With the EU about to activate its sustainable aviation fuel blending mandate, the European Union Aviation Safety Agency (EASA) has released an assessment of Europe’s preparedness to deliver required volumes of SAF up to 2030. The blending requirement will escalate from a minimum 2% of jet fuel composition by the end of 2025 to 70% in 2050 for supplies dispensed at airports across the EU’s 27 member states. EASA concludes that by 2030, when the blending mandate reaches 6%, there will be sufficient European production of SAF through multiple pathways to meet the requirements of the ReFuelEU Aviation regulation, which governs the mandate. But it warns rapid action is needed to ensure that a sub-target, initially 0.7%, is achieved for the use of increasingly important synthetic aviation fuels, or e-fuels.

The EASA report, titled ‘State of the EU SAF market in 2023’, examines the capacity of member states to produce the new fuels during the next five years, based on an assessment of the sector’s performance in 2023 and the addition of updated projections. The report, EASA’s first on this subject, also provides real or estimated reference prices for multiple types of current and future aviation fuels and outlines emerging trends in European SAF production.

“This first report on SAF provides a comprehensive analysis and valuable insights to the potential of sustainable aviation fuels for commercial airline operations in Europe,” commented Maria Rueda, EASA’s Strategy and Safety Management Director. “It will be a key component on the journey towards a more sustainable and environmentally friendly aviation sector.”  

EASA says the minimum SAF volume required by 2030 under the ReFuelEU Aviation (RFEUA) programme is around 2.8 million tonnes based on forecast jet fuel consumption of 46 million tonnes and a mandated blending level which, by then, will be at least 6%.

But it qualifies that the SAF production market is “inherently volatile”, impacted by factors including high capital expenditure, feedstock supply chain limitations and the risks to investors of supporting technologies in their early stages. “While many projects are announced,” adds EASA, “some may not reach commercialisation.”

The report presents three scenarios for SAF production in EU countries: Operating, Realistic and Optimistic.

The ‘Operating’ scenario covers only those facilities currently producing SAF, which are expected to deliver just over 1 million tonnes of product by 2030.

The ‘Realistic’ assumption estimates 3.2 million tonnes of SAF will be produced and distributed by facilities already operating, under construction or with small pilot plants either activated or being built. This includes co-processing of SAF in existing refineries.  

The ‘Optimistic’ case predicts 5.5 million tonnes, including pipeline production from projects which have announced elements including technology, feedstock, SAF capacity, commissioning year, location and technical partners, but are yet to be built.

“The Realistic case led to the exclusion of facilities which had not gone through final investment decision (FID) at the time of assessment,” says the report, singling out e-fuels as an example.

“There is a strong pipeline of synthetic aviation fuel projects in the EU, estimated at 1.1 million tonnes by 2030, but at the time of assessment none of these facilities had gone through FID. They were therefore not included in the realistic capacity estimation.”

The report says the hydrotreated esters and fatty acids (HEFA) process, which encompasses converted vegetable oils, waste oils, greases and fats, is the major SAF production pathway, supported by additional supplies through co-processing facilities.

It adds that immature technology for other SAF production pathways including Alcohol-to-Jet (AtJ) and Fischer-Tropsch (FT) prevented them from delivering commercial volumes of the fuel during the study reference year, 2023.

As well, says EASA, the AtJ process of converting alcohols into SAF disqualifies the fuel in the EU because the alcohols are fermented from food crops including corn and sugarcane. 

Additional pathways, including Sun-to-Liquid (StL) and Hydrothermal Liquidation (HtL), are also under development. “However,” says the report, “they remain immature, with only a couple of pilot plants announced, and their contribution to commercial SAF volumes is projected to be negligible by 2030.”

Synthetic aviation fuel is required to comprise at least 1.2% of jet fuel makeup by 2030, which EASA calculates to be a 600,000-tonne requirement. Also known as Power-to-Liquid, or PtL, this process combines water with renewable electricity to extract green hydrogen, which is then combined with captured CO2 to create SAF and other products such as renewable diesel.

“Within the EU,” says the EASA report, “more than 15 synthetic aviation fuel production facilities have been announced, primarily in countries with considerable renewable electricity capacity or infrastructure.”

