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Long-lived contrails form predominantly not in cloud-free skies, but within already existing ice clouds. This is the conclusion reached by a team of scientists from Forschungszentrum Jülich, the University of Cologne, the University of Wuppertal, and Johannes Gutenberg University Mainz.
Research team identifies common environmental conditions for the formation of contrails and provides initial insights into their impact on the climate
Long-lived contrails form predominantly not in cloud-free skies, but within already existing ice clouds. This is the conclusion reached by a team of scientists from Forschungszentrum Jülich, the University of Cologne, the University of Wuppertal, and Johannes Gutenberg University Mainz (JGU). Using extensive observational data, the researchers were able, for the first time, to systematically determine the atmospheric conditions under which long-lasting contrails form – whether in cloudless skies, in very thin and barely visible ice clouds, or in more clearly visible ice clouds, known as cirrus clouds. The result: more than 80 percent of all persistent contrails form within pre-existing clouds, mostly within natural cirrus clouds. The effects of this on the climate are not yet clearly understood. The study, now published in Nature Communications, provides important insights for further research – and, beyond that, strong arguments for taking cloud cover into account when planning flight routes adapted to climate considerations.
Effect of natural and man-made ice clouds on the climate
Contrails are a visible signature of daily air traffic in the sky. They form when the hot exhaust gases from aircraft engines mix with the cold air at an altitude of around ten kilometers. In dry air, most contrails dissipate quickly. In cold and humid air, however, they can persist for several hours and develop into extensive ice clouds or cirrus clouds. Cirrus clouds are thin ice clouds that occur at altitudes of about eight to twelve kilometers and often appear as fine, fibrous veils in the sky. The overall climate impact of these cirrus clouds formed from contrails is greater than that of the direct CO₂ emissions produced by air traffic.
The decisive factor for their climate impact is whether the man-made clouds form in a blue, cloudless sky or within existing natural cirrus clouds. High ice clouds, whether natural or man-made, exist at cold temperatures below -40°C. Although they often appear optically very thin, they can act like a blanket that prevents heat from escaping from the atmosphere into space, thereby contributing to the greenhouse effect. Only when the clouds are very dense and the sun is barely visible does the amount of sunlight reflected back into space become large enough to produce a cooling effect on the climate.
Accordingly, artificial clouds formed by contrails affect the climate differently depending on their environment: Under clear conditions – such as blue skies or very thin cirrus clouds – they tend to contribute to warming, because they trap some of the Earth's radiation that would otherwise escape into space, while allowing sunlight to pass through. In dense, clearly visible cirrus clouds, however, the opposite effect can occur: Contrails reflect more sunlight than they absorb heat radiation, leading to a slight cooling effect. How exactly contrails and natural cirrus clouds influence each other is still poorly understood.
"Our results show that we need to take a more differentiated view of the climate impact of contrails in the future," says Professor Andreas Petzold from the Institute of Climate and Energy Systems – Troposphere (ICE-3) at Forschungszentrum Jülich. "If most persistent contrails occur within natural clouds anyway, it might be more effective to plan climate-relevant flight routes not only according to clear skies but also with regard to existing ice cloud structures."
For the study, the research team used measurement data on temperature and water vapor collected by commercial aircraft flying over the North Atlantic between 2014 and 2021. These aircraft are part of the European research infrastructure IAGOS (In-service Aircraft for a Global Observing System, https://www.iagos.org/), which is co-coordinated by Forschungszentrum Jülich. IAGOS aircraft are equipped with instruments that continuously collect atmospheric data during scheduled operations – something unique worldwide.
Mainz contribution to the study: model calculations of radiative forcing
The data evaluation was supplemented by model calculations on radiative forcing. "Our analysis shows that contrails in thick cirrus clouds actually have hardly any effect," says Professor Peter Spichtinger from JGU, who contributed this aspect to the study. "However, additional effects in more complex scenarios – such as those arising from multiple layers of contrails and cirrus clouds on top of each other – are difficult to estimate and will be investigated in more detail in the future."
The results of the study are being incorporated into ongoing international activities of the World Meteorological Organization (WMO), the International Civil Aviation Organization (ICAO), the European Aviation Safety Agency (EASA), and the aviation industry. The goal is to develop a sustainable flight-planning strategy to reduce climate-relevant contrails by designing flight routes with climate impact in mind. IAGOS aircraft will continue to play a key role in evaluating such strategies in the future.
Professor Dr. Peter Spichtinger
Institute for Atmospheric Physics
Johannes Gutenberg University Mainz
55099 Mainz
phone: +49 6131 39-23157
e-mail: peter.spichtinger@uni-mainz.de
https://www.staff.uni-mainz.de/spichtin/
A. Petzold et al., Most long-lived contrails form within cirrus clouds with uncertain climate impact, Nature Communications 16, 9695 (2025),
DOI: 10.1038/s41467-025-65532-2
https://doi.org/10.1038/s41467-025-65532-2
https://www.iagos.org/ – IAGOS (In-service Aircraft for a Global Observing System)
https://www.fz-juelich.de/de/ice – Institute of Climate and Energy Systems (ICE)
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