Klaus Gierens

The climate effect of aviation is significant and expected to increase. Reducing the sector's environmental footprint to contribute to global temperature targets will require not only investments in airframe and engine technologies but also operational strategies such as eco-efficient flight routing. The D-KULT project (Demonstrator Climate and Environmentally Friendly Air Transport), funded under the German Federal Aviation Research Programme (LuFo) demonstrated the operational feasibility of integrating climate-response estimates of non-CO ₂ effects into flight trajectory optimisation in practice. The project addressed a multi-objective optimisation problem. Flight trajectories were optimised to minimise climate effects based on climate response estimates from numerical models while maintaining operational and economic efficiency. Operational constraints such as meteorological hazards, regulatory requirements, airspace and airport capacity were incorporated to ensure real-world applicability. D-KULT developed an end-to-end information chain integrating aviation weather forecasting, flight planning, air traffic control, and climate benefit assessment to enable eco-efficient flight routing for testing purposes. Achieving this complex operational and environmental objective required close collaboration across multiple disciplines and substantial upgrades to the majority of participating components. Advanced aviation weather products were generated that estimate the climate response to emissions under prevailing meteorological conditions. Flight planning tools have been extended to incorporate this information in addition to the standard data used in the flight planning optimization algorithms. In this way, flight planning tools can calculate emissions and corresponding climate effects along flights, both as part of pre-tactical and tactical (pre-take-off and in-flight) eco-efficient flight optimisation. Developments within D-KULT were tested through a large-scale national contrail avoidance flight trial campaign (TF-100 with 108 flights), including enhanced satellite-based contrail detection methods and assessment of estimates of climate benefits. Further evaluation focused on operational integration, examining air traffic control procedures, and workflow implications in a high-fidelity simulator environment. This manuscript provides a comprehensive overview of the project, describes new developments and major challenges on implementation pathways, summarizes key findings and gives recommendations derived from practical implementation. Issues that still need to be solved are: Aviation weather forecast should be based on improved cloud microphysics representing ice supersaturation correctly, humidity measurements at cruise altitudes are required for data assimilation, the geographical and temporal scope of implemented response tools needs to be extended. Since different meteorological input data and different response tools lead to differing assessments of climate benefits and damages, such sensitivities need to be considered in accordance with a thorough uncertainty analysis. For future implementation practical obstacles are the automation of procedures, the increase of workload for controllers if climate sensitive sectors need to be kept clear of aircraft, and the subject of standardization.

A recently proposed method to statistically treat trade-offs and associated risks in aviation mitigation options (Prather et al., 2025) leaves, to this author's opinion, issues open for discussion. In particular, the method lacks a rigorous mathematical derivation and gives no precise mathematical definition of the central notion “trade-off ratio”. The derivation, which is given here, leads to an inequality that expresses the postulate that the total (summed-up) metric value, after a mitigation measure has been taken, must be smaller than the value of the same metric without mitigation. Surprisingly, neither the metric pair itself nor the basic (effective) radiative forcings appear in the inequality. However, if one enforces their presence or if one does not cancel them out like in the original method, uncertain quantities appear on both sides of the inequality, which renders it inadequate for its purpose. In contrast, the method in its “sensitivity form” may work in hindcast mode (that is, to estimate the probability whether a mitigation method was successful), if the changes of the effective radiative forcings due to the measure taken can be determined with sufficient precision, using a climate model with well defined weather patterns over the course of a year or longer.

A recently proposed method to mathematically treat trade-offs and associated risks in aviation mitigation options (Prather et al., 2025) leaves, to this author's opinion, many issues open for discussion. The method is critically reviewed and the equations are derived and justified. Issues that remained vague in the recent paper are clarified. Unfortunately, close inspection proves this method to be inadequate for its purpose. An alternative formulation is proposed with transparent and understandable derivations. The unfounded assumptions basic to the original method are discussed and their effects on the final result are shown. It turns out that with the current data basis the proposed risk-analysis method for mitigation in aviation suffer from a certain degree of arbitrariness. Alternative approaches that exploit ensemble weather forecasts seem more promising.

