In meteorology, it was long assumed that aircraft contrails mainly contribute to global warming: long-lasting contrails persist in the sky, especially under cloudless conditions, where they exert their warming effect. We have now investigated this effect in more detail in a joint study with Forschungszentrum Jülich, Johannes Gutenberg University Mainz, and the University of Wuppertal. The results showed that contrails usually dissipate quickly in dry, cloudless air. Surprisingly, eighty per cent of all long-lasting contrails instead form within existing natural ice clouds – or cirrus clouds – at an altitude of approximately five to twelve kilometres. Such clouds appear as delicate, fibrous veils in the sky. In this cold, humid air, the contrails can persist for several hours as the hot exhaust from the aircraft engines mixes with the cirrus clouds.
These so-called contrail cirrus clouds have a greater overall impact on the climate than direct CO2 emissions from air traffic: they retain some of the heat radiated from the Earth in the atmosphere and thus contribute to global warming. However, the ambient conditions determine whether the effect is always warming or, in some cases, slightly cooling. If contrail cirrus clouds form in a cloudless sky or in thin ice clouds, they usually intensify the greenhouse effect. Sunlight penetrates the thin ice clouds and is absorbed by the Earth’s surface. The resulting heat is then trapped by the ice clouds like an insulating mantle, leading to continued atmospheric warming. If, however, they occur in very dense clouds so that the sun is barely visible, the sunlight is reflected by the cloud and hardly reaches the Earth’s surface – the cooling effect predominates.
If most long-lasting contrails occur within natural clouds anyway, it may be more effective to plan climate-friendlier flight routes not only according to cloud-free skies, but also with regard to existing ice-cloud structures. To achieve this, our research findings are 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.