The quest for guilt-free flying may have been knocked off course by a broad study that has concluded there is “no clear or single net zero alternative to jet fuel”.

The four most viable alternatives “offer some carbon savings but are not ideal”, according to the review by the Royal Society academy of scientists.

Replacing jet fuel with biomass, for example, would require half the UK’s farmland just to sustain current passenger levels.

But the government is planning for levels to soar by 70% by 2050, representing an additional 200 million passengers.

Switching to sustainable fuel is key to its “jet zero” strategy to turn aviation green, which it touts as a plan to offer “guilt-free flying”.

Flying is responsible for 8% of UK emissions and around 2.4% globally, and also releases other forms of pollution.

The lack of alternatives makes the carbon intensive industry one of the hardest to decarbonise as the world works towards net zero emissions by 2050.

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“The requirements for an alternative to jet fuel, to kerosene, is energy density, has to be sufficient to sustain short and long haul flights, it must be produced globally at scale, it must be cost-competitive and it must be implementable by 2050,” said Professor Graham Hutchings, chair of the report’s working group.

Other options, such as hydrogen, ammonia and synthetic fuels require a massive increase in renewable energy production, or are expensive or require substantial modifications to existing aircraft.

Producing enough green hydrogen – which is created by splitting water molecules into oxygen and hydrogen with renewably generated electricity – would require more than doubling or tripling the UK’s renewable capacity.

A fuel from biomass can be used in the same aircraft engine but there are concerns about its sustainability.

Suitable crops could be rapeseed, fast-growing poplar trees and miscanthus, the Royal Society said.

But because of how much land would be needed to grow them, there has been a growing interest in using biowaste such as used cooking oil.

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The UK is “highly reliant” on importing raw material for biofuel, known as feedstocks, with 423 million litres of used cooking oil imported from China alone in 2021.

Converting waste from the 250 million litres of vegetable oil produced in the UK would produce only 0.3 to 0.6% of the UK’s annual jet fuel needs.

The government wants five “sustainable aviation fuel” (SAF) plants under construction by 2025.

A Department for Transport spokesperson said its SAF programme is “one of the most comprehensive in the world”.

“Our Jet Zero Strategy sets out how we can achieve net zero emissions from UK aviation by 2050, without directly limiting demand for aviation.

“Sustainable Aviation Fuels and hydrogen are key elements of this, and we will ensure that there is no impact on food crops.”

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The Royal Society report did not consider battery-powered aircraft as they are “unlikely to have been developed to give the energy density required for most commercial flights in the timescale available to reach net zero by 2050”.

A spokesperson for Airlines UK, the industry’s trade body, said “there is no magic bullet”.

“But by modernising airspace to make flying more efficient, by introducing new zero-emission technology like hydrogen aircraft and by upscaling the use of sustainable aviation fuels this decade, it can be achieved.”

Cait Hewitt, policy director at campaigning group Aviation Environment Federation, said the “elephant in the room” is “the need to fly less”.

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