Contrails: Why a few night flights in winter ruin the climate balance

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The white lines in the sky are seen by many as a symbol of wanderlust – or as a basis for conspiracy theories. What is certain is that they are a significant burden on the Earth's atmosphere. An analysis by the environmental organization Transport & Environment (T&E) shows how concentrated the problem of climate-damaging contrails actually is. While the aviation industry mostly focuses on reducing CO2 emissions, the study highlights an often underestimated lever: the avoidance of non-CO2 effects through minimal adjustments to flight routes.

At the same time, the researchers found that a tiny fraction of air traffic is responsible for a large part of warming.

Contrails form when aircraft fly in cold and humid air layers. Under these conditions, water vapor from the engines freezes into ice crystals on soot particles. While most of these trails disappear after minutes, some spread under special atmospheric conditions and remain in the sky for hours as artificial veil clouds. These act like a gigantic heating blanket, retaining heat emitted by the Earth.

The effect is massive: According to the T&E study, contrails contribute between one and two percent to global warming. This influence is roughly as large as that of all CO2 emissions from air traffic.

Seasonal Hotspots and Nocturnal Heat Traps

The data for 2019, reviewed by experts, reveals an extreme imbalance. Only three percent of all flights, according to the analysis, caused 80 percent of the warming due to contrails. These particular "climate offenders" among flights can be precisely localized in time and geography.

Night flights in the autumn and winter months are considered particularly critical. Although these flights accounted for only ten percent of European air traffic between October and March, they caused a quarter of the total warming from contrails in Europe during the analysis period. The reason: In the cold season, atmospheric conditions are particularly favorable for long-lasting contrails. At night, moreover, the cooling effect of solar reflection, which the emerging clouds offer during the day, is absent.

According to T&E, the solution to this problem sounds simple and could be implemented without major technical revolutions. Since the critical air layers usually have only a small vertical extent, small changes in altitude of about 600 to 1200 meters are often sufficient to prevent the formation of warming clouds. Such adjustments could already be considered in flight planning based on weather forecasts or tactically instructed by air traffic control during the flight. Since a large part of the harmful contrails form during times of low traffic volume, the additional burden on air traffic controllers would be manageable.

Successful Practical Tests Have Already Been Conducted

The German Aerospace Center (DLR) and Eurocontrol's Maastricht Upper Area Control Centre (MUAC) have already demonstrated that this theoretical approach works in practice. In field trials in 2023, they were able to prove that long-lasting contrails can indeed be effectively avoided through targeted, minor changes in altitude. This already tested procedure marks a crucial milestone for the industry and proves that a significant reduction in the climate impact of air traffic is achievable.

The study makers paid particular attention to the North Atlantic. This region is considered a hotspot because many long-haul flights take place at high altitudes where the air is extremely cold. Although significantly fewer aircraft operate there than over mainland Europe, the climate effect per kilometer flown is particularly high. Long-haul flights lasting over five hours accounted for only ten percent of European departures in 2019 but were responsible for 40 percent of contrail warming. This offers an ideal opportunity for large-scale test runs, as the lower traffic density allows more room for route changes.

Political Course Set for the Future

To leverage the climate potential, T&E is calling on the EU to quickly integrate non-CO2 effects into air traffic legislation. Currently, the system is primarily designed to minimize direct greenhouse gas emissions through the shortest possible routes. However, a slight diversion that consumes minimally more fuel is significantly more climate-friendly overall. The prerequisite is that it prevents the formation of a long-lasting heat cloud. The organization therefore recommends creating incentives for airlines and air traffic control that actively contribute to the avoidance of contrails. These impacts would need to be firmly integrated into the climate metrics of European aviation.

Given that solutions are already available, T&E researcher Alexander Kunkel urges for haste: With the right political framework, Europe could pave the way to realize a comprehensive avoidance of this unnecessary climate burden in the next five to ten years. Waiting for future technologies is not necessary for this.

(nie)