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

<jats:p>Abstract. Aviation contributes to anthropogenic climate change inter alia through carbon dioxide (CO2) and nitrogen oxides (NOx) 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 NOx 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 &lt; 5 % or meteorological uncertainties &lt; 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. NOx-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.</jats:p>