Condensation Trails: What Impact Do They Have on the Climate?

This article was originally published in our "Décryptage Mobilité" newsletter. To receive future articles by email as soon as they are published, Subscribe now. By Célia Foulon – Consultant

You’ve probably noticed those long white trails that form in the sky after an airplane passes by. Known as contrails, they form at high altitudes at the exhaust of jet engines if the surrounding air is cold and humid enough. They are caused by the condensation of water vapor emitted by engines around condensation nuclei present in soot and combustion gases ^[1]. These long trails are not insignificant and can sometimes persist for several hours, covering a significant portion of the sky: these are known as induced high-altitude cirrus clouds, which are very thin clouds covering large areas. Persistent contrails and induced cirrus clouds contribute to global warming (especially cirrus clouds) : They absorb some of the radiation coming from the Earth and re-emit it toward the ground, while being too thin to have an albedo effect that could counteract this warming. While their impact is difficult to quantify precisely, it is generally estimated that this would double aviation’s radiative forcing (see the diagram below, taken from ^[1]).

RF: radiative forcing => radiative forcingAIC: aircraft-induced cloudiness => cloud cover caused by aircraftContrail cirrus: cirrus clouds formed by contrailsPersistent contrails: persistent condensation trails

One point of scientific controversy concerns the metric itself : Radiative forcing (RF) measures the impact of past emissions at a given time t, whereas the metric typically used to measure the future impact of emissions on the climate is global warming potential (GWP). A factor of 2 in RF does not automatically translate to a factor of 2 in GWP because the concept of lifetime comes into play … In any case, the impact of these contrails is never taken into account by airlines or the DGAC when calculating the carbon footprint of flights. The aviation sector’s contribution to climate change is therefore underestimated in public debate, as it takes into account only the kerosene burned (combustion in general, or combustion plus upstream processes in some cases). While the priority for the aviation industry is indeed to reduce CO2 emissions resulting from the combustion of kerosene in order to limit the long-term impact (CO2 remains in the atmosphere for a century), Do contrails and induced cirrus clouds have a very short lifespan? (from a few minutes to a few hours) and could thus be mitigated quickly, with an immediate impact ^[2].

In the coming years, alternative fuels (derived from biomass or synthesized via the Power-to-Liquids process) should significantly reduce the formation of contrails due to the near absence of particles in the combustion products. But to reduce the impact of contrails right now, scientists at Imperial College London propose flying planes at a slightly different altitude, where humidity levels would not be conducive to the formation of contrails. Recognizing that flight paths are chosen to optimize flight time (and thus kerosene consumption and flight costs) and that airlines are often reluctant to change their aircraft’s routes, Scientists estimate that less than 2% of flights—which produce the most persistent contrails—could be rerouted by ±2,000 feet (a unit commonly used in aviation, equivalent to approximately 600 m) to reduce the impact of contrails from commercial aviation by nearly 60%^[3]. According to them, these deviations would result in only a 0.014% increase in fuel consumption and thus a significant overall benefit. Two questions remain, however: Is it possible, before takeoff, to identify the 2% of flights affected and determine the correct altitude at which to fly? Would airlines and/or air traffic control authorities agree to reroute some of these flights solely for environmental reasons? While the answer to the first question is probably yes, the answer to the second is less clear …

Sources: ^[1]Nature Communications^[2]Le Monde^[3]ACS Publications