COMMENTARY: The climate impact of contrails – why targeting a small share of flights could make a big difference

The climate impact of persistent contrails from aircraft is receiving growing attention from policymakers, with the EU currently developing policies to integrate them into climate regulations. In a recent study, researchers from Chalmers University of Technology and Göteborg University in Sweden, together with Imperial College, developed methods and tools to study the climate impact of persistent contrails compared to carbon dioxide emissions from aviation. They studied their impact from nearly half a million flights across the North Atlantic and from global aviation as a whole, and analysed the potential climate benefits of rerouting flights as a strategy to avoid contrail formation. In this article, Christian Azar and Daniel Johansson summarise key insights from the study.  

When comparing the climate impact from contrails compared to that of CO₂, one may use the Global Warming Potential (GWP) metric. It measures the integrated warming effect of a kg of, say, methane emissions divided by the integrated warming effect of a kg of CO₂, over a particular time horizon, typically 100 years. This is the method used by countries and international organisations like the UN when comparing greenhouse gases.

For persistent contrails, this is not directly applicable because one does not emit a tonne of persistent contrails. Instead, one may compare warming effect of the contrails associated with a particular flight to the CO₂ emitted during that flight.

Aggregated to the global scale, we then find the warming effect of persistent contrails versus CO₂ emissions from aviation is 33% using the GWP metric with a 100-year time perspective.

This means the total warming effect from global aviation during one year – considering both CO₂ and persistent contrails – is 1.33 times that of the CO₂ emissions alone by using this metric. It should be noted that the uncertainty for the warming effect of persistent contrails is large. We estimate the uncertainty range for the total warming effect from contrails and CO₂ from global aviation over the warming from CO₂ emissions from global aviation is 1.11 to 2.11 (representing the 90% interval, using the GWP 100 metric).

Furthermore, the choice of time horizon in the GWP calculation is critical. If one uses a short time horizon, for example 20 years instead of 100 years, then the importance of persistent contrails appears to be much higher. The reason for this is that one would then only value the warming effect of carbon dioxide over a very short period of time compared to its full life time, which stretches beyond a thousand years. The apparent relatively higher persistent contrail value when using a 20-year time horizon thus depends on an undervaluation of the full warming effect of CO₂.

Now GWP is just one possible approach to compare persistent contrails with CO₂. Another approach is to look at the climate damages caused by the formed contrails and compare that with the climate damages from the associated CO₂ emissions. This is sometimes called a social cost approach. (The climate damages are defined here as the combined economic and non-economic losses that affect societies and ecosystems expressed in monetary terms.)

In our paper, we focused on this latter approach and find that social cost of contrails is 15% of the social cost of carbon associated with global aviation (for the year 2019). Hence the climate impact, when taking into account both CO₂ and contrails, is 1.15 times that of CO₂.

Once again, the uncertainty range here is large and the social cost of contrails to carbon dioxide could lie somewhere in the range 1.02-2.92. This uncertainty depends primarily on the discount rate and the uncertainty in the warming effect of persistent contrails. The discount rate describes how future costs or damages are valued compared with costs and damages today. A higher discount rate means future climate damages are given less weight, while a lower rate places more importance on long-term impacts.

The heterogenous nature of contrails

It should be noted that the contrail contribution from individual flights varies even more. The impact depends on meteorological conditions, and when and where they are formed. For instance, while some flights do not cause any persistent contrails at all, others may cause warming that is an order of magnitude as large as the CO₂ emissions from that flight.

We illustrate in Figure 1 below the climate damage from contrails (expressed in economic terms) compared to the climate damage from CO₂ emissions from nearly half a million flights across the North Atlantic in 2019. We divided all flights into 50 groups (bins) in order of increasing contrail impact.

In our assessment, we find that about 14% of the flights caused persistent contrails with negative energy forcing, so they are cooling the planet (see the left-hand panel in the figure), and that 38% of the flights cause persistent contrails that warm the planet (see the right-hand panel).

Cooling may only occur if contrails are present during daytime, so that they reflect incoming radiation, but it is also required that the contrail is formed above darker areas.

Hence, from a policy perspective, it would be very inefficient to apply a simple adjustment factor – constant across all flights – to CO₂ emissions to account for non-CO₂ effects from aviation. Rather, policies should be directed at those flights that are expected to cause significant amounts of warming contrails.

