Adam Morton • 21.02.24 •  3 mins

Adam Morton, ATI Head of Technology – Sustainability & Strategy, considers the imperative of addressing aviation’s non-CO₂ emissions while capturing opportunities for UK aerospace technology to play a leading role.

What are non-CO₂ emissions?

The Jet Zero Strategy launched in 2022 outlined the UK government’s approach to achieving Net Zero in aviation by 2050. This rightly focused on minimising the carbon dioxide (CO₂) emitted by aircraft, but also acknowledged other atmospheric emissions can contribute to climate effects. Non-carbon dioxide (non-CO₂) emissions is a collective term for all emissions other than carbon that are emitted by aircraft in flight. The term includes persistent contrails, water vapour, oxides of nitrogen (NO_x), sulphur compounds and particulates. The ATI’s FlyZero project reported non-CO₂ emissions from aircraft have complex and interacting impacts on climate change over dramatically varying timescales. There is growing recognition in the sector that climate impacts from aviation’s non-CO₂ emissions, may match or even exceed the effects of carbon itself.

Why are non-CO₂ emissions important?

Since October 2021 when IATA member airlines committed to Net Zero, there has been added impetus to decarbonise flights. It is widely accepted that Net Zero can only be achieved through a combination of sustainable aviation fuel (SAF), more efficient conventional aircraft, zero-carbon emission aircraft, better air traffic management, more efficient operations, and technical and non-technical offsets.

With aerospace OEMs investing heavily in delivering on their own improvements, it is important that we fully understand the non-CO₂ implications of different decarbonisation solutions. Adoption of alternative fuels, such as hydrogen and SAF, must not cause unintended negative consequences and should improve aviation’s overall greenhouse gas footprint.

On average, carbon emissions remain in the atmosphere for hundreds of years contributing to climate change over centuries. The behaviour of carbon emissions is well understood and we are increasingly confident in predicting the warming related to carbon. In contrast, non-CO₂ emissions research has produced much more variance in results which gives us a low degree of confidence in predicting their climate effects.

Non-CO₂ emissions vary in terms of how long they persist and how damaging they could be. To complicate things further, the impact also varies depending on where the emissions are produced and at what height. For example, contrails are formed by water vapour which stays in the stratosphere for weeks, but typically disperses within hours in the troposphere. The climate effects of NOx are very uncertain, partly due to interactions with other gases in the atmosphere. The direct interaction of soot and sulphur aerosols with radiation, or the role in cloud formation, is even less well understood.

There is broad academic and industry agreement that significantly more work is necessary to understand the individual climate contributions, and mitigations of non-CO₂ emissions. The UK is a leader in relevant areas of climate science and the funding recently announced by Natural Environment Research Council (NERC) will help to advance the fundamental research needed in this space.

Are there trade-offs to be made?

Mitigation of climate effects is likely to involve trade-offs between technology, operations and fuel burn. There is a general consensus within the sector that, without significant mitigation, persistent contrails will be the single largest non-CO₂ contributor to aviation’s warming impact. At the same time, there is strong evidence avoiding Ice Supersaturated Regions (ISSRs) can help mitigate persistent contrail formation. Tactical re-routing could offer opportunities to reduce this climate impact but more fundamental science, data and validated models is needed to fully understand the benefits of this approach. As information about non-CO₂ effects matures and the data becomes more complete, other trade-offs are likely to emerge that will need to be navigated if our industry to mitigate the climate impacts.

Do non-CO₂ emissions need to be reported?

No targets or commitments are in place for non-CO₂ climate effects although some aircraft non-CO₂ emissions, such as NOx and soot, are subject to internationally agreed standards in order to protect air quality. The EU is moving to mandate reporting of non-CO₂ emissions from all aircraft operators from 1st January 2025. While industry bodies are pushing for a broader review of these reporting requirements, airlines are already taking steps to prepare. They also anticipate, that over time, it will be linked to the EU ETS scheme with resultant financial implications. While the UK has yet to confirm its policy on non-CO₂ emissions, it is reasonable to expect alignment with the EU and other global players.

What role can the UK play in addressing non-CO₂ emissions?

World class capabilities exist across UK aerospace companies, operators, navigation service providers and academic institutions. We now have the opportunity to increase certainty of non-CO₂ effects and to use that information to drive targeted technology improvements. Under the NERC funded programme and with ATI support, game changing work based in the UK will make an invaluable contribution to domestic and international advancements in this area.