| 14 Aug 2026
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 (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 < 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. 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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Journal of Environmentally Compatible Air Transport System.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
The manuscript addresses an important and timely topic: the potential mitigation of aviation's climate impact through pre-tactical flight trajectory optimization and the associated implications for air traffic management.
The combination of climate-impact modelling, trajectory optimization, and an assessment of airspace-related constraints represents a valuable contribution to the development of climate-oriented ATM concepts.
The study has several notable strengths. It investigates pre-tactical contrail mitigation across Northern European airspace and integrates climate-impact modelling with flight trajectory optimization. Furthermore, it provides a quantitative evaluation of the potential climate mitigation benefits based on 145,092 flights across 16 selected days in 2023. The analysis also considers changes in aircraft concentrations and minimum horizontal separation indicators, thereby providing an initial assessment of the effects of optimized trajectories on traffic distribution. Importantly, the authors acknowledge several limitations concerning the operational implementation of the proposed concept, demonstrating an awareness of the challenges associated with translating theoretical optimization results into operational ATM procedures.
However, several aspects require further clarification before conclusions regarding operational feasibility can be sufficiently substantiated. The main concerns relate to the distinction between geometric traffic compatibility and actual operational feasibility, the proposed allocation of responsibilities between the Network Manager and local Flow Management Positions, and the representativeness of the investigated airspace and traffic sample. In particular, the proposed Concept of Operations assigns the selection of climate-optimized flight plan modifications to FMPs within individual ANSPs. From an operational ATM as well as from the Single European Sky (SES2+) perspective, this allocation deserves reconsideration.
The identification, prioritization, and network-wide coordination of climate-optimized flight plan alternatives would more appropriately be considered a central Network Manager function, while local FMPs should retain their primary responsibilities concerning safe and efficient traffic management, capacity constraints, and the operational requirements of airspace users.
This distinction is important because climate optimization should be integrated into the existing ATM framework without creating potentially conflicting responsibilities or additional local decision-making obligations.
The manuscript would benefit from a clearer separation between the theoretical climate mitigation potential, the geometric compatibility of optimized trajectories, and their actual implementation within existing ATM responsibilities and procedures.
The study addresses a highly relevant challenge for future aviation: reducing non-CO₂ climate effects while maintaining safe and efficient air traffic operations.
The investigation of pre-tactical trajectory adaptations is particularly relevant because it offers a potentially less elaborate alternative to tactical interventions during flight. The integration of meteorological uncertainty, climate-impact modelling, and traffic-related indicators provides useful insights into the potential benefits of contrail mitigation.
However, the scientific contribution should be positioned more clearly as an assessment of climate mitigation potential and selected airspace constraints rather than a comprehensive demonstration of operational feasibility. A further relevant aspect concerns the institutional and procedural integration of climate optimization into the existing European ATM framework. The proposed allocation of responsibilities between Network Manager, FMPs, and airspace users is fundamental to the practical implementation of the concept and should therefore receive greater attention.
In particular, the authors should explain why the selection of climate-optimized flight plan modifications is assigned to individual FMPs rather than being coordinated primarily at network level.
The climate-impact modelling is generally well described and provides a reasonable basis for investigating potential mitigation benefits.
Still, the operational assessment is subject to important methodological limitations. The study primarily uses aircraft concentrations within predefined four-dimensional grid cells and minimum aircraft separations to characterize changes in traffic conditions. While these indicators are useful for identifying changes in spatial traffic distribution, they do not directly represent operational ATC complexity, sector capacity, or procedural feasibility. Actual ATC operations are influenced by additional factors, including sector configurations, traffic-flow interactions, vertical transitions, aircraft performance, coordination requirements, applicable separation standards, and controller workload. This distinction is particularly relevant because investigated mitigation measures involve vertical trajectory adaptations, potentially introducing additional level changes and associated coordination requirements.
Furthermore, the proposed pre-tactical process, approximately 3–12 hours before departure, does not by itself guarantee that optimized trajectories can be accommodated under the operational conditions prevailing at the time of execution. The manuscript explicitly states that operational and capacity constraints from FMPs are not considered during the selection of flights for optimization. Consequently, the resulting trajectories represent potential optimization solutions rather than operationally validated flight plan modifications.
An additional methodological concern relates to the proposed distribution of responsibilities. Assigning the selction of climate-optimized flight plan modifications to local FMPs may introduce additional decision-making tasks that are not necessarily aligned with their primary operational responsibilities and furthermore adds more coordination effort between FMPs and ATC units. A network-level approach coordinated by the Network Manager could provide a more appropriate framework for identifying and prioritizing climate-related trajectory adaptations, while allowing local FMPs to focus on safety, capacity management, and the accommodation of airspace user requirements. This would also help to address cross-border dependencies and the potential displacement of traffic complexity between adjacent ANSPs.
These issues do not necessarily require additional modelling within the present study, but the interpretation of results shall be put into context with regard to the the proposed operational concept.