However, it adds, the requirement for large volumes of renewable electricity, the costs of producing the power and limited sources of eligible carbon in many key locations drives up the costs of e-fuels.  

“The resulting production price is currently non-competitive with other forms of SAF production, particularly HEFA. Therefore, the development of synthetic aviation fuels is likely to be driven by the RFEUA sub-mandate that requires their supply.”

Of eight current or future aviation fuel categories, the report lists market prices or production cost estimates in 2023, reflecting vast premiums for non-fossil product. EASA used price reporting agency (PRA) references to provide benchmarks for price assessments or a basis for estimates.

“Whereas 2023 prices for conventional aviation fuels and aviation biofuels were available through multiple PRA indexes,” said EASA, “the market for other RFEUA aviation fuel types was either non-existent or not yet liquid enough to determine actual reference market prices for 2023. For these fuel types, a bottom-up production cost estimation was developed to provide indicative results.”

The report listed the average 2023 market price of conventional aviation fuel at €816 ($850) per tonne and aviation biofuels at €2,768 ($2,880) per tonne, the latter with an estimated production cost of €1,770 ($1,840) per tonne.

Because none of the other fuel categories had a market price in 2023, production cost estimates were produced by EASA.

Production costs for recycled carbon aviation fuels were estimated to average €2,125 per tonne, while advanced aviation biofuels averaged €2,675 per tonne, low carbon hydrogen for aviation averaged €4,700 per tonne, synthetic low carbon aviation fuels averaged €5,300 per tonne and renewable hydrogen for aviation averaged €6,925 per tonne.

But by far the largest estimates were those for synthetic aviation fuels, the average production cost of which EASA lists as €7,500 per tonne.

The same average estimate – €7,500 per tonne – also applied to synthetic fuel produced with CO2 captured at the industrial source of emission or that made from biogenic CO2.

The most expensive production cost estimated was for fuel produced with CO2 captured from the atmosphere, averaging €8,225 per tonne – more than 10 times the 2023 average market price of conventional jet fuel.

“To be able to meet the synthetic aviation fuel minimum shares, announced synthetic aviation fuel facilities would need to reach FID within the next couple of years,” says the EASA report.  

“On top of this, continuous scale-up in SAF capacity would be needed to comply with RFEUA by 2035, as the minimum SAF share required increases from 6% to 20% by that date.”

The report, developed with the support of consultancy ICF, is a precursor to a first EASA annual technical report due in 2025.

Photo: OMV

Tony Harrington

Correspondent

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Although research is ongoing, it is widely accepted aviation’s contribution to global warming goes well beyond that caused by carbon dioxide emissions alone. One of the main non-CO2 impacts is the net warming effect from the formation of contrails and contrail cirrus caused when aircraft engines emit particulates (soot) at altitude in ice-supersaturated regions. The main culprit is the aromatic content of jet fuel, and fuels with a higher concentration of aromatics and especially naphthalene, a bicyclic aromatic compound, cause higher particulate emissions because aromatics burn slower than other hydrocarbons. Sustainable aviation fuels, on the hand, have negligible concentrations of aromatics because they are hydrotreated. Their use therefore could have beneficial consequences, but they are currently only permitted in blends up to a maximum of 50% with conventional fossil jet fuel so the aromatics issue remains. A report by CE Delft for the Dutch government proposes the aromatic content of jet fuel be monitored or controlled within the proposed ReFuelEU Aviation SAF regulation in such a way that it is decreased to ensure the non-CO2 climate impact of aviation is reduced.

A report on non-CO2 impacts by European aviation agency EASA in 2020 identified lowering the aromatic content of jet fuel as a way of reducing the sector’s climate impact and analysed several policy options but concluded current uncertainty and lack of information about aromatic content was a significant barrier to monitoring the effectiveness of such a policy.

