Contrail avoidance and its role in aviation climate impact mitigation

This project will use state-of-the-art weather and climate models to assess the climate impact of contrail avoidance for current and future generation aircraft.

The aviation sector faces mounting pressure to reduce its climate impact, with ambitious targets set nationally and internationally. The UK’s 2021 Sixth Carbon Budget, for example, aims to cut national emissions (including those from international aviation and shipping) by 78% by 2035 (UK Government, 2021). Similarly, the International Air Transport Association (IATA) pledged in October 2021 to achieve global net-zero carbon emissions by 2050 (IATA, 2021). Beyond CO2 emissions reductions, aviation must also address non-CO2 effects, which are currently estimated to account for around 66% of the aviation effective radiative forcing (ERF) (Lee et al., 2021).

Across the several net-zero emission pathways proposed, a key part involves a transition to alternative fuels, such as sustainable aviation fuel (SAF) and hydrogen, together with targeted strategies for non-CO₂ effects mitigation (Rap et al., 2021; Dray et al., 2022), including through contrail avoidance (Mannstein 2005; Teoh et al. 2020). This consists in rerouting aircraft to minimise the formation of persistent contrails, the thin line-shaped clouds that form behind an aircraft under liquid water saturation conditions due to mixing between the warm and moist exhaust and the cool ambient air (Karcher, 2018). When the ambient relative humidity exceeds ice saturation, contrails can persist and spread, leading to the formation of contrail cirrus, responsible for the largest aviation warming effect on climate.

Contrail avoidance relies on integrating real-time weather data into flight planning in order to avoid the ice supersaturated regions (ISSRs) where contrails are likely to form and persist. A recent meta-analysis of contrail avoidance modelling studies suggested that half of the contrail length could be avoided at a penalty of 1% increase in fuel burn (Dray et al., 2022). In 2021, the first-ever operational contrail avoidance trial which took place in the Maastricht Upper Area Control (MUAC) region, concluded that contrail avoidance could be an efficient method for mitigating the climate impact of aviation, with persistent contrails potentially avoided by flying up to 2000 ft lower or higher (Sausen et al., 2024). In another trial in the US, Google Research together with American Airlines used Artificial Intelligence based predictions, combined with Breakthrough Energy’s pycontrails open-source contrail model to avoid contrail formation during 70 test flights over 6 months. Their post flight analysis using satellite images found a 54% reduction in contrail cover, with an associated fuel burn penalty of 2% (Google, 2024). However, these first trials also highlighted a series of important open questions that need to be addressed before any large-scale contrail avoidance operational implementation. In particular, the main concern remains the current poor skill of forecast models to predict persistent contrails (Sausen et al., 2024; Hofer et al., 2024), together with the potential negative impact of the additional emissions caused by the flight rerouting.

Objectives

The aim of this project is to investigate the climatic effect of contrail avoidance strategies for current and future generation aircraft. The modelling approach will involve the use and development of the state-of-the-art UK Met Office Unified Model, including its recent contrail cirrus parameterisations and weather forecast models. While relatively flexible to allow for your interests, the project is likely to involve:

• Assessing the UM model skill to forecast ice supersaturated regions, using a series of in-situ observations (i.e. IAGOS) and reanalysis products;
• Quantify the reduction in contrail cirrus ERF for different contrail avoidance strategies designed for current and future generation aircraft;
• Quantify the total aviation ERF for different contrail avoidance strategies for current and future generation aircraft
• Exploring the role of different climate metrics for setting aviation climate mitigation targets

Potential for high impact outcome

There are still large uncertainties in our understanding of the current aviation impact on climate and how it might evolve in the future. With access to state-of-the-art models and support from our world leading research groups, this project will address these uncertainties and will therefore have important implications for future climate projections. This will be of interest to both the general public and to policy makers working in climate mitigation and the transport sector. It is expected that findings from this project will be published in high impact journals and will be presented at international conferences.

Training

The student will work under the supervision of Dr Alex Rap and Prof Piers Forster and will be a member of the Physical Climate Change research group in SEE. The project provides an exciting opportunity to be trained in and exploit the new UK Earth System Model (UKESM) via collaborations with UK Met Office staff. The student will also be part of the Priestley International Centre for Climate that brings together world leading expertise in all the key strands of climate change research at the University of Leeds. Through the high level specialist scientific training associated with this project, the student will develop a comprehensive understanding of aviation impacts on climate and will work with state-of-the-art climate models. In addition, the student will learn how to communicate science and how to write high impact journal publications.

The successful PhD student will also have access to a broad spectrum of training workshops put on by the Faculty that includes an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva. A full list of training opportunities is available here.

Eligibility requirements

A good first degree, Masters degree or equivalent in a quantitative science discipline (e.g. Physics, Mathematics, Chemistry, Atmospheric Science, Engineering) and a keen interest in global environmental problems. While a substantial part of this project involves computer modelling, prior experience is not essential – we provide high level specialist scientific training during the PhD.

References

 

1. Dray et al., 2022. Cost and Emissions Pathways towards Net-zero climate impacts in aviation. Nature Climate Change 12(10), 956-962.
2. Google, 2024. Project Contrails. [Online]. Available from: https://sites.research.google/contrails/
3. Hirst, 2021. Aviation, decarbonisation and climate change. House of Commons Library Research Briefing, 8826, 20 September 2021.
4. Hofer et al., 2024. How well can persistent contrails be predicted? An update, Atmos. Chem. Phys., 24, 7911–7925.
5. IATA, 2021. Net-Zero Carbon Emissions by 2050, International Air Transport Association, Press release No 66, 4 October 2021.
6. Karcher, 2018. Formation and radiative forcing of contrail cirrus, Nature Comm., 9, 1824.
7. Lee et al., 2021. The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, Atmospheric Environment, 244, 117834.
8. Mannstein et al., 2005. A note on how to avoid contrail cirrus. Transportation Research Part D: Transport and Environment, 10(5), 421-426.
9. Rap et al., 2021, The impact of contrails and NOx emissions for FlyZero aircraft concepts with alternative fuel and energy systems, Fly Zero Report FZ_RB_001, ATI.
10. Sausen et al., 2024. Can we successfully avoid persistent contrails by small altitude adjustments of flights in the real world?, Meteorol. Z., 33(1), 83-98.
11. Teoh et al., 2020. Beyond Contrail Avoidance: Efficacy of Flight Altitude Changes to Minimise Contrail Climate Forcing, Aerospace, 7(9), 121.
12. UK Government, 2021. UK enshrines new target in law to slash emissions by 78% by 2035. Press release, 20 April 2021.