Contrails contribute to warming the Earth. Therefore, airlines should in the future be urged by the EU, depending on weather conditions, to select flight routes that produce fewer contrails.
The contrails form depending on the weather — namely when, at low temperatures and high humidity, water condenses on the exhaust particles and ice crystals form. In very humid environments, contrails can persist for hours and grow into ice clouds, called cirrus clouds. “Depending on the location and time of day, long-lasting contrails can have a cooling or warming effect on the atmosphere, with the warming effect probably prevailing,” acknowledges the German air traffic industry as well.
According to findings of the Federal Environment Agency, contrails together contribute to the greenhouse effect roughly as much as the CO₂ emitted by air traffic. In individual cases, under suitable weather conditions, the effect of contrails can even exceed the CO₂ effect by a multiple. Therefore, the EU Commission intends to integrate contrail formation into the emissions trading system in the medium term and has developed the concept NEATS, the Non-CO₂ Aviation Effects Tracking System.
The NEATS process has already begun, with the introduction of reporting obligations. Since January, airlines for intra-European flights must document the extent of contrail formation in the air layers traversed, based on weather conditions. From 2027 this will apply to all departures from the EU. By the end of March 2026, companies must present their first annual report for 2025.
Air Transport Industry Criticizes the Requirements
Based on this collection, airlines should in the future optimize their flight routes according to the current weather conditions to minimize contrail formation. The Federal Association of German Air Transport (BDL) calls the EU’s move “ambitious, but not practically feasible,” especially since “reliable scientific data” are lacking. The currently available systems for modeling non-CO₂ effects have not yet adequately demonstrated “to what extent the results of these models actually reflect the real climate impact of a flight.”
Scientists are precisely working on this now. D-KULT is a research project that is running at the German Aerospace Center (DLR) until the end of the year. The acronym stands for “Demonstrator Climate and Environmentally Friendly Air Transport.” As part of the project, scientists are developing optimization procedures to minimize CO₂-, non-CO₂- emissions and noise, while also keeping costs as low as possible and taking the requirements of air traffic into account.
“This is an optimization problem with conflicting objectives,” says the DLR. In the framework of a Europe-wide, thus far unique field trial, more than 100 regular flights of German airlines were rerouted in 2024 with the aim of avoiding contrails to the best possible extent.
“An important goal of the project was to make the process manageable with all involved actors,” says atmospheric physicist Sigrun Matthes from the DLR. That has been achieved. From a meteorological point of view, however, much still needs to be improved. In particular, humidity in the upper troposphere, i.e., where contrails form, is currently not adequately represented in weather models, because it is not decisive for traditional weather forecasting.
Avoiding Contrails Through Route Changes
The next major task then consists in developing a system that quantifies the climate impact of the contrails produced so that it can be integrated into flight route planning and thereby efficiently avoided. The climate impact could subsequently also be integrated into the emissions trading system.
It is about route changes of at most a few minutes
Sigrun Matthes, atmospheric physicist
To what extent contrail formation was actually reduced on the experimental flights is not yet known. Critics’ concerns that long detours and possibly increased kerosene consumption may occur are countered by the DLR scientist: “It’s about route changes of at most a few minutes.”
The BDL, in the meantime, sees in the data collection, reporting and verification system for non-CO₂ effects in aviation so far a kind of “black box.” The data, models and instruments used have not yet been sufficiently tested for accuracy, the association writes in a statement.
Moreover, data collection is very complex. Airlines would have to collect and store their flight data themselves at great effort in order to feed them into NEATS later. In addition, avoiding certain areas in the densely trafficked European airspace could lead to massive capacity bottlenecks.
While the industry’s arguments that there are scientific uncertainties and that the administrative burden is substantial are not entirely unfounded at present, another critique by the BDL of the system seems somewhat helpless: the “avoidance of non-CO₂ effects” must “precede pricing,” the association states—and thus ignores the fact that pricing is precisely intended to avoid contrails.
