory overload and incomplete paper trails across borders and supply chains.
AEF’s conclusion: SAF may have potential to reduce emissions, especially if we can produce affordable and surplus renewable energy in the UK in the future, but there are considerable hurdles. Until these challenges can be overcome, it looks unlikely that SAF production can scale without relaxing the environmental standards that have been set. Given this uncertainty, relying on SAF to justify the increased emissions stemming from airport expansion is both questionable and unproven.
Further reading
What does SAF mean for airport expansion? From the Grantham Institute at Imperial College