Non-CO2 effects of aviation: Time to finally address aviation’s full climate impact

March 31, 2022

by Carlos Lopez de la Osa

When discussing the climate impact of aviation, decarbonisation receives much attention, but CO2 is only the tip of the iceberg. With the ending of Covid restrictions, we’re seeing the sky lined with the white trails of more and more aircraft. These represent non-CO2 effects such as those derived from persistent contrail cirrus and nitrous oxides, and make aviation’s total climate impact two to four times worse than the impact of CO2 alone, according to the European Union Aviation Safety Agency (EASA).

These effects have been flying under the regulatory radar for a long time, but legislators in Europe are starting to take note and are gearing up for mitigation.

T&E’s Summit on the non-CO2 effects of aviation brought together industry, policymakers and researchers, who proved that, in spite of complexity and uncertainties, we can and should act now to solve this hidden yet enormous issue, to help us unlock savings in our fight against climate change.

Non- CO2 effects: what are they, and why do they matter?

On top of CO2, aircraft engines emit other gases (NOx, SO2 and H2O) and particulate matter (soot). When emitted at high altitudes, these emissions affect atmospheric physical and chemical properties, resulting in an increase in greenhouse gases and the potential formation of persistent contrail cirrus.

The consequence is a net warming effect, which may be up to three times worse than the warming caused by aviation’s CO2 emissions.

Nevertheless, these effects are also short lived, meaning that acting against them would quickly reduce aviation’s climate impact, scoring important wins in our fight against climate change.

What has been done so far?

The first piece of evidence highlighting the importance of aviation’s non-CO2 effects came from the UN’s Intergovernmental Panel on Climate Change (IPCC) back in 1999. Since then, a lot of excellent research has been carried out to better understand the effects of these emissions and how to tackle them.

The European Commission was first tasked with addressing the non-CO2 emissions of flying in 2008, and commissioned a landmark report  to the European Union Aviation Safety Agency (EASA). The report, published in 2020, analyzed the latest available science, quantified non-CO2 climate impact of aviation as twice that of CO2, and proposed some mitigation measures.

However, when reading the EU’s 2030 climate package, published last year and known as “Fit for 55”, it’s easy to see that there is barely any mention of non-CO2 effects or the proposed policy measures to address them.

Given this context, and at a time when bold, ambitious climate action is needed, T&E’s Summit came at the right moment. It gave an opportunity for policymakers, industry and researchers to meet to discuss how to address this hidden, yet enormous issue quickly and effectively.

Setting the scene: where are we more than one year after EASA’s report

The EASA report and ongoing research has confirmed that  two thirds of aviation’s effect on climate is caused by non-CO2, with contrail cirrus alone accounting for up to 57% of the total impact. This is why we need to act now.

How? William Todts, T&E’s Executive Director, talked about the importance of gathering legislators, researchers and industry to design the policies and solutions that will allow us to solve the problem. But this will not be sufficient. Dr Marc Stettler, from Imperial College London, stressed that the aviation industry must be provided with the right incentives to tackle non-CO2 effects, and must further collaborate with research institutions, ensuring data is shared for scientific studies. Only then will we be able to accelerate the pace of action.

Can we already act now?

A number of industry representatives and researchers gave a glimpse of state-of-the-art technology and solutions to deal with non-CO2 effects. The main takeaway was clear: in spite of uncertainties and the complexity of the problem, we can start doing many things today. So why wait?

Persistent contrail formation is mainly the result of soot and other emissions on cold, high humidity atmospheric areas known as Ice Super Saturated Regions (ISSRs). But solutions exist to tackle the problem.

First, Dr. Patrick Le Clercq (DLR) explained how jet fuels with high aromatics[1] and naphthalene[2] concentrations and low hydrogen to carbon ratios increase soot formation, which in turn leads to persistent contrail cirrus. Widely sharing information on fuel properties would be essential to better understand this phenomenon and to create effective policies.

One way to reduce aromatics and naphthalene in jet fuel is to perform hydrocracking or hydrotreating[3] on fossil jet fuels, as explained by Alain Quignard, IFPEN. He added that reducing aromatics content of fossil jet fuel down to 8-10% can be achieved without significant costs and could lower non-CO2 effects significantly.

