Aviation: International scientists warn of warming impact of contrails and risks of delaying action

Contrails - the white lines in the sky created by planes - have a net warming effect on the planet, estimated to be at least as important as the one caused by aviation’s CO₂ emissions. The climate impact of these effects was first recognised by the IPCC 25 years ago. Nonetheless, little effort has been made in the last decades to reduce them. This is why a coalition of over 50 scientists from across the world are gathering together to call upon global decision makers to implement solutions to tackle contrails - without these coming at the expense of decarbonisation efforts.

If decisive action is taken to mitigate contrails, these effects may be reduced in a faster and more cost-effective way than other climate issues, thanks to slightly rerouting a small percentage of flights. Indeed, only a minority of flights (3%) generate 80% of contrail warming. Tweaking the flight paths of these very few flights can reduce global contrail warming by more than half before 2040, new research by green group Transport & Environment (T&E) finds. Contrail avoidance could cost less than €4 per flight. Additionally, the climate benefit of reducing contrails could be at least 15 times larger than the impact of the CO₂ emissions from the extra fuel burn, caused by deviating the flight.

Moreover, the effects of contrails on the climate are short lived, so addressing them would deliver short term climate benefits, very much needed in the race to meet the goals set out in the Paris agreement, the letter explains.

The letter urges policy makers to take the issue seriously by applying several levers. A first step would be to raise awareness among the general public of the climate impact of contrails. Airline passengers should be informed of the full climate impact of flying when booking a flight and companies performing business flights should include non-CO₂ in their corporate reporting.

The scientists call for a policy framework, underpinned by a robust monitoring system, to reduce warming contrails and other non-CO₂ effects. This should come hand in hand with large scale contrail-avoidance trials, supported by more research. This will ensure that mitigation measures are quickly adopted, as soon as they are ready.

Krisztina Hencz, aviation policy manager at Transport & Environment (T&E), says: “Contrails represent aviation's climate opportunity of the decade. There are very few climate solutions that can be implemented so quickly, at so little cost. The problem was recognised by the UN 25 years ago. But to date, the problem has received very little attention. So it’s time for policymakers to take control of this issue and implement the solutions they have at hand. It's a win win for them and the planet."

Professor Anthony Patt, Professor of Climate Policy, Department of Environmental Systems Science at ETH Zürich, explains: “Preventing contrails has turned out to be among the lowest hanging fruit among our options to stop the rise in global temperatures. However, because safety dictates that air traffic be tightly regulated, the minor changes to flight routes that are needed require coordinated government leadership.”

The scientists note that, while avoiding contrails would lead to significant climate benefits, it does not make it any less urgent to reduce the CO₂ emissions of the aviation sector, whose impact on the climate remains as significant as before.

Signatories of the letter include (in alphabetical order):

Professor Jillian Anable, Institute for Transport Studies, University of Leeds

Professor Alexander Archibald, University of Cambridge

Dr Mona Bachmann, University of Copenhagen

Dr Magnus Bengtsson, Hot or Cool Institute gGmbH

Dr Peter Berrill, Institute of Environmental Sciences, Leiden University

Katharina Bohnenberger, Institute for Socio-Economics, University of Duisburg-Essen

Dr Kobe Boussauw, Associate Professor of Spatial Planning and Mobility, VUB Brussels

Dr Olivier Boucher, IPSL, Sorbonne Université / CNRS

Professor Christian Brand, Emeritus Professor of Transport, Energy and Climate Change , University of Oxford

Dr Cameron Brick, University of Amsterdam

Professor Federico Butera, PhD h.c., Professor Emeritus of Technical Environmental Physics, Politecnico di Milano

Dr Sven Buyle, Principal research fellow, University of Antwerp

Stefano Caserini, University of Parma, Department of Engineering and Architecture

Pierpaolo Cazzola 

Dr Alex Chapman, Senior Economist, New Economics Foundation

Dr Ying Chen, Assistant Professor in Atmospheric Science, University of Birmingham

Dr Nicolas Clerbaux, Royal Meteorological Institute of Belgium, Remote Sensing from Space

Professor An Cliquet, UGent

Professor Wolfgang Cramer, Professor and Director of Research CNRS, Medit. Institute for Biodiversity and Ecology, Aix-en-Provence, France

Professor Edouard Davin, Professor of Climate Scenarios for Sustainable Development, University of Bern

Professor Cesare Hall, Professor of Aerothermal Engineering, University of Cambridge

Karl-Martin Hentschel, Mathematician and author of „Handbuch Klimaschutz“

Dr Charlie Gardner, Associate Senior Lecturer, Durrell Institute of Conservation and Ecology, University of Kent

Professor Dr Xavier Fettweis, Department of Geography, University of Liège

Chris Kesteloot, Emeritus Professor, Division of Geography and Tourism, Department of Earth and Environmental Sciences, KU Leuven

Dr Jarlath Molloy

Patrick Le Clercq, Head of department, multiphase flows and alternatives fuels, German Aerospace Center (DLR)

Dr Kristian Nielsen, Copenhagen Business School

Professor Dr Pao-Yu Oei, Professor for Economics of Sustainable Energy Transition, Head of Research Group FossilExit, Europa-Universität Flensburg (EUF)

Professor George Papadakis, Professor of Aerodynamics, Department of Aeronautics, Faculty of Engineering, Imperial College London

Professor Anthony Patt, Professor of Climate Policy, Department of Environmental Systems Science, ETH Zürich

Pauline Petereit, PhD

Dr Robert Pietzcker, Head of National Energy Transitions Team, Energy Systems Group, Potsdam Institute for Climate Impact Research

Joachim Piret, Department of Water and Climate, VUB

Professor Hans-O Poertner, Alfred-Wegener-Institut

Dr Philippe Quirion, Director of Research, CNRS

Professor Edward Richardson, Professor of Applied Thermodynamics, University of Southampton

Dr María Victoria Román, Basque Centre for Climate Change

Professor Sandra Rousseau, Center of Economics and Corporate Sustainability, KU Leuven

Dr Kevin Sartor, Département d'aérospatiale et mécanique, University of Liège

Gianni Silvestrini, Scientific Director Kyoto Club, QualEnergia, KeyEnergy, Responsible Master Ridef Politecnico Milano, President Exalto

Professor Steven Sleutel, Faculty of Bioscience Engineering, Department Environment, Ghent University

Dr Chris Smith, VUB

Dr Sophie Szopa, Senior scientist in atmospheric chemistry, Laboratoire des Sciences du Climat et de l'Environnement, LSCE-IPSL 

Professor Wim Thiery, Professor of Climate Science, VUB

Professor Jean Pascal van Ypersele, Professor of climate sciences, UCLouvain and Uliège, former IPCC Vice-Chair

Professor Kwinten van Weverberg, Professor in Climate Science, Ghent University, Royal Meteorological Institute of Belgium

Professor Frank Venmans, Assistant Professor, London School of Economics

Professor Sara Vicca, Associate Professor, University of Antwerp

Professor Christiane Voigt, Professor of Atmospheric Physics, University Mainz

Professor Tommy Wiedmann, Professor of Sustainability Research, Sustainability Assessment Program, School of Civil and Environmental Engineering, UNSW Sydney

Luxembourg Climate Policy Observatory (Observatoire de la politique climatique)