Global Impact of Aviation Contrails

Avoiding contrails is one of the recent trends in ATM. Aviation contrails are considered a significant non-CO2 environmental factor worth avoiding even with a CO2 increase (lower-level cruise or horizontal avoidance, both burning more fuel). This paper is a study of the global impact of aviation on global warming considering contrails and CO2 trade-offs. In the literature, there are two concepts on why contrails are detrimental to the environment: (i) Daytime persistent contrails have a positive effect by reflecting the Sun’s rays back, whereas the contrails persisting into nighttime need to be avoided because they block the cooling of the planet by radiation—the overall impact is negative; (ii) too much humidity is injected into the tropopause by aircraft regardless of the type of contrails, persistent or not, and even by the flights without contrails. In hypothesis (ii), contrail avoidance is not the issue, since humidity is generated by the turbine engines regardless of the visibility of the water molecules (ice crystals or water droplets). Regarding hypothesis (i), the study analysed the Earth’s reflections contributing to albedo and the Earth’s emissions at the top of the atmosphere in infrared (day and night) over 25 years (2000–2025) from CERES data and found correlations with the two pandemic years, when the number of flights was significantly reduced, to understand the real environmental impact of aviation. The conclusion is that most of the time, contrails increase the Earth’s albedo, having a positive environmental impact. The damage to the environment comes mostly from 2% of flights, mainly over Europe, and the paper puts forward some practical proposals to regulate these flights, instead of complex contrail avoidance applied at the ATM level for all flights.