In addition to facing criticism for its CO₂ emissions, which contribute to global warming, the aviation industry is now under scrutiny for the condensation trails left behind by aircraft. The contrails have gained notoriety in recent years since some conspiracy theorists have accused them of everything from spreading cancer and sterilizing populations to spreading the virus responsible for Covid-19.

In reality, these "contrails" are simply a by-product of kerosene combustion. They form at the engine outlet when aircraft are flying "at a certain altitude and under certain atmospheric pressure and temperature conditions," said Patrick Le Clercq, head of the multiphase flow and alternative fuels department at the University of Stuttgart's Institute for Combustion Technology.

For some years now, scientists around the world have been paying close attention to the "non-CO₂" effects of air transport – in other words, aviation pollution other than that linked to carbon dioxide emissions. "Two-thirds of aviation-related global warming comes from these non-CO₂ effects," said Le Clercq.

Worse still: "Within these effects, condensation trails have the greatest and most visible impact" on the environment, pointed out Matteo Mirolo, a specialist in sustainable aviation with the NGO Transport & Environment. Condensation trails are formed at jet engine outlets by the condensation of water around the soot released into the atmosphere after kerosene combustion.

The phenomenon "is part of the formation of clouds, cirrus clouds, linked to human activity and not to nature," said Le Clercq. He added: "In a small number of cases, these clouds cover a sizeable area of the globe, especially in North America and Northern Europe, where there is heavy air traffic." These clouds are precisely the problem. "They form a veil over part of the globe, which modifies the radiation balance," continued Le Clercq.

It is important to understand that, at night, this veil acts like a lid, preventing the Earth from releasing some of the heat stored during the day back into its atmosphere, and this pressure cooker effect has consequences. "When we assess the effect of these trails over several years, we see that they contribute to global warming," said the researcher. "At the moment, there is no consensus in the scientific community on the impact of contrails on the environment. Results are scattered," said Le Clercq. "More impact than CO₂ for some, less than CO₂ for others." Nevertheless, the proof is in the pudding that contrails do contribute to global warming, to a greater or lesser extent.

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