The manuscript provides a description of the modelling framework, meteorological datasets, and selection of representative study days. The methods used for trajectory optimization seem too simple. The operational representativeness of the underlying traffic dataset requires further consideration.
The study evaluates 145,092 flights across 16 selected days in 2023. Although this represents a substantial number of flights, the temporal sample remains limited. The selection of study days is primarily based on seasonal and climate-related characteristics. These criteria are appropriate for investigating contrail mitigation potential but do not necessarily ensure that the sample captures operationally critical traffic conditions.
The authors should therefore clarify the extent to which the selected traffic sample can support generalized conclusions regarding operational capacity constraints. The availability of the underlying traffic data and the implementation details of the experimental trajectory optimizer should also be documented as far as possible to support independent reproducibility.
The manuscript is well structured, and the scientific approach and results are presented clearly. The figures and quantitative analyses provide useful insights into the distribution of climate effects and the consequences of trajectory adaptations.
The terminology relating to traffic concentration, congestion, airspace capacity, and operational feasibility should be used more consistently. In particular, geometric traffic concentration should not be directly related with available ATC capacity since operational capacity is no nominal value but related to complexity metrics to be handled by controllers. Better related to traffic concentration are traffic counts or demand.
The manuscript would also benefit from a clearer distinction between the proposed concept of operations and the operational elements actually represented in the simulation.
The conclusions should be aligned more closely with the limitations acknowledged in the methodological discussion.
Major Comment 1 — Allocation of Responsibilities within the Proposed Concept of Operations, Section 2.1, Concept of Operations
The proposed Concept of Operations assigns the selection of climate-optimized flight plan modifications to FMPs within individual ANSPs according to CONCERTO. Even if DSNA, ENAV, NATS and others support flight plan modifications by FMPs, generally ANSPs shall accommodate those flights plans being initiated or adapted by AUs. If these are only routing or profile adaptation proposals to mitigate climate sensitive areas without any flight performance computations, this task should be done by NM. From a network-oriented perspective, the identification, prioritization, and coordination of climate-optimized flight plan alternatives could more appropriately be organized as a central Network Manager function. Such an approach would support network-wide consistency, account for cross-border dependencies (e.g. related to the trajectory shown in Fig. 3), and reduce the risk of transferring traffic complexity or capacity constraints between individual ANSPs. Local FMPs could then focus on assessing the operational acceptability of proposed modifications within their respective areas of responsibility, while airspace users would retain their role in evaluating and accepting flight plan alternatives.
The authors clarify the rationale for assigning the selection of climate-optimized trajectories to local FMPs rather than considering a network-level coordination approach? Is this being done related to other capacity restrictive events, like e.g. convection?
Major Comment 2 — No sufficient description of trajectory optimization approach, Section 2.3.3, Flight trajectory optimizer
The authors refer to “two different approaches” to trajectory optimization, although these appear to represent two distinct mitigation concepts (CO₂ and non-CO₂ mitigation versus airspace avoidance). Furthermore, they introduce an “intermediate approach using a very simple experimental trajectory optimizer”. Later, they explain that “up to 100 alternative trajectories are created”, with a manually adjusted tuning factor k to balance the avoidance of climate-sensitive areas against fuel consumption. This trade-off essentially represents the optimization objective, which appears to be defined manually in the proposed approach.
In this context, it would be helpful if the authors could clarify to what extent the proposed method constitutes trajectory optimization in the conventional sense, particularly considering the relatively large number of alternative trajectories generated. Moreover, the computational effort associated with generating and evaluating these trajectories may pose challenges for operational implementation, both in terms of computational efficiency (especially related to vertical adaptations regarding flight performance) and the coordination required among the relevant stakeholders (AUs, NM, and ANSPs).
These aspects raise questions regarding the operational feasibility of the proposed approach and how the feasibility claims made in the study are supported by the methodology. A more detailed discussion and clarification of these points would therefore strengthen the manuscript.
Major Comment 3 — Geometric Traffic Indicators versus Operational ATC Feasibility, Section 3.2.2, Airspace congestion and aircraft separations
The assessment of operational impacts relies primarily on aircraft concentrations within predefined four-dimensional grid cells and minimum horizontal aircraft separations. These indicators provide useful information about changes in spatial traffic distribution. However, they do not directly quantify operational ATC complexity or sector capacity. In particular, the approach does not explicitly account for sector configurations, controller workload, traffic-flow interactions, coordination requirements, or the procedural implications of additional vertical trajectory changes.
The authors shall clarify to what extent the selected indicators can support conclusions regarding operational feasibility? It would be sufficient to distinguish between geometric traffic compatibility and the ability of the ATM system to accommodate the proposed flight plan trajectory modifications under actual operational conditions.
This clarification would not necessarily require additional simulations but should be reflected in the interpretation of the results and the conclusions. The last bullet in 3.3 Limitations is taking this up, but only very superficial.
