Beyond carbon emissions: The dual climate benefit of SAF

To understand the scale of this challenge and the potential of sustainable aviation fuel (SAF) to deliver on two fronts, we spoke with Stephen Arrowsmith, Chief Expert in Environmental Protection and Andreas Busa, Sustainable Aviation Officer at the European Union Aviation Safety Agency (EASA), and with Reetu Sallinen, R&D Associate at Neste.

The “hidden in plain sight” climate impact

While CO₂ is long-lived and accumulates in the atmosphere, non-CO₂ emissions from aircraft trigger distinct, often short-term atmospheric processes and impacts.

“Aviation’s climate impact extends beyond carbon dioxide emissions. During flight, aircraft emits a range of other substances, commonly referred to as non-CO₂ emissions,” explains Reetu Sallinen.

These include emissions like Nitrogen Oxides (NOₓ), which affect atmospheric chemistry, and soot particles that act as a basis (‘nuclei’) for ice crystals. However, a potentially significant factor in the climate impact of these emissions is the formation of persistent contrail-cirrus clouds.

“Line-shaped visible condensation trails behind aircraft form when hot, moist exhaust gases mix with cold, humid air. During this process particles present in the exhaust gas act as nucleation sites for ice particles, which are condensing from water vapor present during this mixing. Under certain atmospheric conditions, these contrails persist and spread into cirrus-like cloud cover, which can change the radiative balance creating a net-warming effect," Sallinen notes.

Measuring the magnitude

How significant are these effects? The scientific community often cites a landmark 2021 study by Lee et al., which estimated that non-CO₂ emissions account for approximately two thirds of aviation's total climate impact.

However, Stephen Arrowsmith notes that the uncertainty distributions show that the non-CO₂ terms contributed about eight times more than CO₂ to the overall uncertainty in the effects of aviation emissions on the climate. This underlines the importance of improving scientific understanding to bring down uncertainties associated with these best-estimate figures closer to the CO₂-related impact. It is also critical in order to ensure informed decision-making in the assessment and interpretation of options to reduce the overall future climate impact from aviation.

“The Lee et al. study was based on calculating effective radiative forcing (ERF) terms up to 2018, which is a backward-looking perspective on emissions. When considering the future effect of mitigation measures, the weight of impact between CO₂- and non-CO₂ emissions is very much dependent on the metric and time horizon used as well as the assumed background atmosphere conditions and future development of the aviation sector,” Arrowsmith points out.

Despite the complexity and need for further research, the consensus is clear: non-CO₂ effects are a key piece of the climate puzzle that needs to be addressed.

Building on existing policies

While the focus on non-CO₂ emissions is growing, the industry is not starting from scratch. Policies to mitigate these emissions at the source have been in place for decades.

“For instance, we have global aircraft engine emissions certification standards that include NO_X and most recently Particulate Matter,” Arrowsmith notes.

This technical oversight is core to EASA’s function. As the centerpiece of the EU's strategy for aviation safety and environmental protection, this independent agency has had an environmental remit since it was created over 20 years ago. It provides technical support for policymaking and ensures uniform standard-setting across the EU single market.

EASA’s work extends to support optimizing the entire fuel landscape. Andreas Busa notes that the agency is managing a project dedicated to ‘optimizing aviation fuels and how they could minimize the negative climate impacts’. This includes exploring ‘advanced aviation turbine fuel’ pathways, such as SAF, optimized fossil fuels and co-processed fossil fuels designed to significantly reduce fuel aromatic and sulfur content.  

The SAF advantage: Cleaner combustion

When it comes to renewable solutions, SAF proves to be a dual-impact solution to reduce climate impact. Beyond reducing lifecycle GHG emissions, SAF’s chemical composition inherently reduces pollutants that drive non-CO₂ related warming.

"Compared to conventional fossil jet fuel, many SAFs – such as Neste MY Sustainable Aviation Fuel™ – contain practically zero aromatic content,” Sallinen explains.

Aromatic content of the fuel is one of the best known precursors of fuel sooting tendency. By not containing aromates, the combustion of SAF leads to significantly lower particulate emissions, which in turn means a reduced likelihood of forming persistent contrail-cirrus clouds.

From theory to flight: ECLIF3 and PACIFIC

The benefits of SAF are being validated by cutting-edge research. The ECLIF3 project has been instrumental in investigating how 100% SAF performs in real aircraft engines, focusing on soot emissions and contrail formation.

Building on this knowledge is the ongoing PACIFIC project (Particle emissions, Air Quality and Climate Impact related to Fuel Composition and Engine Cycle). The project examines how fuel composition influences particle emissions and their climate and air quality impact.

Arrowsmith notes that these projects are vital for “gathering empirical data in terms of how different types of fuel affect emissions in order to enhance scientific understanding and impact assessments that inform decision-making”.

A holistic approach to decarbonization

EASA recently established the Aviation Non-CO₂ Expert Network (ANCEN), bringing together stakeholders from policy, science, airlines, manufacturers, air navigation service providers, fuel producers and NGOs to provide technical support and develop a common understanding on effective action within Europe and internationally to mitigate the overall climate impact of aviation.

While operational measures – such as rerouting flights to avoid regions prone to the formation of contrail-cirrus clouds – are being explored, they face challenges regarding airspace congestion and fuel burn trade-offs. SAF, by comparison, offers a ‘no regrets’ solution that delivers benefits regardless of the flight path.

"In order to achieve the future environmental goals agreed to by both states and industry, we need all stakeholders to work together to facilitate the safe uptake of SAF and its benefits, including utilizing existing infrastructure and working on the approval of 100% drop-in SAF," Arrowsmith emphasizes.

Looking ahead, the integration of the non-CO₂ benefits of SAF into climate strategy represents a maturation of the aviation industry’s efforts to reduce its climate impact.