Contrail conundrum: How plane trails secretly heat up the night

Aeroplanes leaving contrails can surprisingly influence the Earth's climate. Research indicates that these white lines in the sky act in two ways: they cool the surface during the day while impeding heat emission at night. This information comes from the "arXiv" service.

During flights in humid air, soot particles emitted by jet engines lead to the formation of ice crystals, which transform into persistent contrails. Contrails can linger in the atmosphere for many hours. Previous analyses suggested they may significantly impact the Earth's warming more than the carbon dioxide emissions from jet fuel itself.

Research conducted by Irene Ortiz and her team from Carlos III University of Madrid aimed to determine the net effect of contrails during two winter periods—January 2023 and 2024. The study's authors used artificial intelligence tools. These tools identified and analysed the net warming effect for 700,000 contrails from satellite images covering Europe, North Africa, and the surrounding oceans.

The results suggest that contrails produced by aeroplanes have a net cooling effect during the day by reflecting sunlight back into space, according to the study's authors.

At the same time, they also have a drawback in the context of climate warming—they hinder the emission of heat into space. However, this has little significance during the day, as they significantly limit the Sun's heat intake.

The situation looks worse at night. When the Sun is not shining, contrails hinder the emission of heat from the Earth's surface, contributing to global warming. The study also noted that 62 per cent of the observed contrails formed at night, so the net impact of all contrails are warming. However, the study was limited to the Northern Hemisphere in winter when nights are longer than days.

The study's authors suggest that daytime flights could reduce the climate impact by limiting the Sun's heat intake. However, it must be admitted that they will not eliminate carbon dioxide emissions, which also contribute to global warming. Contrails persist on average for 2 to 3 hours, but some can last 14 hours or longer. For this reason, contrails formed in the afternoon and persisting at night may have a net warming effect.

One solution could be to direct aeroplanes to avoid areas with humid air where contrails form most easily. This could be achieved, among other ways, by flying higher or lower. However, this approach would be associated with lengthening the flight route and/or increasing fuel consumption. The research team plans to repeat the study at other times of the year to see if and how the overall impact of contrails on the climate changes.