How to scale up contrail avoidance in Europe?

Contrails - the white lines in the sky left by aircraft - contribute to global warming in a way that is comparable to aviation’s CO₂ emissions. Most contrails are short-lived and disappear within a few minutes. However, if a plane flies through regions with very cold and humid air, contrails can stay in the atmosphere for hours and form clouds that act like a giant blanket. They trap heat that would normally escape from Earth into space.

But only a fraction of flights (3%) generate 80% of contrail warming. Tweaking the flight paths of just a handful of flights to avoid contrails is a low-cost climate fix that places a negligible burden on both industry and passengers. Simulations and real life tests have proven that this solution can reduce contrail formation and warming, with very limited extra fuel burn.

For a snapshot in time, explore the globe below, depicting regions where cooling (a type of contrail that sometimes forms during early day), warming and very warming contrails would form. To avoid contrails, aircraft need to avoid flying through the orange and pink regions shown.

Contrail management involves both airlines and air traffic management, the latter with a key role to play in enabling airlines to re-route flights to avoid the warming impact of contrails.

Contrail avoidance is one of aviation’s biggest climate opportunities. Contrail management could be the most effective lever to reduce aviation’s climate impact until 2050. This report seeks to understand how contrail avoidance can be performed at scale, with minimum disruption to the air traffic network, by targeting the flights that contribute the most to contrail warming.

Seasonality and time of day are major factors in contrail formation. Flights operating at night and in the winter and autumn produce a disproportionate share of warming contrails and therefore offer opportunities for effective contrail mitigation, as these periods coincide with lower overall traffic.

Flights operating at night account for a disproportionately large share of contrail-induced warming

Explore the dashboard below to study the relationship between flight distance, time of day and contrail formation.

What stands out is that night flights have a disproportionate contrail climate impact in Europe - the number of aircraft in European airspace during the night is low and still, contrail warming is significant. Around 40% of European contrail warming was caused by flights from 8 pm to 4 am (UTC) that accounted for 20% of traffic in 2019.

But why is that? During the daytime, the contrails function like an umbrella, reflecting some of the sun's incoming solar radiation back into space, which provides a cooling effect. Simultaneously, they act like a blanket, trapping heat emitted by the Earth and preventing it from escaping into space, contributing to warming.

At night, however, when there is no incoming solar radiation, the "umbrella" effect becomes irrelevant. The contrails then serve only as a blanket, trapping the Earth's heat and amplifying their warming impact. This dual behaviour explains why contrails have a stronger warming effect at night compared to during the day.

This suggests that targeting late-evening and night flights to do contrail avoidance yields a disproportionately large reduction in warming. Many warming contrails are produced at times when overall traffic is already lower, reducing congestion concerns.

Contrail avoidance at night is a clear opportunity, so what about the season?

The dashboard below shows air traffic and contrail warming by month across different airspaces.

Again, Europe has a clear opportunity: October to March accounted for only 45% of European traffic in 2019, but for 75% of contrail warming. In other words, flights in late autumn and winter show significantly higher warming potential than summer. This means that: avoidance in autumn and winter is higher-impact and operationally easier than in late spring and summer.

Our analysis finds that for a region, avoiding contrails during just a few weeks a year would already bring substantial climate benefits for Europe.

Not all flights cause the same amount of contrail warming. In some regions, flights tend to create much stronger contrail warming than in others. The map below highlights these areas: darker regions show where avoiding a contrail would bring the greatest climate benefit.

The North Atlantic appears as a major contrail hotspot, with lower overall traffic than Central Europe, meaning it is a good region to start performing contrail avoidance manoeuvres. For this reason, contrail warming over the North Atlantic receives special attention from research and is also considered for contrail avoidance trials. Eastern and Northern Europe show similarly elevated values - indicating potential for scaling up contrail avoidance. It is important to note that high traffic density does not preclude contrail avoidance, as trials in some of Europe’s busiest airspace demonstrate.

Finally, it is important to identify which types of flights generate the most warmingThe chart below shows that long-haul flights accounted for less than 10% of departures, yet they were responsible for around 40% of total contrail warming from European departures in 2019. This highlights the highly disproportionate contrail climate impact of long-haul flights.

Contrail-sensitive regions can extend over several hundred kilometres horizontally and span hundreds of metres vertically. These are the layers of cold, moist air in which persistent, warming contrails are most likely to form.

The core objective of contrail avoidance is therefore intuitive: minimise the distance an aircraft spends inside such regions while also keeping the additional fuel burn low.

