FRANKFURT (dpa-AFX) - Passenger aircraft contribute significantly to global warming. In addition to pure CO2 emissions, aerosols, nitrogen oxides and, to a very large extent, the well-known contrails also pollute the climate. Together, these "non-CO2 effects" are responsible for more than 60 percent of the climate damage caused by aircraft, according to researchers.
For several years now, research has therefore been conducted in Germany into how air masses in which particularly long-lived contrails form can be flown around. In view of the lack of alternatives to combustion engines, this seems a relatively simple way of quickly reducing the considerable environmental impact of air traffic. But now there is controversy.
How do contrails form?
The trails form when water vapor from aircraft engines meets cold air and condenses into ice crystals. These attach themselves to the soot particles also emitted by the turbines. At low temperatures and high humidity, the trails are particularly long-lasting and can develop into artificial cirrus clouds. This occurs mainly at high altitudes of at least ten kilometers, the usual cruising altitude of commercial aircraft.
How harmful are contrails to the environment?
That depends on several factors, including the time of day. Like natural clouds, their artificial counterparts trap warm air masses in lower layers of the atmosphere. This is particularly a problem at night. During the day, however, artificial clouds can actually shield the Earth from the sun's heat rays. The impact on the climate also depends on the season and the geographical position of the aircraft.
What options are there for avoiding this?
In addition to alternative fuels with fewer soot particles, route optimization seems to be the most suitable way to reduce contrails. This would require aircraft to fly detours with additional kerosene consumption in order to avoid humid upper air layers. The respective environmental effects of the routes must be measured and offset against each other. When it comes to fuels, scientists at Future Cleantech Architects (FCA) recommend the targeted use of sustainably produced synthetic fuels (SAF) on flights where contrails are particularly likely to occur.
Are detours with higher kerosene consumption worthwhile?
The European environmental protection organization Transport & Environment estimates that only 3 percent of all flights are responsible for 80 percent of climate-damaging contrails. Their routes, mainly over the North Atlantic, would have to be changed on short sections, with jets consuming 5 percent or less additional fuel. The benefits for the climate would be between 15 and 40 times greater than the disadvantages caused by the additional kerosene burned. In practice, these figures have hardly been substantiated to date.
What are the problems with implementation?
The German Aerospace Center (DLR) is running the "D-Kult" research project, in which more than 100 regular flights by German airlines were diverted in 2024 to avoid contrails. The flights in question had to be selected and planned with considerable personnel effort at Lufthansa, Condor, and DHL, among others. There is a lack of reliable data for automated flight planning, such as weather conditions along the entire route. According to participants, it is not yet possible to make reliable statements about the climate effects that have actually been avoided.
Very long detours were not planned because this would have jeopardized the stability of the entire network. Even minor delays can lead to major problems and additional flights if slots are missed or night flight restrictions are violated, warns Tuifly, which is also involved in the project. The German Air Traffic Control (DFS) also points out the limited airspace in Europe and over the Atlantic: Changes to original flight plans to so-called yo-yo routes (up and down) would inevitably lead to a domino effect and result in increased coordination efforts by air traffic controllers.
What is the European Union planning?
Since the beginning of the year, the EU has required European airlines to determine and report the non-CO2 effects of their European flights. However, according to the German industry association BDL, the necessary reporting system, NEATS (Non-CO2 Aviation Effects Tracking System), will not be fully available until the end of the year. BDL chief executive Joachim Lang criticizes that the content of the tool is still a "black box" for airlines. Due to a lack of research results, the expected NEATS findings on climate impact are unreliable and, because of conservative assumptions, likely to be too high.
Why does the aviation industry want a delay?
The BDL fears further burdens for European airlines in competition with providers from Turkey or the Arab world, for example. The EU wants to integrate non-CO2 effects into the existing emissions trading system for CO2 certificates. This would mean that airlines would have to purchase and pay for "pollution rights" for condensation trails in addition to those for kerosene. It is also unclear who would pay for the additional fuel required for climate-friendly detours. The BDL is calling for a moratorium until more reliable studies are available. These are to come from, among other sources, the already approved follow-up project SD-KULT, which is scheduled to start in 2026./ceb/DP/zb
