The main problem with flying is not the flying itself, but the burning of fossil fuel. So the industry’s biggest hope is to change the fuel: keep flying, but power it with something cleaner.

Brilliant scientists and engineers are working hard on it, and the airlines are counting on these alternatives arriving, at enormous scale, in time to meet their “Net Zero by 2050” pledge. The honest position today is that they are not close.

If food were fuel

Planes need fuel the way we need food, and aviation needs to change its diet. It helps to compare the options to what is on our plate.

Fossil-based jet fuel is like factory-farmed beef: a very efficient source of energy, but with real environmental costs, and in need of a greener substitute. There are three main substitutes on the table. Here is what each can and cannot do.

Sustainable Aviation Fuel: the veggie burger

SAF (Sustainable Aviation Fuel) is the veggie burger of aviation: a substitute that delivers similar energy and, depending on how it is made, can be a better environmental choice than the steak. It is easy to use, because you simply blend it into ordinary jet fuel, and it is already used as a “drop-in” replacement today, just in tiny amounts.

A word on the name. “Sustainable” here is the industry’s label, not a guarantee. Whether a batch is anything close to sustainable depends entirely on what it is made from and how, and a lot of it falls short, which is why you will often see the word written in quote marks, including by us.

Its appeal is that it need not add new fossil carbon to the atmosphere. It is made by recycling carbon that is already above ground, from used cooking oil and animal fats, forestry and farm waste, or even carbon captured from the air. Airlines claim this can cut the fuel’s carbon by “up to 80%”, and they treat it as their silver bullet: SAF alone is 65% of the “Net Zero by 2050” plan drawn up by their global trade body, IATA (the International Air Transport Association).

But: SAF is expensive to produce, and nowhere near enough is being made to dent total aviation emissions, which keep rising. UK climate charity Possible, in its report Missed Targets, tracks how far the industry has always fallen short: in 2007, IATA aimed for 10% of aviation fuel to be SAF by 2017; that target was quietly dropped, and by 2025 SAF was still only about 0.6% of the fuel supply. Meanwhile, aviation’s total fuel use keeps climbing, so SAF is not even keeping pace with the extra fuel burned as the industry grows.

Why SAF is not a silver bullet

Here is the key point most people miss: SAF still releases carbon when it burns, but because that carbon was recently taken from the atmosphere or comes from genuine waste, over its full life cycle, it can add less than fossil fuel does. Burning cleaner, it may also reduce the contrails that contribute to global warming. So why is it not the answer? Two reasons. First, the saving is smaller and far less certain than the ‘up to 80%’ headline suggests. Second, there is barely any of it: the genuinely sustainable ingredients are scarce, cleaner production swallows ever more limited renewable energy, and SAF is still only 0.6% of the fuel airlines burn. A real cut on such a tiny sliver of fuel does little to aviation’s total emissions.

What is SAF actually made from?

This is where it gets uncomfortable. Much of today’s SAF is made from used cooking oil and animal fat, and neither are as clean or as simple as it sounds.

Used cooking oil is in such demand that there is not enough of it. Europe now imports around 80% of the “used” cooking oil it turns into fuel, and how much of that is genuinely used, no one can say for sure: Europe’s own auditors say the schemes meant to certify it cannot guarantee the oil is really waste. The figures invite suspicion. Malaysia, a big palm-oil producer, exports three times more “used” cooking oil than it collects. The fear is that virgin palm oil, which is not allowed as a green fuel because it drives rainforest destruction and is, some argue, worse for the climate than the jet fuel it replaces, is being relabelled as waste to get around the ban. In one proven case, a fraud investigation in Indonesia led to arrests over crude palm oil mislabelled as “waste” and shipped to Europe’s biggest fuel producers. Once it is refined into jet fuel, telling the difference is almost impossible.

Animal fat, known as tallow, is the other main ingredient, and it is already spoken for: nearly half of Europe’s animal fat already goes into fuel, and the rest makes soap, cosmetics, and pet food, industries that then switch to, of all things, palm oil. So burning it does not create a new clean resource; it shuffles the problem sideways. And far from discouraging it, turning that waste into fuel gives the meat industry a profitable new market.

And there is a question your airline never asks you: are you happy for your flight to be part-powered by animal fat at all? SAF made from tallow is not vegan, and beef or pork fat sits badly with many Hindu, Muslim, and Jewish flyers. Consumer labelling tells you more about the ethics of a jar of chocolate spread than your airline tells you about your fuel.

So flying on “sustainable” fuel is, at best, marginally better than flying on freshly drilled fossil fuel, and at worst no better at all. It still puts carbon back into an overloaded atmosphere, and it is not a licence for guilt-free flying.

Electric aircraft: the watermelon

Flying on batteries is the watermelon of fuels: like batteries, watermelons are heavy and bulky, and you could not run far or fast on a diet of them alone.

Electric and hybrid-electric aircraft are already used in flight training and should soon manage limited passenger flights, with hydrogen-electric engines in testing too. They will be good for very short regional hops, like connecting an island to the mainland, and even for private jets, the greediest fuel-burners per passenger, since almost half of private-jet flights are under 500 km.

Private jet flights, the greediest jet fuel consumers per passenger, could also benefit from shifting to ‘a diet of watermelons’, seeing almost half of private jet flights are under 500km.

Heart Aerospace is developing the ES-30, a 30-seater with a roughly 200 km all-electric range, and ZeroAvia is designing hydrogen-electric engines to retrofit small regional planes.

But: these solutions only really apply to short-haul. The International Council on Clean Transportation estimates electric aircraft will cut just 0.2% of expected passenger-flying emissions in 2050.

Hydrogen: the popcorn

Burning hydrogen is the popcorn of fuels: to get enough energy from it, you need mountains of the stuff. Liquid hydrogen takes up about four times the volume of jet fuel, so the tanks, currently in the wings, would need to be four times bigger. Hydrogen has drawn huge attention and investment as a zero-emission option, since burning it produces only water vapour.

But: that four-times volume, plus very different handling and combustion, means both the aircraft’s shape and its engines must be completely redesigned before we can fly on it safely, which will take many years. Airbus has been designing a hydrogen aircraft, ZEROe, since 2020; it was meant to fly by 2035, but in 2025 Airbus pushed the timeline back to the 2040s and cut the project’s budget, blaming slow progress on hydrogen infrastructure. So it is unlikely to play a real part in decarbonising aviation by 2050. Storage is another wall: hydrogen must be kept at minus 253°C, at the airport and on the plane, and the network to deliver it barely exists. Because liquid hydrogen is so bulky, even if it does arrive, it is likely to suit only small, short-range aircraft, not the long-haul flights that do most of the damage. Universal Hydrogen, a start-up built around the idea, ran out of money and shut down in 2024.

The bottom line

Changing the fuel is real, and worth backing. But none of the three is ready at the scale and speed the climate needs: SAF is tiny and oversold, electric works only for short hops, and hydrogen is years away at best. Better than nothing, nowhere near enough on its own, and never a reason to fly more. The one thing that works today, with no new technology, is flying the most polluting trips less.

What can you do?

Your choices as a flyer are a signal, and the most polluting flights are the ones worth thinking twice about.

• Take the quiz: How Big Is Your Bumprint? and share your result.
• Use our Passenger Action Checklist to plan a lower-impact trip.
• Get Involved, and back the measures that would make the whole system cleaner.

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