The climate effect of aviation is significant and expected to increase. Reducing the sector’s environmental footprint to contribute to global temperature targets will require not only investments in airframe and engine technologies but also operational strategies such as eco-efficient flight routing, focusing on reducing non-CO2 effects. The D-KULT project (Demonstrator Climate and Environmentally Friendly Air Transport), funded under the German Federal Aviation Research Programme (LuFo), aims to demonstrate the feasibility of optimising flight trajectories with respect to climate effect. The project addresses a multi-objective optimisation problem in which flight trajectories minimise climate effects while maintaining operational and economic efficiency. Operational constraints such as meteorological hazards, regulatory requirements, airspace and airport capacity need to be incorporated to ensure real-world applicability. This work provides a comprehensive overview of the project, describing new developments and major challenges on implementation pathways and summarizes the key findings. D-KULT developed an end-to-end information chain integrating aviation weather forecasting, flight planning, air traffic control, and climate benefit assessment to enable eco-efficient flight routing for testing purposes. Achieving this complex operational and environmental objective required close collaboration across multiple disciplines and substantial upgrades to the majority of participating components. Novel aviation weather products were generated that estimate the climate sensitivity of emissions under prevailing meteorological conditions. Flight planning tools have been extended to take this information into account in addition to the standard data in the flight planning optimization algorithms. In this way, flight planning tools can calculate emissions and corresponding climate effects along flights, both as part of strategic (pre-departure) and tactical (pre-take-off and in-flight) eco-efficient flight optimisation. Developments within D-KULT were tested through a large-scale national contrail avoidance flight trial campaign, including enhanced satellite-based contrail detection methods and assessment and workflow implications in a high-fidelity simulator environment. Results demonstrate substantial progress toward operational climate-optimised aviation but also highlight remaining challenges, including uncertainties in weather forecast and non-CO2 climate effects, automation needs along the workflow and increased controller workload in dense airspaces. A key requirement for operational implementation is transparent information of prediction uncertainties, enabling informed decision-making when rerouting for climate benefit. These remaining research of achievable climate benefits. Further evaluation focused on operational integration, examining air traffic control procedures, and development needs form the basis for the planned successor programme to D-KULT.

Contrail lifetime is constrained mainly by the sedimentation of ice crystals into lower levels that are subsaturated, by the blowing out of the ice crystals from the parent ice-supersaturated regions (ISSRs) as a result of the (horizontal) wind and by the reduction in supersaturation down to subsaturation due to large-scale subsidence. The first of these processes can be characterised by a sedimentation timescale. The second and third processes can together be characterised by a synoptic timescale. The synoptic timescale is determined in this paper by trajectory calculations for air parcels that initially reside in ice-supersaturated regions and which leave these either with the wind or where the ice supersaturation itself vanishes. It is crucial to know the timescales of contrails because their individual effect on the climate depends on their lifetime. The distinction between the two timescales is particularly important for planning flights that use alternative fuels, in order to mitigate contrail effects. This works in particular if sedimentation is the predominant contrail termination process – that is, if the sedimentation timescale is shorter than the synoptic one. Here we show that both timescales are of the order of a few hours. Actually, in nature, the three mentioned processes act simultaneously. The combined timescale is half of the harmonic mean of the two timescales in separation. Furthermore, we found as a side result that ISSRs emerge only in areas where the normalised geopotential height Z^∗ is at least 0.98. For contrail-avoiding flight planning, this means that contrail avoidance in regions with Z^∗ < 0.98 is not necessary.

Contrails can last for many hours in the sky if they form in ice supersaturated regions (ISSRs). Contrail formation is possible once the ambient air is sufficiently moist and cold (below −40 °C). Contrails are persistent when the ambient relative humidity with respect to ice is at least 100 %. Cirrus clouds and contrails consist of ice crystals, which move with the wind. Ice supersaturation is an immaterial feature, which does not generally move with the wind. However, the movement of ISSRs relative to the winds is currently unknown. We analyse the movement of ISSRs and the wind using data for aviation weather forecasts, the WAWFOR package, from the German Weather Service (DWD). Our results demonstrate the kinematic differences and similarities of the movement of ISSRs in comparison to the wind. We show that the ISSRs on average move slower than the wind at the same location, the direction of movement is usually quite similar and the distributions of both velocities follow Weibull distributions. The almost identical direction of the movements is beneficial for contrail lifetimes, but the lifetime of contrails is not generally determined by the lifetime of ISSRs. Contrails are blown out of ISSRs with the wind. We assume that our study can be used as a basis for further analyses of the movement of ISSRs. Furthermore, our method of analysis is applicable to other extended features or areas, for instance to areas where aircraft non-CO₂ emissions would have a particularly large climate effect.

