Abstract. Aviation-induced condensation trails (contrails) contribute significantly to anthropogenic radiative forcing. While navigational contrail avoidance has been proposed as a strategy to mitigate this climate impact, the operational viability of such maneuvers relies on the ability to verify their efficacy. Current verification methodologies often employ contrail models (such as CoCiP) driven by reanalysis weather data; however, these assessments are limited by the variable fidelity of the underlying meteorological datasets. In this work, we address this uncertainty by leveraging satellite observations to refine reanalysis estimates for specific contrail events. We demonstrate that this approach significantly improves the agreement between reanalysis data and in-situ measurements obtained from the IAGOS program, thereby offering a more robust framework for evaluating avoidance strategies.

Contrail cirrus clouds are a major non-CO2 contributor to aviation climate forcing. Current models link contrail ice particle numbers to climate impact, yet the upstream link to fuel composition remains poorly explored. Here we show that fuel hydrogen content governs soot number emissions and thereby provides a mechanistically grounded predictor of contrail ice particle numbers, with the first in-flight validation of this relationship showing agreement within ~±20%. Increasing fuel hydrogen content by 1.4 percentage points, from 13.8% to 15.2%, substantially reduces soot and ice particle numbers, contrail optical depth, lifetime and coverage, resulting in a calculated 52 [35, 67] % reduction in contrail energy forcing. Global deployment scenarios reflecting alternative fuel policies indicate substantial long-term mitigation potential from high-hydrogen content fuels. By 2050, their deployment could reduce the aviation-induced increase in global surface temperature by 21 [16, 27] %, offsetting a substantial proportion of the climate effects of projected aviation growth. Our results can establish fuel hydrogen content as a physically based metric linking fuel composition to aviation climate impacts, informing future fuel standards and mitigation strategies for climate-compatible aviation.

Global aviation contrail climate forcing could match or exceed the forcing from aviation’s cumulative CO2 emissions. Aircraft engine exhaust contains various particles, including non-volatile particulate matter (nvPM), volatile particulate matter (vPM), and ambient aerosols. At cruise altitudes, these particles can activate to form water droplets if the relative humidity in the plume exceeds their activation threshold, where they subsequently freeze into contrail ice crystals. The initial number of contrail ice crystals is primarily driven by the nvPM in “soot-rich” conditions, where the nvPM number emissions index (EIn) exceeds a threshold of around 1014 kg-1; while vPM and ambient aerosols become more likely to activate to form contrail ice crystals under “soot-poor” conditions (nvPM EIn < ~1014 kg-1) (Yu et al., 2024). However, existing global contrail simulation workflows do not currently account for the potential activation of vPM, which may lead to an underestimation of the contrail climate forcing. This underestimation could likely be more significant for flights powered by: (i) cleaner lean-burn combustors, where their nvPM EIn at cruise is typically below 1012 kg-1; or (ii) sustainable aviation fuel (SAF), which can reduce the nvPM EIn by up to 70%.An analytical model describing the microphysical pathway of contrail formation from nvPM and ambient aerosol particles was developed by Kärcher et al. (2015), which has since been extended to account for the potential activation of vPM in forming contrail ice crystals. Here, we aim to integrate this extended model into the contrail cirrus prediction model (CoCiP) to: (i) provide an updated estimate of the global annual mean contrail net radiative forcing (RF) for 2019; and (ii) quantify the simulated differences in contrail climate forcing between flights powered by conventional (rich-burn) and cleaner lean-burn combustors.By accounting for vPM activation, our preliminary results estimates that the 2019 global contrail net RF could increase by up to 35%, depending on the assumed vPM properties (EIn and particle size distribution). On average, the contrail climate forcing from lean-burn combustors could be around 50% to 90% lower than that from conventional rich burn combustors. When compared with the simulation without vPM activation, the increase in contrail warming effects due to vPM activation in the exhaust of lean-burn combustors becomes significant only when the ambient temperature is at least 10 K below the Schmidt-Appleman criterion threshold temperature. Further work is ongoing to quantify the contrail mitigation potential from a fleetwide adoption of SAF and cleaner lean-burn engines.ReferencesKärcher, B., Burkhardt, U., Bier, A., Bock, L., and Ford, I. J.: The microphysical pathway to contrail formation, Journal of Geophysical Research: Atmospheres, 120, 7893–7927, https://doi.org/10.1002/2015JD023491, 2015.Yu, F., Kärcher, B., and Anderson, B. E.: Revisiting Contrail Ice Formation: Impact of Primary Soot Particle Sizes and Contribution of Volatile Particles, Environ Sci Technol, 58, 17650–17660, https://doi.org/10.1021/ACS.EST.4C04340, 2024.

Mitigating contrail-induced warming by re-routing flights around contrail-forming regions requires accurate and stable forecasts of the state of the upper troposphere and lower stratosphere. Forecast stability (i.e. consistency between forecast cycles with different lead times) is particularly important for ‘pre-tactical’ contrail avoidance strategies that adjust routes based on forecasts with lead times as long as 24–48 h. However, no study to date has systematically quantified the degree to which forecast stability limits the effectiveness of pre-tactical avoidance. This study addresses this gap by comparing contrail forecasts generated using European Center for Medium-Range Weather Forecasts (ECMWF) HRES weather forecasts with lead times up to 48 h to contrail hindcasts generated based on ECMWF ERA5 reanalysis. An analysis of forecast errors show low pointwise consistency between persistent-contrail-forming regions in forecasts and reanalysis, with pointwise error rates similar to those found in previous comparisons of contrail-forming regions in reanalysis and reality. However, we also show that spatial errors in the locations of contrail-forming regions are relatively small, both when forecasts are compared to reanalysis and when reanalysis is compared to in-situ measurements. Finally, we show that designing a trajectory optimizer to take advantage of relatively small spatial errors allows flight trajectory optimizations based on contrail forecasts to reduce contrail climate forcing evaluated based on reanalysis by 80%–90% at the 8–24 h lead times most relevant to flight planning, with fuel penalties under 0.4%. Our results show that forecasts with lead times relevant to flight planning are stable enough to be used for pre-tactical contrail avoidance.

The global annual mean contrail climate forcing may exceed that of aviation's cumulative CO2 emissions. As only 2 %–3 % of all flights are likely responsible for 80 % of the global annual contrail energy forcing (EFcontrail), re-routing these flights could reduce the occurrence of strongly warming contrails. Here, we develop a contrail forecasting tool that produces global maps of persistent contrail formation and their EFcontrail formatted to align with standard weather and turbulence forecasts for integration into existing flight planning and air traffic management workflows. This is achieved by extending the existing trajectory-based contrail cirrus prediction model (CoCiP), which simulates contrails formed along flight paths, to a grid-based approach that initializes an infinitesimal contrail segment at each point in a 4D spatiotemporal grid and tracks them until their end of life. Outputs are provided for N aircraft-engine groups, with groupings based on similarities in aircraft mass and engine particle number emissions: N=7 results in a 3 % mean error between the trajectory- and grid-based CoCiP, while N=3 facilitates operational simplicity but increases the mean error to 13 %. We use the grid-based CoCiP to simulate contrails globally using 2019 meteorology and compare its forecast patterns with those from previous studies. Two approaches are proposed to apply these forecasts for contrail mitigation: (i) monetizing EFcontrail and including it as an additional cost parameter within a flight trajectory optimizer or (ii) constructing polygons to avoid airspace volumes with strongly warming contrails. We also demonstrate a probabilistic formulation of the grid-based CoCiP by running it with ensemble meteorology and excluding grid cells with significant uncertainties in the simulated EFcontrail. This study establishes a working standard for incorporating contrail mitigation into flight management protocols and demonstrates how forecasting uncertainty can be incorporated to minimize unintended consequences associated with increased CO2 emissions from re-routes.

The radiative forcing (RF) of contrail cirrus is substantial, though short-lived, uncertain, and heterogeneous, whereas the RF from CO₂ emissions is long-term and more predictable. To balance these impacts, we calculate the social costs of CO₂ and contrail cirrus using a modified Dynamic Integrated Climate Economy (DICE) model, spanning three discount rates, two damage functions, and three climate pathways. The main case estimate of the global social cost ratio of contrail cirrus to aviation CO₂ emissions ranges from 0.075 to 0.57, depending on assumptions. Accounting for uncertainty in contrail cirrus RF and climate efficacy further widens this range. We also quantify flight-specific social costs of contrail cirrus by analyzing nearly 500,000 flights over the North Atlantic, revealing substantial variability due to meteorological conditions. While uncertainty is considerable, our findings suggest that carefully implemented operational contrail avoidance could offer climate benefits even when the social cost of additional CO₂ emissions is considered.

Navigational avoidance as a contrail mitigation strategy has the potential to reduce the climate impact of aviation by as much as half. The effective implementation of avoidance strategies requires forecasts of the state of the upper troposphere and lower stratosphere that are stable (consistent across a range of lead times) and accurate (true to reality). However, the optimal criteria for evaluating whether a contrail forecasting system is sufficiently stable and accurate remain unclear. Here, we argue that forecast stability is best evaluated holistically by asking whether estimated decreases in contrail warming, for a given set of flight trajectories and deviations, are consistent across forecasts with different lead times. We use real-world flight trajectories taken from operational ADS-B datasets and deviations generated using aircraft performance models with a contrail-aware trajectory optimization routine to evaluate the stability of contrail predictions based on ECMWF IFS HRES forecasts. We find a high degree of stability with contrail warming reduced by over 80% even with lead times as long as 48 hours, sufficient to enable pre-tactical contrail avoidance at the flight planning stage. Finally, we show that we obtain large reductions in contrail warming despite frequent pointwise differences in the locations of ice supersaturated regions across forecast cycles because forecasts agree on the broad regions where ice supersaturation occurs.

Global simulations suggest the mean annual contrail cirrus net radiative forcing is comparable to that of aviation's accumulated CO2 emissions. Currently, these simulations assume non-volatile particulate matter (nvPM) and ambient particles are the only source of condensation nuclei, omitting activation of volatile particulate matter (vPM) formed in the nascent plume. Here, we extend a microphysical model to include vPM and benchmark this against a more advanced parcel model (pyrcel) modified to treat contrail formation. We explore how the apparent emission index (EI) of contrail ice crystals (AEI(ice)) scales with EInvPM, vPM properties, ambient temperature, and aircraft/fuel characteristics. We find model agreement within 10 %-30 % in the previously defined "soot-poor" regime. However, discrepancies increase non-linearly (up to 60 %) in the "soot-rich" regime, due to differing treatment of droplet growth. Both models predict that, in the "soot-poor" regime, AEI(ice) approaches 10(16) kg(-1) for low ambient temperatures (< 210 K) and sulfur-rich vPM, which is comparable to estimates in the "soot-rich" regime. Moreover, our sensitivity analyses suggest that the point of transition between the "soot-poor" and "soot-rich" regimes is a dynamic threshold that ranges from 10(13)-10(16) kg(-1) and depends sensitively on ambient temperature and vPM properties, underlining the need for vPM emission characterisation measurements. We suggest that existing contrail simulations omitting vPM activation may underestimate AEI(ice), especially for flights powered by lean-burn engines. Furthermore, our results imply that, under these conditions, AEI(ice) might be reduced by (i) reducing fuel sulfur content, (ii) minimising organic emissions, and/or (iii) avoiding cooler regions of the atmosphere.

Observations of contrails are vital for improving our understanding of the contrail formation and life cycle, informing models, and assessing mitigation strategies. Here, we developed a methodology that utilises ground-based cameras for tracking and analysing young contrails (< 35 min) formed under clear-sky conditions, comparing these observations against reanalysis meteorology and simulations from the contrail cirrus prediction model (CoCiP) with actual flight trajectories. Our observations consist of 14 h of video footage recorded over 5 different days in Central London, capturing 1582 flight waypoints from 281 flights. The simulation correctly predicted contrail formation and absence for around 75 % of these waypoints, with incorrect contrail predictions occurring at warmer temperatures than those with true-positive predictions (7.8 K vs. 12.8 K below the Schmidt–Appleman criterion threshold temperature). When evaluating contrails with observed lifetimes of at least 2 min, the simulation's correct prediction rate for contrail formation increases to over 85 %. Among all waypoints with contrail observations, 78 % of short-lived contrails (observed lifetimes < 2 min) formed under ice-subsaturated conditions, whereas 75 % of persistent contrails (observed lifetimes > 10 min) formed under ice-supersaturated conditions. On average, the simulated contrail geometric width was around 100 m smaller than the observed (visible) width over its observed lifetime, with the mean underestimation reaching up to 280 m within the first 5 min. Discrepancies between the observed and simulated contrail formation, lifetime, and width can be associated with uncertainties in reanalysis meteorology due to known model limitations and sub-grid-scale variabilities, contrail model simplifications, uncertainties in aircraft performance estimates, and observational challenges, among other possible factors. Overall, this study demonstrates the potential of ground-based cameras to create essential observational and benchmark datasets for validating and improving existing weather and contrail models.

Sustainable aviation fuel (SAF) could reduce aviation's CO2 and contrail climate forcing. This study quantifies the contrail mitigation potential and fuel supply chain costs of a uniform SAF distribution scenario, assuming all departing flights use a 10% SAF blend by mass. Building on this, we propose three SAF allocation strategies that optimize the same SAF supply to maximize contrail mitigation, while considering real-world supply chain constraints and additional costs. A seasonal strategy - providing SAF to all flights from October to February at higher blend ratios (28%) - achieves the highest benefit-to-cost ratio (1.7-7.2) and lowest abatement cost (€14-61/tCO2e). It raises annual reductions in contrail energy forcing (EFcontrail) from 7-8% (uniform vs no-SAF scenario) to 12-13%, with supply chain costs rising by 0.5% relative to the uniform scenario. Two diurnal strategies - one targeting flights after 16:00 local time and another adding a constraint of selecting flights with >250 km of persistent contrails - have lower benefit-to-cost ratios (0.2-2.4) and higher abatement costs (€42-675/tCO2e). Their 1-2% rise in supply chain costs relative to the uniform scenario outweighs the additional contrail climate benefits, as annual EFcontrail reductions only rise from 7-8% (uniform scenario) to 9-17%.

Clouds produced by aircraft (contrails) are responsible for over half of the positive radiative forcing from aviation, leading to the proposal of contrail avoidance as a method for mitigating the climate impact of aviation. This requires accurate prediction of the radiative properties of individual contrails, which themselves are highly dependent on the contrail microphysical properties, lifetime and macrophysical evolution along with the background atmospheric state. In-situ observations have also shown an impact of the generating aircraft and its fuel type on the properties and evolution of contrails. However, these observations are typically made close to the aircraft, with fewer observational constraints for the properties of the longer-lived contrails that drive the majority of the radiative forcing.Coupling satellite observations of contrails with flight data, we track contrails formed by individual aircraft over the North Atlantic. We find a strong link between aircraft type and contrail lifetime, with newer, more fuel-efficient aircraft forming longer-lived contrails. This relationship is not driven by the aircraft properties, but rather by operational differences in aircraft flight patterns, with the newer types flying higher in this region and so producing contrails with longer lifetimes. We present some encouraging initial evidence of reductions in aircraft soot emissions affecting contrail lifetime.

Contrails are estimated to account for the majority of the present-day warming by the aviation industry. Their formation relies on the availability of aerosol in the exhaust plume, upon which water vapour can condense and subsequently freeze to form contrail ice crystals Most modern aircraft operate in the soot-rich regime, releasing soot particles with a number emission index (EIn) of between 1014 and1016 (kg-fuel)-1. Under these conditions, the number concentration of soot particles and contrail ice crystals scales linearly. For this reason, existing global contrail simulations typically assume that the number concentration of ice crystals and soot particles are equivalent. However, the use of alternative fuels such as sustainable aviation fuel (SAF) and liquid hydrogen, and the adoption of cleaner lean-burn combustors in the existing fleet are likely to drive the soot EIn into the soot-poor regime < 1013 (kg-fuel)-1. Here, (semi) volatile material and entrained ambient particles can compete with soot for plume supersaturation and the relationship between the number concentration of soot particles and contrail ice crystals is non-linear. These effects are not currently accounted for in existing contrail models used to simulate regional and global contrail climate forcing. In this work, we extend the parcel model proposed by Kärcher et al. [1] to account for the activation of volatile particulate matter (vPM) in the soot-poor regime and integrate this into the contrail cirrus prediction model (CoCiP) [2]. We explore the relationship between the soot EIn and the apparent ice emissions index (AEI) in the soot-rich and soot-poor regimes, evaluating the model’s sensitivity to different aerosol properties, including particle hygroscopicity and characteristics of the particle size distribution. Preliminary results show a linear relationship between the soot EIn and AEI in the soot-rich regime, consistent with previous work [1]. However, in the soot-poor regime, the AEI: (i) could be up to two orders of magnitude larger than the soot EIn; (ii) increases with decreasing ambient temperatures and (iii) depends on the assumed particle properties of the (semi) volatile and ambient particle modes. These results suggest that existing global contrail simulations may underestimate the contrail climate forcing for a small subset of flights with soot EIn < 1014 (kg-fuel)-1.The model developed in this work will be implemented in a global contrail simulation, incorporating activation of vPM. A sensitivity analysis will also be performed, and the results validated with in-situ measurements from the recent Emissions and Climate Impact of Alternative Fuels (ECLIF) III experimental campaign [3].References[1] Bernd Kärcher et al., Journal of Geophysical Research: Atmospheres, 2015, 120, 7893–7927.[2] Ulrich Schumann, Geoscientific Model Development, 2012, 5, 43–580.[3] Raphael Märkl et al., [EGUsphere preprint].

The current best-estimate of the global annual mean radiative forcing (RF) attributable to contrail cirrus is thought to be 3 times larger than the RF from aviation's cumulative CO2 emissions. Here, we simulate the global contrail RF for 2019–2021 using reanalysis weather data and improved engine emission estimates along actual flight trajectories derived from Automatic Dependent Surveillance–Broadcast telemetry. Our 2019 global annual mean contrail net RF (62.1 mW m−2) is 44 % lower than current best estimates for 2018 (111 [33, 189] mW m−2, 95 % confidence interval). Regionally, the contrail net RF is largest over Europe (876 mW m−2) and the USA (414 mW m−2), while the RF values over East Asia (64 mW m−2) and China (62 mW m−2) are close to the global average, because fewer flights in these regions form persistent contrails resulting from lower cruise altitudes and limited ice supersaturated regions in the subtropics due to the Hadley Circulation. Globally, COVID-19 reduced the flight distance flown and contrail net RF in 2020 (−43 % and −56 %, respectively, relative to 2019) and 2021 (−31 % and −49 %, respectively) with significant regional variations. Around 14 % of all flights in 2019 formed a contrail with a net warming effect, yet only 2 % of all flights caused 80 % of the annual contrail energy forcing. The spatiotemporal patterns of the most strongly warming and cooling contrail segments can be attributed to flight scheduling, engine particle number emissions, tropopause height, and background radiation fields. Our contrail RF estimates are most sensitive to corrections applied to the global humidity fields, followed by assumptions on the engine particle number emissions, and are least sensitive to radiative heating effects on the contrail plume and contrail–contrail overlapping. Using this sensitivity analysis, we estimate that the 2019 global contrail net RF could range between 34.8 and 74.8 mW m−2.

Contrails are condensation trails left behind by aircraft at high altitudes when hot exhaust gases mix with cold air, causing water vapor from engine exhaust to freeze into ice crystals, can persist and evolve into cirrus clouds. This study investigates the impact of the aviation industry on contrail formation and its effect on global warming using a physics-based algorithm called Contrail Cirrus Prediction Model (CoCiP) with 2019 airline data. The aviation climate impact due to major greenhouse mechanisms is analyzed from an in-service fleet-level perspective to uncover the impact on global energy forcing due to factors such as aircraft market class, operator region, origin-destination pair, and more.

Aviation emissions that are dispersed into the Earth's atmosphere affect the climate and air pollution, with significant spatiotemporal variation owing to heterogeneous aircraft activity. In this paper, we use historical flight trajectories derived from Automatic Dependent Surveillance–Broadcast (ADS-B) telemetry and reanalysis weather data for 2019–2021 to develop the Global Aviation emissions Inventory based on ADS-B (GAIA). In 2019, 40.2 million flights collectively travelled 61 billion kilometres using 283 Tg of fuel, leading to CO2, NOX and non-volatile particulate matter (nvPM) mass and number emissions of 893 Tg, 4.49 Tg, 21.4 Gg and 2.8 × 1026 respectively. Global responses to COVID-19 led to reductions in the annual flight distance flown and CO2 and NOX emissions in 2020 (−43 %, −48 % and −50 % respectively relative to 2019) and 2021 (−31 %, −41 % and −43 % respectively), with significant regional variability. Short-haul flights with durations < 3 h accounted for 83 % of all flights but only for 35 % of the 2019 CO2 emissions, while long-haul flights with durations > 6 h (5 % of all flights) were responsible for 43 % of CO2 and 49 % of NOX emissions. Globally, the actual flight trajectories flown are, on average, ∼ 5 % greater than the great circle path between the origin and destination airports, but this varies by region and flight distance. An evaluation of 8705 unique flights between London and Singapore showed large variabilities in the flight trajectory profile, fuel consumption and emission indices. GAIA captures the spatiotemporal distribution of aviation activity and emissions and is provided for use in future studies to evaluate the negative externalities arising from global aviation.

Abstract. The global annual mean contrail net radiative forcing may exceed that of aviation’s cumulative CO2 emissions by at least two-fold. As only around 2–3 % of all flights are likely responsible for 80 % of the global annual contrail climate forcing, re-routing these flights could reduce the formation of strongly warming contrails. Here, we develop a contrail forecasting model that produces global predictions of persistent contrail formation and their associated climate forcing. This model builds on the methods of the existing contrail cirrus prediction model (CoCiP) to efficiently evaluate infinitesimal contrail segments initialized at each point in a regular 4D spatiotemporal grid until their end-of-life. Outputs are reported in a concise meteorology data format that integrates with existing flight planning and air traffic management workflows. This “grid-based” CoCiP is used to conduct a global contrail simulation for 2019 to compare with previous work and analyze spatial trends related to strongly warming/cooling contrails. We explore two approaches for integrating contrail forecasts into existing flight planning and air traffic management systems: (i) using contrail forcing as an additional cost parameter within a flight trajectory optimizer; or (ii) constructing polygons of airspace volumes with strongly-warming contrails to avoid. We demonstrate a probabilistic formulation of the grid-based model by running a Monte Carlo simulation with ensemble meteorology to mask grid cells with significant uncertainties in the simulated contrail climate forcing. This study establishes a working standard for incorporating contrail mitigation within existing flight planning and management workflows and demonstrates how forecasting uncertainty can be incorporated to minimize unintended consequences associated with increased CO2 emissions of avoidance.

Aviation contrail cirrus has important climate impacts. To construct efficient policies to reduce the uncertain, heterogeneous and short-lived climate impacts of contrail cirrus and balance these against the certain, homogeneous and long-lived climate impact of CO2 emissions the climate impact of CO2 and contrail cirrus need to be placed on a common scale. We analyze the social cost of CO2and of contrail cirrus as well their ratio using an updated version of the Dynamic Integrated Climate Economy (DICE) model with three different discount rates and three different future climate pathways. The social cost of contrail cirrus is less sensitive to the discount rate and less affected by the long-term temperature pathway than the social cost of CO2. However, the social cost of contrail cirrus is strongly dependent on specific meteorological conditions, which makes the social cost associated with individual flights vary by several orders of magnitude.

Abstract. Observations of contrail are vital for improving understanding of contrail formation and lifecycle, informing models, and assessing contrail mitigation strategies. Ground-based cameras offer a cost-effective means to observe the formation and evolution of young contrails and can be used to assess the accuracy of existing models. Here, we develop a methodology to track and analyse contrails from ground-based cameras, comparing these observations against simulations from the contrail cirrus prediction model (CoCiP) with actual flight trajectories. The ground-based contrail observations consist of 14 h of video footage recorded on five different days over Central London, capturing a total of 1,619 flight waypoints from 283 unique flights. Our results suggest that the best agreement between the observed and simulated contrail formation occurs at around 35,000–40,000 feet and at temperatures at least 10 K below the Schmidt-Appleman Criterion threshold temperature (TSAC). Conversely, the largest discrepancies occurred when contrails are formed below 30,000 feet and at temperatures within 2.5 K of TSAC. On average, the simulated contrail width is 17.5 % smaller than the observed geometric width. This discrepancy could be caused by the underestimation of sub-grid scale wind shear and turbulent mixing in the simulation, and model representation of the contrail cross-sectional shape. Overall, these findings demonstrate the capability of ground-based cameras to inform weather and contrail model development when combined with flight telemetry.