Probabilities of success for contrail avoidance analysed

Like road and sea transport, air traffic also produces carbon dioxide emissions that contribute to global warming. In addition, aviation also contributes to warming through more short-lived effects, mainly through the formation of contrails and the emission of nitrogen oxide compounds (NOx). This warming is considerable - current estimates indicate that the energy input from non-CO2 effects in a few years will be of a similar order of magnitude to that from CO2 effects since the beginning of commercial aviation in the 1940s.

Important research results for climate-friendly aviation consider the reduction of CO2 emissions from aircraft. However, less attention has so far been paid to reducing non-CO2 effects. There are several reasons for the slower progress in this area, such as the complexity of the processes that lead to the formation of contrails. Another problem is the different time scales of the individual effects: CO2 circulates between the atmosphere, the oceans and the continents and is gradually removed from the atmosphere over centuries, - while nitrogen oxide-induced increases in ozone and reductions in methane (both of which affect the climate) are expressed over months to decades. Contrails, on the other hand, last only a few hours at cruising altitudes. This enormous range of time scales has so far hampered efforts to compare the effects of CO2 emissions from aircraft with non-CO2 effects and to evaluate mitigation strategies.

A key question here is the extent to which specific mitigation measures, which have positive and negative effects on the climate, reduce global warming overall. For example, whether the reduction of contrails or NOx emissions benefits the climate as a whole if the measure simultaneously increases CO2 emissions from aircraft. A scientific article by a US research group (Prather et al., 2025) on this question has been published in the current issue of Nature.

Prof. Christiane Voigt from the Institute of Atmospheric Physics at the German Aerospace Centre (DLR) comments on the results in the same issue of Nature. She says: "The new findings take the field a step forward. The authors of the Prater et al. study introduce a risk-based trade-off assessment - a type of decision-making process widely used in other sectors - which here explicitly incorporates uncertainty. More specifically, their method determines the probability that a mitigation measure that has both a positive and a negative impact on the climate will have a net positive outcome overall. This makes it possible, for example, to estimate more precisely the extent to which flight routes with slightly higher fuel consumption at lower altitudes are worthwhile in order to avoid areas with climate-warming contrails. This risk assessment is a useful tool for further developing mitigation strategies for non-CO2 effects."

Optimisation of flight routes for climate protection

Aircraft generally fly cost- or fuel-efficient routes. Therefore, changing such routes to avoid contrails, for example by adjusting altitude, can lead to a slight deviation from optimal flight performance and thus to an increase in CO2 emissions. A statistical analysis has analysed the effects of optimising 85,000 flight routes to avoid contrails. It shows that the climate impact of contrails can be reduced by 73 per cent, with an increase in CO2 emissions of less than one per cent. The method used by the Prather et al. research group calculates a very high probability of success for this reduction strategy: more than 99 per cent probability of a positive net climate effect. However, the operational feasibility of contrail avoidance still needs to be proven in real-life trials. Areas with an increased occurrence of contrails, for example in Europe, North America and on the transatlantic routes between the two continents, are suitable for the practical testing of this new method. Further research is required to further limit the uncertainties for contrails and nitrogen oxide effects, for example by improving weather and contrail forecasting models.

References

Prather et al. (2025): Trade-offs in aviation impacts on climate favour non-CO₂ mitigation. Nature 643, 988–993

Voigt (2025): Solving aviation’s climate-action conundrum. Nature 643, 921-923