Valérie Guénon, Safran, presented the VOLCAN research project, aimed at analysing the effects of low aromatics jet fuel on aircraft engines and systems, and ensuring the highest levels of aircraft safety are maintained when using these fuels, whilst reducing non-CO2 effects as well.

This great potential in reducing aromatics has not gone unnoticed by legislators. Jutta Paulus, MEP for the Greens, talked about the use of Sustainable Aviation Fuels (which often have no aromatics) in the mid to long term, and short-term solutions such as mandating reduced aromatics on fossil jet fuels.

On top of reducing aromatics and naphthalene to reduce soot, avoiding areas with particular climatic conditions, known as  Ice Super Saturated Regions (ISSR), is a key axis to reduce non-CO2 effects. Laurent Laluque (Thales), and Guillaume Beurnaux, from the airline Amelia, described how they are doing this today. Thales’ scientific models provide an estimate of the total climate impact of a flight, including the impact of ISSR and strategies to avoid them. This information is then passed on to the airline Amelia, which uses it to plan its flight routes and minimise persistent contrail cirrus formation.

Air Traffic Control also has a role to play in the avoidance of ISSR, according to Ilona Sitova, Eurocontrol, who shared the results of the trials over the Maastricht Upper Area Control Center (MUAC), reiterating the need for more research in order to make better informed decisions.

But of course, all of such solutions then need to be brought into legislation. Olaf Hölzer-Schopohl, from the German Environment Agency UBA, detailed how different metrics can be developed to integrate non-CO2 effects in the EU’s carbon market (the EU ETS), thus incentivising airlines to tackle these effects.

Bringing solutions to life

A big challenge remains. How can we successfully integrate existing and future solutions in the EU’s climate package?

First of all, monitoring, reporting and verification (MRV) of non-CO2 effects must be put in place, as suggested by Jane Amilhat, from the European Commission’s DG RTD. This should be backed up by political dialogue to decide on fundamental aspects such as metrics to better quantify non-CO2 impact on climate, said Jesper Van Manen, from the Dutch government.

To conclude, S&D MEP Milan Brglez stated that no tangible measures to deal with aviation’s non-CO2 effects can be seen in the Fit for 55 package, despite solid evidence accumulated during 15 years to give legislators a push in the right direction. The proposal to mandate SAF use is welcome, given the reduced aromatics, but uptake will be too slow given the scale of aviation’s climate problem.

Aviation’s climate impact is not fully captured, and we must prevent these non-CO2 effects from being swept under the carpet for any longer. Solutions do exist, research should be sped up, and legislation such as the EU ETS and ReFuelEU are well placed to solve this issue.

What’s next?

The summit on the non-CO2 effects of aviation was a unique opportunity to review the current situation and solutions. Policymakers, industry representatives and researchers agreed on the need for further research to better understand the subject, and on the benefits of mid-/long-term measures such as the use of SAFs. However, differing views arise on the way forward for short-term and policy solutions. While some actors argue for a cautious approach, others defend the need to act now.

Uncertainties exist, and will do for decades. However, as pointed out by Clara de la Torre (DG CLIMA), uncertainties should be taken as risks to be assessed when designing policies, rather than preventing policy action.

As a first step, several measures could be implemented in the FF55 package, with ReFuelEU mandating the reduction of aromatics and naphthalene in fossil jet fuel, and the addition of non-CO2 effects in EU ETS. Beyond policy, funding for research and the development solutions must be increased.

Dealing with non-CO2 effects can be perceived as a complex task, but flying an aircraft safely and reliably is no easy feat either, and the aviation industry has made it possible every day. The European aviation sector must embrace this innovative, can-do attitude, and lead the world in fighting the hidden part of the iceberg of aviation’s climate problem. The planet and all of us have a lot to gain.


[1] Aromatics are cyclic hydrocarbons contained in jet fuel. Jet A specifications require a maximum content of 25%, with typical concentrations ranging between 10 and 20%. Although there is no minimum requirement, a de facto minimum limit of 8% exists for safety reasons.

[2] Bicyclic hydrocarbons present in jet fuel, with maximum concentration of 3% as per Jet A specifications, and typical concentrations around 1-3%.

[3] Hydrocracking, hydrotreating: refinery processes that use hydrogen to alter the chemical composition of hydrocarbons in fossil fuels, for example the reduction of aromatics and naphthalenes concentrations in fossil kerosene.

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