How can several billion passengers travel by plane each year without contributing to climate change? This question is at the heart of the new edition of the Paris Air Show, the major aeronautical event, which began on Monday 19 June. More sustainable appliances, eco-friendly fuels, electrification... While the aviation sector has set itself a goal of carbon neutrality for 2050, in line with the Paris Agreement, airlines are displaying several solutions to meet the challenge.

For his part, French President Emmanuel Macron has shown himself ready to get his hands on the wallet. On Friday 16 June, the head of state announced an envelope of 2.2 billion euros to help the French flagships of the sector – Airbus, Safran, Thales and their subcontractors – to invent this "green aircraft".

The stakes are high. "The aviation sector accounts for 2 to 3% of global CO2 emissions," explains Isabelle Laplace, a specialist in air transport sustainability issues at the National School of Civil Aviation in Toulouse. "To which must be added other emissions with effects still relatively unknown. Among them, for example, contrails – these white clouds that form when planes pass by – and which cause a temporary greenhouse effect," adds the specialist. However, these impacts will continue to grow if nothing is done since air traffic is expected to more than double by 2050, according to projections by the International Air Transport Association (IATA).

Lighter aircraft, more efficient engines

To reduce the climate bill of the aviation sector, "the first lever of action is to play on the energy efficiency of aircraft," explains Isabelle Laplace, referring to work on engines or materials that make up aircraft. "We've been doing it for fifty years. Today, an aircraft consumes 80% less fuel than it did in the 1970s." Over this period, however, these technological advances had not prevented the sector's carbon footprint from increasing in the face of the explosion of air traffic.

CFM, a joint venture between the American General Electric and the French Safran, is working for 2035 on an engine that would reduce fuel consumption by another 20%. But if "progress can still be made at this level", this, alone, will not "achieve carbon neutrality", slices the specialist.

Biofuels, a solution to limited resources

In addition to these technological improvements, the aviation industry relies mainly on SAF, the "sustainable aviation fuel" – or CAD "sustainable aviation fuels" in French. "This is certainly the most readily available solution in the short term. Especially since SAF have the advantage of being used in existing devices. There is no need to change the structure of the planes," notes Isabelle Laplace.

Behind the term SAF hide several types of fuels. The first are biofuels, or agrofuels. According to the French Ministry of Ecological Transition, they would reduce greenhouse gases in the aviation sector by 80 to 85%. Traditionally made with wheat, rapeseed, sugar beet and even palm oil, they are often singled out by environmentalists, accused of causing deforestation, intensive cultivation and land-use changes.

To mitigate these accusations, the airlines claim to use only so-called second-generation biofuels, that is to say forest and agricultural waste and residues such as straw or cooking oil. "But this is far from solving all the problems," laments Jérôme Du Boucher, aviation manager for the European NGO Transport & Environment France, which examines solutions to decarbonize our travel.

"Companies mostly use used oil because it's the cheapest solution. Unfortunately, there is a lack of transparency on the part of sellers, especially in Southeast Asia, and there is a suspicion that some are fraudulently using palm oil instead, thus participating in deforestation," he said.

Beyond that, there is the question of available resources. "We are not going to triple our consumption of oil or agricultural products to make waste available for aviation," continues Jérôme du Boucher. "But to supply fuels to the entire sector, you would need huge volumes of biomass." An observation shared by Isabelle Laplace: "it seems impossible to me that biofuels will one day be enough to fly all planes, especially since demand will certainly continue to increase in parallel in all sectors, especially in the automotive sector."

Other types of SAF are synthetic fuels, e-fuels. "They are made with green hydrogen and carbon monoxide (CO)," explains Isabelle Laplace. They also have the advantage of emitting much less CO2 than kerosene, but they emit other pollutants such as nitrogen oxides (NOx). "It is also a technology that is now only mastered on a small scale and in very small quantities. We are only at an experimental stage," says the researcher.

Despite these challenges, things are accelerating at European level. At the end of April, the European Parliament agreed on a regulation, entitled ReFuelEU Aviation, which stipulates that 2% of SAF will have to be mixed with kerosene from aircraft in circulation by 2025. This figure will increase to 6% by 2030, 20% by 2035, 34% by 2040 and reach 70% in 2050.

The Eldorado of hydrogen

Another source of energy raises many hopes in the sector: green hydrogen. Airbus has promised an aircraft equipped with a hydrogen engine by 2035. For its part, the small French company Blue Spirit Aero founded in 2020, unveils at Le Bourget its four-seater hydrogen aircraft supposed to reach a cruising speed of 230 km / h.

See also Paris Air Show: flying taxis, combat drones... These aircraft that will mark the 2023 edition

"Obviously, it appears to be a miracle solution since the plane would only emit water vapor," notes Isabelle Laplace. But there are many limitations. "Today, green hydrogen, made from a carbon-free source of electricity, exists only on a small scale. More than 95% of this energy still comes from fossil natural gas," insists Isabelle Laplace. "And as for the SAF, there is the question of available resources," continues Jérôme Du Boucher. "We know that the demand for clean electricity will increase significantly in the coming years. However, we cannot install photovoltaic panels everywhere at the expense of biodiversity."

Not to mention the many logistical challenges: unlike SAF, hydrogen would require renewing aircraft and therefore launching a new production line. A way would also have to be found to store it on board aircraft – hydrogen is four times larger to store than kerosene and must be kept at -250°C.

Electric for short hauls

Last solution considered: electric. The French start-up Voltaero, which develops small hybrid fuel-electric aircraft, presented Sunday its first aircraft in its final form, a five-seater that it hopes to produce in series by 2025 and which has already collected more than 200 pre-orders.

Electric at take-off and landing, the aircraft is equipped with a small thermal engine that only starts once in flight, to recharge the batteries if necessary. These can also be plugged into the mains when the device is on the ground. "A promising device but which would be limited to short hauls because of the weight of the batteries," notes Isabelle Laplace. "A solution that can therefore be ecological in some areas of the world where rail is non-existent or in archipelagos but it will be better to take the train when possible."

For the two specialists, the scenario to come in the coming years is the same: "the sectors will all develop in the more or less long term," they believe. "These fuels will coexist and be used according to uses."

"In any case, the decarbonization of air traffic will only go through a reduction in air traffic," concludes Jérôme Du Boucher, blaming the "rebound effect". According to a study published in September 2022 by the French Environment and Energy Management Agency (Ademe), technological progress in aviation will not be enough because all this will not go fast enough. Among the avenues mentioned by the institution to limit the use of the plane: capping the number of flights in airports or taxing airline tickets, while recalling that in the world, only 1% of the population is responsible for 50% of aircraft emissions.

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