In a new study, researchers from the University of Cologne, Forschungszentrum Jülich and the Universities of Mainz and Wuppertal show that more than 80 per cent of all long-lived contrails do not form in cloudless skies, but within existing natural ice clouds, known as cirrus clouds. To date, the impact of these embedded contrails on the climate has hardly been researched. A new study published in the scientific journal Nature Communications, titled 'Contrails inside cirrus clouds predominate with uncertain climate impact', provides new evidence that could influence the future planning of climate-adapted flight routes.
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 not yet received much research attention.
So far, research has shown that 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. On the other hand, if they occur in very dense clouds where the sun is barely visible, sunlight is reflected by the clouds and hardly reaches the Earth’s surface, resulting in a cooling effect.
The processes that occur when contrails overlap with natural cirrus clouds and the resulting 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 Professor Andreas Petzold from the Institute of Climate and Energy Systems – Troposphere (ICE-3) at Forschungszentrum Jülich. Professor 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.”
“The idea for this publication originated from the Master’s thesis of Neelam Khan, a MA student in Physics of the Earth and Environment,” says Professor Dr Susanne Crewell from the Institute of Geophysics and Meteorology at the University of Cologne. “Together with our colleagues from Andreas Petzold’s group at Jülich, we developed the study, with special involvement from the students. Ms Khan is continuing the work as part of her doctoral thesis.” For the study, the researchers 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.
The results of the study are being incorporated into the 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 Professor Andreas Petzold at Forschungszentrum Jülich. Other German partners include the Karlsruhe Institute of Technology (KIT), Max Planck Society, the German Aerospace Centre (DLR), and the Leibniz Institute for Tropospheric Research (TROPOS). The German Lufthansa Group has also supported IAGOS since its inception.
Media Contact:
Professor Dr Susanne Crewell
Institute of Geophysics and Meteorology
+49 221 470 5286
susanne.crewell(at)uni-koeln(dot)de
Press and Communications Team:
Jan Voelkel
+49 221 470 2356
j.voelkel(at)verw.uni-koeln(dot)de
For publication:
https://www.researchsquare.com/article/rs-6837438/v1