This is because the warming impact of a flight generally depends on the distance flown through the contrail-sensitive airspace. At the extreme, the most warming flights may form persistent contrails for many thousands of kilometres along their route.

Reducing the distance flown through contrail-sensitive airspace can be achieved in three ways:

Any of these strategies require coordination among air traffic management stakeholders, as they all constitute changes to the planned flight trajectory. However, because contrail-sensitive regions are typically not very deep and because lateral manoeuvres can add significant distance or route complexity, vertical deviations are often operationally the simplest option. Additionally, they can often reduce contrails at minimal additional fuel burn.

The fact that many contrails can be avoided with a vertical movement raises the question of how far that vertical movement needs to go. The above chart shows how likely a vertical manoeuvre is to steer an aircraft clear of a contrail-sensitive region at different flight levels. For each altitude, it displays three options: climbing, descending, or choosing the optimum of the two, taking into account that aircraft cannot always climb or descend. It highlights that persistent contrails can often be avoided with relatively modest vertical deviations.

Critics sometimes suggest that contrail avoidance is difficult to implement effectively due to the significant impact it could have on air traffic control when performed at scale, particularly during busy periods such as peak summer travel days.

We argue, however, that the focus should shift from where contrail avoidance is hardest to where it is arguably easiest and most impactful. Therefore, contrail avoidance should be scaled up gradually, starting from less busy periods and less complex airspace. This allows science, legislation, and operational experience to co-evolve responsibly.

The above chart shows when contrail warming happens compared with how busy air traffic is. The orange line shows air traffic, measured as total flight distance per hour, compared to the busiest hour of the year. The pink line shows how contrail warming adds up over time. Time in the chart is ordered from very busy traffic periods on the left to quieter periods on the right. As you move to the right, air traffic falls, but contrail warming continues to increase.

The key message is that most contrail warming does not happen during the busiest flying hours. Instead, a large share occurs when traffic levels are lower, especially in autumn and winter, and at night.

For example, the chart shows that around 70% of contrail warming occurs when air traffic is below 60% of its annual peak level. This is because the busiest flying hours mostly happen during summer days, while contrail warming is strongest in cooler, darker conditions typical of autumn and winter. Focusing on the quietest periods such as autumn and winter nights where traffic is below 40% of the annual peak would still allow for addressing 15% of total annual contrail warming in European airspace.

While airspaces with high traffic generally cause high contrail warming, some regions witness higher contrail warming at lower traffic levels, providing an opportunity for smart targeted policies to do a lot of heavy lifting.

The above chart highlights airspaces with high contrail warming per flown distance and low traffic density. Flight Information Regions (FIRs) in Northern and Eastern Europe, as well as North Atlantic FIRs such as Shanwick (UK and Ireland), Gander (Canada), New York (USA) and Santa Maria (Portugal) stand out. This reflects the fact that colder or oceanic climates may be more prone to contrail formation, and that planes flying in these airspaces, particularly long-haul aircraft, tend to form more contrail warming per distance flown.

These regions have lower traffic density than FIRs such as Brussels or Langen, making them interesting for contrail avoidance. At the same time, it is important to stress that low traffic density does not necessarily imply low traffic complexity. In oceanic airspaces, for instance, traffic densities are lower but radar coverage is limited, which is why each aircraft requires larger separation and more strategic planning and coordination with fewer available routes.

These findings illustrate that different airspaces have different contrail and traffic profiles, and contrail-avoidance strategies may need to be tailored to local conditions. These strategies will also depend on workload and staffing levels at ANSPs among other things. This is why collaboration between airlines, the Network Manager, ANSPs and other aviation stakeholders is essential to better understand how to best scale up contrail avoidance.

Contrail avoidance is a key opportunity to reduce aviation’s climate impact. Air traffic management has a key role to play in making it a reality. As shown above, contrail avoidance can be efficiently integrated into air traffic management when designed carefully. Still, it is paramount to state that safety always has priority and changes to flight trajectories must not compromise safety. Contrail avoidance could start with night flights in autumn and winter in airspaces with low traffic density such as the North Atlantic and focus on vertical deviations. Wherever possible, decisions should be shifted to the pre-tactical planning phase, reducing controller burden. The climate benefits are already significant with pre-tactical contrail avoidance and as satellites, weather forecasts and the familiarity of air traffic controllers with these types of manoeuvres improve, tactical avoidance can gradually be increased. 

In this regard, T&E recommends the following: