Assessing Contrail Climate Impacts and Metrics with OpenAirClim

Abstract

International civil aviation is the cornerstone of global mobility and trade, enabling the efficient movement of people and goods across long distances. At the same time, global air transport operations contribute to anthropogenic climate change through carbon dioxide (CO₂) emissions and a range of non-CO₂ emissions and effects. Particularly important is the increased cloudiness due to the formation of condensation trails, or contrails, which is estimated to account for around half of aviation's total radiative forcing. As the industry continues to grow and its climate impact increases, policymakers are increasingly considering the inclusion of aviation non-CO₂ effects within regulatory frameworks. This poses a significant challenge, because these emissions and effects vary strongly with emission location, altitude, season and time of day, have very different atmospheric lifetimes and efficacies and remain subject to considerable scientific uncertainty.

A number of alternative aviation fuels, propulsion technologies and operational concepts have been proposed to reduce the aviation industry's climate impact. Quantifying their long-term effects requires tools capable of simulating large scenario ensembles while adequately representing both CO₂ and non-CO₂ emissions and effects. Comprehensive climate models offer high fidelity but are computationally expensive, making them impractical for the systematic evaluation of large numbers of aviation scenarios. Response models such as AirClim provide a practical alternative by using pre-computed responses and statistical relationships derived from simulations with more comprehensive climate models. However, existing response models are limited in their applicability to emerging propulsion systems and alternative aviation fuels. This dissertation addresses this challenge through the development and application of the open-source response model OpenAirClim, with a particular focus on contrails. The research has three main objectives: (1) to improve the understanding and modelling of persistent contrail formation within a climatological framework; (2) to enable the consistent assessment of future aviation industry scenarios; and (3) to assess the suitability of climate metrics for representing CO₂ and non-CO₂ climate effects on a common scale within policy frameworks.

The dissertation begins by investigating the environmental and aircraft-design-dependent factors controlling persistent contrail formation. Using 10 % of all hourly reanalysis data on conventional flight levels within the 2010 decade, the relative importance of these factors is analysed on a climatological timescale for a wide range of aviation fuels and propulsion technologies. Ice supersaturation is found to be the dominant limiting factor for all engine and fuel combinations, underscoring the importance of accurately predicting ice supersaturated regions. Nevertheless, the introduction of novel aviation fuels and propulsion technologies is found to have substantial influence on persistent contrail formation. Compared with a state-of-the-art conventional aircraft, envisioned water vapour extraction technologies could reduce the formation of persistent contrails by up to 85 %, whereas hydrogen-powered aircraft could increase formation by around 50 %. The study demonstrates that the probability of persistent contrail formation can be robustly parameterised as a function of aircraft design and flight altitude, which is a necessary first step for developing a consistent climate assessment for different aviation technologies.

Building on this foundation and using dedicated simulations of the general circulation model ECHAM5-CCMod, a new contrail module is developed for the open-source response model OpenAirClim. The module enables the efficient simulation and attribution of stratospheric-adjusted radiative forcing through a modelling chain that links emission inventories, persistent contrail formation, contrail-cirrus coverage and radiative forcing. The framework allows multiple aircraft and fuels to be simultaneously represented within the same emission inventory and scenario. To quantify the climate impact of emerging propulsion technologies and fuels, the module also includes a first representation of the low-soot regime. A comparison with literature demonstrates its ability to reproduce saturation behaviour under high future air traffic scenarios.

The resulting modelling framework is then applied to integrated aviation scenarios developed by the International Civil Aviation Organization (ICAO) to test its ability to simulate complex, multi-fuel and multi-aircraft scenarios. These scenarios represent a wide range of possible technology and fuel transition pathways for the industry. Global mean temperature change from contrails and CO₂ is found not to stabilise within the study horizon for any of the ICAO scenarios, even under the most ambitious transition pathways. The impacts of contrails and CO₂ are found to have opposing trends: While improvements in overall propulsion efficiency and the increasing use of alternative aviation fuels reduce lifecycle net CO₂ emissions, the probability of persistent contrail formation increases. Therefore, the contrail climate benefit of switching to alternative fuels strongly depends on the modelling approach for the low-soot regime. The results demonstrate that an effective reduction in aviation's climate impact requires the introduction of alternative fuels to be accompanied by dedicated non-CO₂ mitigation measures.

Finally, to allow for a complete analysis chain from emissions to policy, the suitability of existing physical climate metrics for representing aviation CO₂ and non-CO₂ emissions and effects is systematically evaluated. The neutrality, temporal stability, policy compatibility and simplicity of alternatives to the conventional Global Warming Potential (GWP) are investigated using the response model AirClim. The historical dominance of the GWP is found to result in a natural bias towards climate metrics that behave similarly. However, more accurate assessments of existing and future aircraft powered by novel aviation fuels can be achieved using the Average Temperature Response (ATR) or Efficacy-weighted Global Warming Potential (EGWP). While the ATR performs best overall, the EGWP represents a practical compromise and has recently been introduced into European aviation policy.

Overall, this dissertation contributes to the scientific understanding of aviation climate impacts by improving knowledge of the factors governing persistent contrail formation, developing a robust response modelling framework for contrails and identifying suitable climate metrics for use in aviation policy. A central outcome is the OpenAirClim contrail module, an open-source and computationally efficient tool that can be readily adapted and extended. Together with ongoing developments of the OpenAirClim framework, this work is an important step towards enabling consistent and transparent assessments of aviation climate impacts across a wide range of industry scenarios, thereby supporting both scientific analysis and policy implementation.