Major Comment 4 — Representativeness of the Investigated Airspace, Sections 2.1 and 3.1.1 & 3.2.2; Figures 1, 2, and 5
The Borealis alliance airspace represents a quite heterogeneous operational environment, including continental free-route airspace, relatively low-density regions, and a structured oceanic airspace system. While this provides a useful environment for investigating climate-optimized trajectories, its suitability for drawing broader conclusions regarding ATM capacity constraints requires further discussion. “Based on the presence of sunlight, flights during night are more often optimized than during day,…” (p. 21, line 417). This indicates, that capacity bottlenecks are generally not to be expected during pre-tactical planning, and that there may even be capacity reserves available to accommodate climate-efficient trajectories.
Moreover, traffic demand and operational complexity differ substantially between the investigated airspaces. Oceanic traffic flows may involve significant traffic volumes but are generally subject to different traffic organization, separation procedures, and coordination requirements than high-density continental airspace. Similarly, several of the investigated continental regions experience lower traffic demand than central European airspaces. The aggregate changes in traffic concentration may therefore partly reflect the specific characteristics of the selected airspace rather than demonstrating generally limited operational constraints. The authors shall discuss whether the selected airspace provides a sufficiently representative basis for assessing the operational feasibility of climate-optimized trajectories.
In particular, how transferable are the findings to more congested European airspaces with higher traffic complexity and more restrictive sector-capacity conditions? A differentiated interpretation of the results according to traffic density and operational environment would strengthen the conclusions.
Major Comment 5 — Representativeness of Traffic Dataset, Sections 2.2.2 and 2.4; Table 1
The analysis is based on 16 selected days in 2023, representing different seasons, weekdays, weekends, and climate-impact conditions. This selection is reasonable for investigating variability in climate mitigation potential. However, the criteria used to select the study days do not necessarily ensure representativeness regarding operational traffic demand and ATC complexity. In particular, the selection may not adequately capture peak traffic periods, high sector loading, or situations in which additional vertical trajectory changes could create significant operational constraints.
The authors shall clarify the extent to which the selected days represent typical and operationally critical traffic conditions? It would also be useful to discuss whether the reported aggregate results are sufficiently robust to support generalized statements regarding airspace capacity and operational feasibility.
Major Comment 6 — Interpretation of Operational Feasibility and Conclusions, Sections 3.2.2 and 4
The manuscript acknowledges that the operational impact of the investigated trajectory adaptations remains inconclusive. Nevertheless, the conclusions suggest that operational constraints may generally not constitute a major obstacle to implementing contrail avoidance strategies. This is desirable when it comes to climate impact mitigation, but this interpretation appears stronger than what can be directly supported by the selected traffic indicators and the operational assumptions of the study. However, they do not establish that the proposed adaptations can be implemented without significant additional coordination, capacity restrictions, or controller workload. The conclusions should therefore distinguish more explicitly between model-based mitigation potential, geometric traffic compatibility, and operational implementation feasibility. A more cautious formulation would strengthen the scientific consistency of the manuscript without diminishing the relevance of the findings.
Minor Comment 1 - Eco-areas
The term “eco-areas” was selected for climate sensitive areas in CONCERTO. Echoes are motivated by radar-echoes, e.g. by convection. Humidity (not precipitation) and temperature do not produce such echoes. The authors shall re-think this term in the context of climate sensitive airspaces, since it is misleading. Those areas cannot be detected by radar, but have to be predicted with (un)certainty, especially in the pre-tactical time horizon.
Minor Comment 2 - Terminology
Consider using the term "traffic concentration indicator" rather than "congestion" where the analysis refers exclusively to aircraft counts within predefined spatial and temporal grid cells.
Minor Comment 3 - Vertical trajectory transitions
The authors should clarify that minimum horizontal separation between planned trajectories is a geometric indicator and does not, by itself, establish the occurrence of operational separation conflicts.
Minor Comment 4 - Interpretation of relative changes
Relative changes in traffic concentration and separation indicators should be interpreted alongside absolute traffic volumes, particularly for airspaces with low baseline demand.
Minor Comment 5 - Operational outlookThe outlook could benefit from a more explicit discussion of how climate optimization might be integrated into existing network-level ATM processes.
The manuscript provides a relevant and technically valuable contribution to the investigation of climate-optimized air traffic operations. The climate-impact assessment and trajectory adaptation (obviously no optimization) approach offer insights into the potential benefits of pre-tactical contrail mitigation.
However, the operational interpretation of the results requires further clarification. The selected geometric traffic indicators do not provide a comprehensive assessment of operational ATC complexity, sector capacity, or procedural feasibility. These concerns do not necessarily require extensive additional simulations or a fundamental revision of the modelling approach. Instead, the manuscript would benefit from a more differentiated discussion of the proposed operational concept, clearer methodological limitations, and appropriate conclusions. The distinction between climate mitgation potential, geometric traffic compatibility, and actual operational feasibility should remain explicit throughout the manuscript.
Addressing these points would substantially strengthen the practical relevance and scientific consistency of the study while preserving its valuable contribution to the development of climate-oriented ATM concepts.