Key takeaways from ICAO’s Symposium on Non-CO2 Aviation Emissions – or we need data, data, data!
This article was published on LinkedIn in October 2024.
The need for more data to reduce uncertainties, coupled with global collaboration and immediate actions, were key messages from the 2024 ICAO Symposium on Non-CO2 Aviation Emissions. Non-CO2 effects, like contrails and nitrogen oxides (NOx), are estimated to have a climate impact on par with aviation’s CO2 emissions—though the two don’t necessarily correlate. For example, fuel-efficient engines may produce more contrails and NOx due to cooler exhaust temperatures and higher combustion temperatures, respectively. The trade-offs between reducing CO2 and mitigating non-CO2 emissions highlight the complexity of aviation’s environmental impact.
I compiled and corrected a transcript of the symposium, and in doing so, I decided to write a summary report (see Publications) for others who might also find it useful. Here are some key takeaways from the event, alongside my personal insights.
Contrails
Contrails form when aircraft emit water vapor into ice-supersaturated air. These trails can evolve into cirrus clouds that trap heat, potentially contributing more to global warming than the CO2 emissions from aviation.
Efforts to mitigate contrail formation by adjusting flight paths are promising. Europe and North America are working on improved forecasts to avoid contrail-prone areas, but accurate prediction requires better temperature and humidity data—something most commercial aircraft don’t collect.
My Take: Contrail avoidance holds promise, but we need more validation to confirm that rerouted flights truly avoid supersaturated regions. Commercial, AI-assisted tools are being developed for this purpose. So far, analysis of past data has shown effects that seem too good to be true. It is much easier to evaluate past weather than to predict what is going to happen in a given region before flight departure.
While this measure could benefit the climate, implementation will be challenging without unified airspace management, such as the Single European Sky.
Contrails, contrail cirrus and other pretty clouds around Zurich airport.
Particulate Matter (nvPM and vPM)
Non-volatile particulate matter (nvPM), mainly soot, plays a key role in contrail formation by acting as ice nuclei. Volatile particulate matter (vPM) interacts with soot or forms new particles, influenced by fuel composition and engine design.
Sustainable Aviation Fuels (SAF) can reduce nvPM, but further research is needed on vPM’s role in contrail formation. Interestingly, jet engines often emit more oil vapor than soot, which may also contribute to contrail formation even when nvPM is low.
My Take: Reducing soot via SAF is beneficial, but clean combustion technology offers more immediate reductions, even with conventional fuels. Oil vapor emissions are another issue requiring attention.
Another challenge is the development of better emission inventories. Current nvPM inventories are highly uncertain and rely on outdated methods for a large part of the current fleet. This data will get better over time as new engines types are certified for nvPM and new reporting points are being added that should help modelers to predict in-flight emissions better. The inventories should also reflect potential increase of emissions as engines age.
NOx Emissions
NOx emissions contribute to both warming and cooling effects in the atmosphere. They form ozone, which warms, but they also destroy methane, a potent greenhouse gas, leading to a cooling effect. The balance is complex and hard to model. The good news is that NOx from today’s fleet may cause less radiative forcing than assumed previously.
My Take: NOx reductions will come from engine technology rather than fuels. Lean-burn engines and other advancements can reduce NOx, but often at the cost of fuel efficiency. Surprisingly, newer engines sometimes produce more NOx than the ones they replace, even if they perform better on regulatory standards. While most of the focus has been on NOx during take-off and landing (for air quality reasons), we need to pay more attention to cruise emissions. More research is needed to fully understand NOx emissions at different flight phases.
CFM Rise, an open rotor concept in development. Figure credit: CFM International.
Novel Engine Technology
New engine designs, including open rotor and hybrid-electric concepts, offer the potential for greater efficiency and lower emissions. However, balancing environmental performance with reliability—something older engine designs excelled at—remains a challenge.
My Take: While new engines types offer efficiency gains, they’ve also had technical issues that can undermine their environmental benefits. We need to consider the emissions cost of these problems, particularly if they lead to additional maintenance flights or delays. Finding the right balance between innovation and reliability is critical.
Hydrogen as Fuel
Hydrogen combustion produces no CO₂ or soot, making it an attractive option for reducing carbon emissions. However, it emits about 2.5 times more water vapor than kerosene per distance flown, which could increase contrail formation. Hydrogen-powered aircraft could double contrail cover compared to kerosene-fueled planes. But changes in contrail properties might reduce their radiative forcing.
My Take: Hydrogen in aviation has been researched for decades, yet we are still far from large-scale adoption. The infrastructure and cost challenges are immense. A lot of the work seems to be driven by FOMO (fear of missing out) by some manufacturers, and boosted by government grants. To illustrate the challenge, last year’s ground test of a medium turbofan engine retrofitted for liquid hydrogen combustion used up all the liquid hydrogen in Germany. I doubt it was the glorified green hydrogen.
From an engineering and economic perspective, hydrogen may not deliver the benefits we are hoping for in aviation by 2050 (or ever).
Monitoring, Reporting, and Verification (MRV)
A strong MRV system is critical for tracking and reducing non-CO2 effects over time. The EU is currently leading with its system for NOx and particulate matter, but we need a globally standardized framework.
My Take: Global consistency in reporting is essential for addressing non-CO₂ emissions. Starting next year, European airlines will have to report these emissions using model-based methods. However, we still lack an agreed-upon metric for non-CO₂ effects, such as contrails. A global effort is needed to streamline this process.
Economic Considerations: A Missed Discussion
One notable gap at the symposium was the lack of discussion around the economic impacts of reducing non-CO2 emissions. SAF, new technologies, and hydrogen require significant investments, but the financial implications were largely ignored.My Take: Addressing the economic costs of reducing non-CO2 emissions is critical. Airlines are already under financial pressure, and adopting new technologies and fuels will only add to their burden. Governments and industry must create incentives and markets to make these changes viable.
Final Thoughts
Scaling SAF remains the one no-regret solution for both CO2 and non-CO2 emissions. Contrail avoidance is captivating the industry, and I’m eager to see its long-term effectiveness. Ultimately, we really do need more data to reduce uncertainties and guide action globally—an area I’m happy to keep contributing to.
If your organization is looking to better understand or address non-CO2 emissions, feel free to reach out—I’d be happy to help you navigate the complexities and develop effective solutions.
