Will travelling by plane ever be carbon neutral? Researchers have their doubts

How can millions of people keep travelling by plane every year without contributing to the climate crisis? That is the question at the heart of this year’s Paris Air Show aeronautical extravaganza which opened near Paris on Monday. Having set an objective of being carbon-neutral by 2050 in line with the Paris Agreement, aviation companies have put forward a number of possible solutions to meet the challenge, from building more sustainable aircraft to finding greener fuels.

For his part, French President Emmanuel Macron showed himself ready to open the state’s coffers. Macron announced a package of €2.2 billion last Friday to help the sector’s flagship companies – Airbus, Safran, Thales and their subcontractors – to find a way to build a game-changing “green plane”.

The stakes are sky-high. 

“The aviation sector represents two to three percent of global CO2 emissions,” explained Isabelle Laplace, specialist in air transport sustainability issues at the National Civil Aviation School in Toulouse. “Add to that other emissions, the effects of which are relatively unknown. For example, condensation trails – those white clouds that form in the wake of planes – which cause a temporary greenhouse effect.”  

And these effects will continue to increase if nothing is done, with the International Air Transport Association projecting that air travel will more than double by 2050.

Lighter aircraft, more efficient engines 

To mitigate the aviation sector’s impact on climate, the first step is to maximise energy efficiency, Laplace explained, noting that work is being done on both the types of engines and the materials used by aircraft. “We’ve been working on this for 50 years. Today, a plane consumes 80 percent less fuel than in the 1970s,” she noted.    

But these technological advances haven’t stopped the sector’s carbon footprint from increasing with the explosion of air travel.

CFM, a joint venture between US conglomerate General Electric and France’s Safran aviation group, has been working on an engine that they hope will reduce fuel consumption by another 20 percent by 2035. But even if such advances were possible, Laplace said this alone “would not allow us to reach carbon neutrality”.

Biofuels, a limited solution 

Beyond technological improvements, the aeronautics industry is focusing on sustainable aviation fuel, or SAF. 

“This is certainly the most readily available solution in the short term,” Laplace said. “Particularly as SAF has the advantage of being usable in the aircraft we have now. There is no need to change the planes’ structure.”

The term SAF covers several different types of fuels. The first are the biofuels, or agrofuels: these would allow a reduction of 80 to 85 percent of greenhouse gas emissions in the aviation sector, according to France’s minister for ecological transition. Traditionally made from wheat, canola, sugar beets or palm oil, the latter is often singled out by environmentalists who say its production causes deforestation, over-cultivation and damaging changes in land use. 

To counter such accusations, aviation companies say they use only “second-generation” biofuels – forestry and agricultural waste products like hay or cooking oil. 

“But all this is far from solving all the problems,” said Jérôme Du Boucher, aviation specialist for the European NGO Transport & Environment France, which researches solutions for decarbonising travel.

“The companies mostly use second-hand oil because it’s the cheapest solution,” Du Boucher said. “Unfortunately, there is a lack of transparency on the part of the sellers, particularly in Southeast Asia, and it’s suspected that some are fraudulently using palm oil instead, thereby contributing to deforestation.”

There is also the question of available resources. 

“We are not going to triple our consumption of oil or agricultural products to make waste products available for aviation,” Du Boucher said. “To supply the whole sector with fuel, we would have to have enormous volumes of biomass.”

Laplace agrees that biofuels are unlikely to be the answer.

“It seems impossible to me that biofuels will ever be enough to one day power every plane in the sky, especially since demand will certainly continue to increase in parallel across all sectors, notably in the automotive sector,” she said.

Other types of SAF are synthetic fuels, or e-fuels.

“They are made with green hydrogen and carbon monoxide,” Laplace explained. These have the advantage of emitting a lot less CO2 than kerosene, but they emit other pollutants such as nitrogen oxides. 

“This is not a technology that we have mastered yet, except on a small scale and in very small quantities,” she said. “We are only at the experimental stage.”

Despite these challenges, things are speeding up at the European level. At the end of April, the European Parliament agreed on a regulation called ReFuelEU Aviation which dictates that 2 percent of SAF will have to be mixed with the kerosene fuelling the planes in circulation between now and 2025. This number will increase to 6 percent by 2030, 20 percent by 2030, 34 percent by 2040 and will reach 70 percent by 2050.

The El Dorado of hydrogen

Another energy source that is fuelling hopes in the sector is green hydrogen. Airbus has promised to have an aeroplane equipped with a hydrogen engine by 2035. For its part, the small French company Blue Spirit Aero, founded in 2020, unveiled at Bourget its four-seater hydrogen plane, which is supposed to reach a cruising speed of 230km/h.

“Obviously, it seems like a miracle solution, as the plane would emit nothing except water vapour,” Laplace noted. But it has many limitations, she said. 

“Today, green hydrogen manufactured using a decarbonised source of electricity only exists on a small scale. More than 95 percent of this energy still comes from natural gas,” a fossil fuel, she noted. 

“And like with SAF, it raises the question of available resources,” Du Boucher added. “We know that the demand for clean electricity will grow considerably in the coming years. But we can’t install solar panels everywhere at the expense of biodiversity.”

Not to mention the many logistical challenges – unlike SAF, using hydrogen fuel would require renovating aircraft and therefore launching new production lines. 

There must also be a way to store it aboard planes – hydrogen is four times bulkier than kerosene and must be kept at -250°C (-418°F).

Electric short-hauls

The last solution the industry is considering is electricity. French start-up Voltaero, which develops small hybrid planes, presented its first aircraft on Sunday, a five-seater that the company hopes to mass produce by 2025 and which has already received more than 200 pre-orders.

Using only electricity during take-off and landing, the plane is also equipped with a small combustion engine that only turns on once in flight to recharge the plane’s batteries if necessary. They can also be plugged into the mains once the plane is on the ground. 

“It’s a promising aircraft, but one that will be limited to short-haul flights due to the weight of its batteries,” Laplace said. “It’s a solution that could make sense ecologically in certain parts of the world where there are no railways, or in archipelagos, but it is preferable to take a train whenever possible.”

Laplace and Du Boucher say that all of these fuels will likely be used over the next few years, but for different purposes. 

“In the end, the decarbonisation of air travel will only happen through a reduction in air travel overall,” Du Boucher said.

According to a September 2022 study by the French Agency for Ecological Transition, technological innovation in the aviation sector will fall short for one simple reason: it won’t come fast enough. 

The agency has outlined several ways to limit the number of planes in the sky, including capping the number of flights at airports or even introducing a tax on tickets – while noting that just 1 percent of the population is responsible for half of the greenhouse gas emissions from the planes that criss-cross the globe.

This is article has been translated from the original in French.