Towards the implementation of climate-optimized air traffic in Northern Europe: Feasibility and airspace constraints of pre-tactical contrail mitigation

Abstract. Aviation contributes to anthropogenic climate change inter alia through carbon dioxide (CO₂) and nitrogen oxides (NO_x) emissions, as well as contrails. Only a small fraction of flights accounts for a disproportionately large share of the contrail-related CO₂-equivalent (CO₂eq), which might be mitigated through minor, spatially limited flight level adjustments. This study investigates such pre-tactical operational contrail mitigation measures for the full air traffic through Northern Europe for 16 days in 2023, aiming to reduce the sum of CO₂ and non-CO₂ effects. The climate impact assessment follows the European Union’s planned monitoring, reporting and verification (MRV) framework for non-CO₂ effects, here employing the 100-year global warming potential (GWP100) as usually applied by the Intergovernmental Panel on Climate Change (IPCC). On average, CO₂ and non-CO₂ effects contribute equally to the total CO₂eq, with contrails and NO_x having similar magnitudes. Weather-related uncertainties from the ECMWF ERA5 reanalysis ensemble are found to be comparably small. Only 6 % of the flights caused 80 % of the cumulative contrail-induced CO₂eq. Trajectory adaptations could reduce about 48 % of the contrail effect in this study, at the cost of a fleet-wide fuel increase of circa 0.4 %. 40 % of the contrail effect could be avoided when limiting to fuel burn changes per optimized flight to < 5 % or meteorological uncertainties < 20 %. Approximately 28 % of the contrail effect could be mitigated without changes to the fleet-wide fuel burn due to the presence of apparently non-fuel-optimal flight plans in the original air traffic dataset. On specific days, particularly during nighttime, fractions of rerouted flights reached up to 17 %. However, only in the domains of 3 out of 11 air navigation service providers were high total contrail CO₂eq produced simultaneously, showing the spatial limitation of contrail formation areas. While flight altitude changes slightly increase the occurrence of significant air traffic concentrations (+3 % for 4–9 aircraft per 2° × 2° × 1000 ft × 10 min grid cell) and of small horizontal separations (+10 % for minimal horizontal separations of 5 NM) in this study, the frequencies of these conditions are still comparable to those observed for the original filed flight plans; thus, airspace capacity on average barely limits climate optimization of flights. NO_x-related climate effects were not substantially reduced, reflecting their weaker spatial variability. Overall, the results demonstrate the feasibility of pre-tactical air traffic management that avoids airspace regions identified as climate-sensitive by the applied climate assessment framework. While the study is purely model-based and, thus, does not provide an assessment of real-world climate effects, it establishes the concepts and procedures required for the future integration of contrail-avoidance strategies and climate-optimized trajectory planning into operational decision-making and regulatory air traffic management frameworks.