Contrail lifetime is constrained mainly by the sedimentation of ice crystals into lower levels that are subsaturated, the blowing out of the ice crystals from the parent ice supersaturated regions (ISSRs) by the (horizontal) wind, and the reduction of supersaturation down to subsaturation due to large-scale subsidence. The first of these processes can be characterised by a sedimentation time-scale. The second and third processes can together be characterised by a synoptic time-scale. The synoptic time-scale is determined in this paper with trajectory calculations for air parcels that initially reside in ice supersaturated regions and which either leave these with the wind or where the ice supersaturation itself vanishes. It is crucial to know the time-scales of contrails because their individual effect on climate depends on their lifetime. The distinction between the two time-scales is particularly important for planning flights with alternative fuels for the purpose of the mitigation of contrail effects. This works in particular if sedimentation is the predominant contrail termination process, that is, if the sedimentation time-scale is shorter than the synoptic one. Here we show that both time-scales are of the order of a few hours. Actually, in nature, the three mentioned processes act simultaneously. The combined time-scale is half of the harmonic mean of the two time-scales in separation. Furthermore, we found as a side result that ISSRs only emerge in areas where the normalised geopotential height, Z*, is mainly at least 0.98. For contrail-avoiding flight planning this means that contrail avoidance in regions with Z*<0.98 is mainly not necessary.

Contrail lifetime is constrained mainly by the sedimentation of ice crystals into lower levels that are subsaturated, the blowing out of the ice crystals from the parent ice supersaturated regions (ISSRs) by the (horizontal) wind, and the reduction of supersaturation down to subsaturation due to large-scale subsidence. The first of these processes can be characterised by a sedimentation time-scale. The second and third processes can together be characterised by a synoptic time-scale. The synoptic time-scale is determined in this paper with trajectory calculations for air parcels that initially reside in ice supersaturated regions and which either leave these with the wind or where the ice supersaturation itself vanishes. It is crucial to know the time-scales of contrails because their individual effect on climate depends on their lifetime. The distinction between the two time-scales is particularly important for planning flights with alternative fuels for the purpose of the mitigation of contrail effects. This works in particular if sedimentation is the predominant contrail termination process, that is, if the sedimentation time-scale is shorter than the synoptic one. Here we show that both time-scales are of the order of a few hours. Actually, in nature, the three mentioned processes act simultaneously. The combined time-scale is half of the harmonic mean of the two time-scales in separation. Furthermore, we found as a side result that ISSRs only emerge in areas where the normalised geopotential height, Z*, is mainly at least 0.98. For contrail-avoiding flight planning this means that contrail avoidance in regions with Z*<0.98 is mainly not necessary.

Contrail lifetime is constrained mainly by the sedimentation of ice crystals into lower levels that are subsaturated, the blowing out of the ice crystals from the parent ice supersaturated regions (ISSRs) by the (horizontal) wind, and the reduction of supersaturation down to subsaturation due to large-scale subsidence. The first of these processes can be characterised by a sedimentation time-scale. The second and third processes can together be characterised by a synoptic time-scale. The synoptic time-scale is determined in this paper with trajectory calculations for air parcels that initially reside in ice supersaturated regions and which either leave these with the wind or where the ice supersaturation itself vanishes. It is crucial to know the time-scales of contrails because their individual effect on climate depends on their lifetime. The distinction between the two time-scales is particularly important for planning flights with alternative fuels for the purpose of the mitigation of contrail effects. This works in particular if sedimentation is the predominant contrail termination process, that is, if the sedimentation time-scale is shorter than the synoptic one. Here we show that both time-scales are of the order of a few hours. Actually, in nature, the three mentioned processes act simultaneously. The combined time-scale is half of the harmonic mean of the two time-scales in separation. Furthermore, we found as a side result that ISSRs only emerge in areas where the normalised geopotential height, Z*, is mainly at least 0.98. For contrail-avoiding flight planning this means that contrail avoidance in regions with Z*<0.98 is mainly not necessary.

Contrails can last for many hours in the sky if they form in ice supersaturated regions (ISSRs). Contrail formation is possible once the ambient air is sufficiently moist and cold (below -40 °C) such that the water vapour in the exhaust of an aircraft engine transiently becomes water saturated, which initiates formation of droplets which immediately freeze. Contrails are persistent when the ambient relative humidity with respect to ice is at least 100 %. Cirrus clouds and contrails consist of material ice crystals, which move with the wind. Ice supersaturation is an immaterial feature, which does not generally move with the wind. However, the exact movement of ISSRs and how they move in comparison to the winds, is currently unknown. We analyse their relation using data from the German Weather Service. Our results demonstrate the kinematic differences and similarities of the movement of ISSRs in comparison to the motion of the air in and through the ISSRs. We show that the ISSRs on average move slower than the wind at the same location, the direction of movement is usually quite similar and the distributions of both velocities follow Weibull distributions. The almost identical direction of the movements is beneficial for contrail lifetimes, but the contrail lifetime is not generally determined by the lifetime of ISSRs. It happens that contrails are blown out of ISSRs with the wind. We assume that our study can be used as a basis for further analyses of the movement of ISSRs and their movement properties. Furthermore, our method of analysis is applicable to other extended features or areas, for instance for areas where aircraft non-CO₂ emissions would have a particularly large climate effect.