Taking a day flight could be a swift way to limit the climate impact of a holiday, as new research suggests vapour trails produced by jets can cool the planet during the daytime.
Soot particles emitted by jet engines can trigger the formation of ice crystals, causing a condensation trail that can spread and last for hours. These persistent contrails, as they are known, help to reflect solar radiation back to space, which has a cooling effect. But they also block heat from escaping from below, causing warming.
Previous studies have found that overall, contrails have a net warming impact on Earth鈥檚 climate that is . But the warming effect of individual contrails is less well understood.
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at Carlos III University of Madrid, Spain, and her colleagues set out to determine the net impact of individual contrails over two one-week periods in January 2023 and 2024. The team used artificial intelligence to identify and analyse the net warming effect of 700,000 contrails from satellite imagery covering Europe, North Africa and the surrounding oceans.
Researchers already knew that contrails that form during the night trap heat, but don鈥檛 reflect any solar radiation, so they have a warming impact. By comparison, daytime contrails both trap heat and reflect sunlight, giving them a lower warming impact.
The team鈥檚 conclusions go a step further, suggesting that most daytime contrails in fact have a net cooling effect. Most of those observed in the study 鈥� 62 per cent 鈥� formed at night, which is why they found that the net effect of all contrails is a warming one, says Ortiz. However, the study only looked at the northern hemisphere in winter, when nights are longer. The team plans to repeat the study at other times of year to see if the overall impact varies.
So, is flying during the day a better choice for the climate? Potentially yes, says at Imperial College London, who wasn鈥檛 involved in the study. 鈥淔lying during the day, the contrails that you will generate will be less warming,鈥� he says.
Switching to day flights wouldn鈥檛 eliminate the climate impact of flying, stresses Nicolas Bellouin at Sorbonne University in France. 鈥淚t doesn鈥檛 mean that the overall impact of your flight will be a cooling of climate of course, because you have carbon dioxide emissions that exert their own warming. But at least you could say that for similar flights taking the same trajectory鈥� you have more likelihood to minimise the climate impact of your flight if you choose a daytime flight.鈥�
However, the climate benefit also depends on how long the contrails last. The average contrail lingers for 2.5 to 3.5 hours, but some can last 14 hours or more. So a contrail that forms in the later part of the day and lasts into the night can have a net warming effect, even if it came from a day flight.
A more promising solution might be re-routing aircraft to avoid persistent contrails forming altogether. They occur when a plane passes through air that is extremely humid, or supersaturated, with the jet exhaust acting as a trigger for clouds to form. Pilots could steer aircraft above or below these supersaturated regions to prevent this happening.
鈥淚t will probably mean a longer flight, because the route will be longer, or the plane will fly lower, where it is a bit less efficient,鈥� says Bellouin, who suggests passengers could expect a long-haul flight across the Atlantic to take an extra 30 minutes or so if planes take a contrail-avoiding route. But if scientists can prove the extra fuel burn is worth it to avoid leaving warming contrails, it might make sense to implement on commercial flights, he argues.
arXiv
