Jülich researchers reveal: Long-lived contrails usually form in natural ice clouds

Contrails form when hot exhaust gas from an aircraft’s engine mixes with the cold air at an altitude of about 10 kilometres. In dry air, most contrails dissipate quickly. In cold, humid air, however, they can persist for several hours and develop into extensive cirrus clouds. Cirrus clouds are high, thin ice clouds at an altitude of about 5 to 12 kilometres, which often appear as delicate, wispy veils in the sky. Until now, researchers had assumed that long-lived contrails form mainly in clear skies, where they exert their warming effect. However, the new study shows that they mostly form within existing natural ice clouds. The climatic implications of this have so far received little research attention.

Impact of contrail cirrus on the climate

What research has shown so far: The cirrus clouds formed from contrails – known as contrail cirrus – 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, thus contributing to global warming.

Whether the effect is indeed warming or, in some cases, slightly cooling depends on the surrounding conditions. If contrail cirrus clouds form in clear skies or in thin ice clouds, they typically enhance the warming greenhouse effect: sunlight passes through the relatively thin ice clouds, is absorbed by the Earth, and the resulting heat is then trapped by the ice cloud like a blanket – further warming the atmosphere. If, on the other hand, they occur in very dense clouds, where the sun is barely visible, the sunlight is reflected by the cloud and hardly reaches the Earth’s surface – the cooling effect prevails.

A more differentiated view of contrails

The processes that occur when contrails overlap with natural cirrus clouds, and their effects on the climate, are still poorly understood.

“Our results show that we need to take a more differentiated view of the climatic impact of contrails in future,” says Prof. Andreas Petzold from the Institute of Climate and Energy Systems – Troposphere (ICE-3) at Forschungszentrum Jülich. Prof. Martina Krämer from the Stratosphere institute division (ICE-4) adds: “If most long-lived contrails occur within natural clouds, it may be more effective to plan climate-friendly flight routes not only according to clear skies but also with existing ice cloud structures in mind”

Commercial flight data as a basis for research

For the study, the Jülich researchers and their university partners used measurement data for temperature and water vapour collected by commercial aircraft 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), which is partly coordinated by Forschungszentrum Jülich. IAGOS aircraft are equipped with instruments that continuously record atmospheric data during regular flight operations — a globally unique capability.

International cooperation to reduce climate impact

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 Union Aviation Safety Agency (EASA), and the aviation industry. The aim is to develop a sustainable flight planning strategy to reduce the climatic impact of contrails in future by planning flight routes in a more climate-friendly way. IAGOS aircraft will continue to play a key role in evaluating such strategies in the future.

The German contribution to IAGOS has been supported for many years by the Federal Ministry of Research, Technology and Space (BMFTR, formerly BMBF) and coordinated by Prof. Andreas Petzold at Forschungszentrum Jülich. Other German partners include the Karlsruhe Institute of Technology (KIT), Max Planck Society, the German Aerospace Center (DLR), and the Leibniz Institute for Tropospheric Research (TROPOS). The Lufthansa Group has also supported IAGOS since its inception.