Figure 1: Cooling and warming persistent contrails across half a million flights over the North Atlantic in the year 2019.
Each bar shows how large the climate damage of persistent contrails is compared with carbon dioxide emissions for different groups of flights. Flights are ordered by their position in the distribution, from those that tend to produce the strongest cooling contrails (left) to those that produce the strongest warming  contrails (right). The whiskers indicate the range of uncertainty across simulations. The left panel shows flights that form cooling contrails, while the right panel shows flights that form warming contrails. Roughly half of the flights fall between either category and produce little or no contrails, so their contribution is close to zero and they are not shown. (Note the change in scales between right-hand and left-hand panels.)

 It can be seen for the top 2% of the flights that generate the strongest persistent contrail warming, the contrail climate impact in terms of damage is more than six times stronger than the climate damages by the CO₂ emissions caused by these flights. At the other end, for the bottom 2% of the flights, where the figure shows a value below -1, the climate benefits of the cooling caused by these persistent contrails are larger than the climate damages from the CO₂ emissions caused by these same flights.

Flight rerouting as a potential mitigation strategy

Persistent contrails are only formed in regions of the atmosphere which are supersaturated with respect to ice. This means that persistent contrails may be avoided if flights are routed so that they fly, typically, below or above those zones.

In our study, we analysed the climate benefits of a flight route strategy, where we compared the benefits of reduced warming from contrails to the climate cost of increased fuel combustion and associated CO₂ emissions. We found that rerouting is climatically beneficial in 34% of all flights, or in other words, in 90% of the flights that cause warming contrails if the changed flight route causes at most an extra 1% in fuel consumption – and associated CO₂ emissions – while avoiding formation of warming persistent contrails.

However, there are several difficulties associated with implementing such a strategy. It would require aviation control managers to collaborate with airlines and meteorological forecasters. Further, forecasts are seldom perfect, so there are also risks that flights are rerouted because air control managers believe in advance that those flights would cause contrails, while in reality they would not.

For that reason, we also carried out a sensitivity analysis, where we analysed what would happen if policymakers would only want to reroute flights in cases where they are 95% certain that it would lead to a climate benefit. The uncertainty analysis is carried out by estimating the climate impact from a particular flight under multiple meteorological conditions, and under various assumptions about the warming effect of the potential contrail that was formed.

Under these conditions, the share of flights suggested for rerouting drops to 22%. This corresponds to around 60% all flights with a warming contrail effect. It may be noted though that these flights still cause as much as approximately 95% of the total climate impact in terms of damages of warming persistent contrails.

The reason for this large fraction is that we would still reroute most of the flights that cause the majority of the persistent contrail climate impact, since relatively few flights, as can be seen in Figure 1 above, contribute to most of the climate impact.

Hence, even under these more restrictive assumptions, a significant share of the warming, and consequently the damages, from persistent contrails may be avoided.

For these reasons, climate and aviation policy needs to be flight specific, and efforts should be directed at flights with strong warming impact. However, more research is needed to analyse the radiative forcing of contrails, their lifetime and interactions with natural clouds, as well as the practical possibilities to implement such a strategy.

Reference:
‘The social costs of aviation CO₂ and contrail cirrus’ published in Nature Communications,16(1), 8558.
Johansson, D. J., Azar, C., Pettersson, S., Sterner, T., Stettler, M. E., & Teoh, R. (2025).

Top photo courtesy of Imperial College London

About the authors

Christian’s background is in physics and he is currently a professor of energy and environment at Chalmers University of Technology, Sweden. His research focuses on climate change mitigation strategies (including energy systems modeling, technology assessment and policy analysis). He has been a policy advisor to two Swedish Prime Ministers, to all Swedish environment ministers between 2002 and 2014, and to the EU commissioner for the Environment (Margot Wallström, 2001-2004). He has published more than one hundred scientific papers and reports on energy, climate and the environment, around a hundred articles for the popular press and has been a lead author of the Intergovernmental Panel on Climate Change (IPCC). In 2009 he was selected the most influential person in Sweden when it comes to environmental issues.

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Daniel is Professor, Physical Resource Theory, Space, Earth and Environment at Chalmers University of Technology, Sweden, and has worked in the field of climate-energy-economic analysis and integrated assessment modelling for around 20 years and has written about 50 peer-reviewed articles and other research papers. His research has covered many different topics such as climate stabilisation scenarios, emission metrics, statistical estimation of climate sensitivity, fuel markets, negative CO2 emissions, food vs fuel, climate impacts of aviation, and the role of autonomous vehicle in future transport systems. In his research he mixes theories and methods from climate science, engineering and economics. He has been active in interacting with industry, policymakers and laymen through research and consultancy projects, workshops, presentations, policy reports and op-eds.

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