Under current jet fuel standards, the aromatic content is limited to a maximum of 25% by volume and a minimum of 8%. In 2011, a jet fuel study for Lufthansa analysed around 2,000 individual batches of jet fuel. About 75% of the batches had an aromatic content between 16% and 20%, with 15 batches analysed at lower than 8%. From analysis of literature and data supplied by a major fuel supplier, the CE Delft research found that a large share of jet fuel sold in Europe has an aromatic content of 15 to 20% by volume. Other data on batches of Jet A-1 produced in a number of European and North American refineries showed a variety between 8% and 25%, with an average of 18.7% aromatics. This is all within the allowed range as set by jet fuel standards, notes the CE Delft report.

The requirement for a minimum 8% content of aromatics has historically been applied for safety considerations, stemming from the role aromatics play in the swell of sealings in the aircraft fuel system. However, the report found contradicting opinions on this, with several modern aircraft and engine types having sealing materials that do not require aromatics for the swell function, and research into new materials is ongoing.

“Moreover, in practice, aircraft sometimes use fuels with an aromatic content of less than 8%, probably unknowingly,” it adds. “All in all, there is consensus on the requirement for an upper limit of aromatics but there is no consensus in the industry for the necessity of a minimum level aromatics in jet fuel. Other than on financial grounds, there are no clear reasons not to reduce the upper limit. Further research is needed to confirm or deny the role of the minimum content of aromatics for their lubrication purpose in aircraft.”

The CE Delft study carried out interviews with industry experts and parties involved in the production and supply chain of jet fuel. It found a gradual reduction in the upper limit would be a possible way to decrease the aromatic and naphthalene content in jet fuel, as some parties indicated the jet fuel market might experience difficulties if a strict or abrupt reduction of the upper limit was enforced. Because of the financial and time investment this would require by refineries, a reduction of aromatics content might lead to a higher cost of jet fuel, although, suggests the report, this increase could contribute to closing the gap in price difference between sustainable aviation fuel and conventional fossil jet fuel.

As they are conforming to jet fuel standards, from the refinery perspective, there is currently no incentive, financially or legally, to modify the production process to diminish the content of aromatics, sulphur and naphthalene in conventional jet fuel, it points out. Airlines might be interested because jet fuel with lower aromatic content has higher energy density and therefore slightly less jet fuel is needed for flying a given distance, and it also reduces their environmental footprint, but likely it would come at a higher fuel price.

Even if fuel suppliers charged airlines for the higher cost, airlines may choose to refuel at other airports in nearby markets. However, says the report: “If the EU were to put a lower maximum aromatics limit for all EU airports in a regulation, with a proper monitoring, reporting and verifying system, there would be little room to bypass the requirements.”

The study analysed how a new system could be set up to monitor aromatic and naphthalene concentrations in aviation fuels, which would allow for the assessment of the impacts of policies like ReFuelEU Aviation on contrail formation and also provide for better estimates of aviation non-CO2 impacts in general, so “filling the data gap that currently exists”.

Under ReFuelEU Aviation, the European Commission is proposing mandatory supply and use of sustainable aviation fuel. SAF could require new standards for aromatic and sulphur content, even if just to confirm the same standards that kerosene has today, suggests the report.

“Monitoring of aromatic and naphthalene content can be internalised with the reporting of SAF shares by the fuel suppliers,” it says. “This approach involves the lowest possible additional cost for the sector and for authorities to monitor, process data and verify. However, the exact design of such a monitoring and reporting system should follow policy or legal requirements for SAF or a lower aromatic content in jet fuel blends.”

The study also looked at how new aviation fuel specifications to reduce contrail formation could be drawn up in the EU and whether the jet fuel ASTM and DEF standards could be set under EU legislation. The report argues there is no EU or international law that would prevent the EU establishing a new standard for aviation fuel, implemented by either a new body or delegating it to an existing body like EASA, to reduce contrail formation and/or air pollution.

“ReFuelEU Aviation offers a great opportunity to establish a system for monitoring the aromatic content of jet fuel, which will increase our understanding of non-CO2 climate impacts and be a building block for addressing contrail formation,” the lead author of the CE Delft report, Jasper Faber, told GreenAir.

However, Adam Durant, CEO of Satavia, whose company is developing weather prediction and navigational avoidance technology designed to help aircraft operators avoid the risk of causing contrails in the first place, argues that while widely-available sustainable aviation fuels are definitely required to decarbonise aviation, the science linking exhaust emissions soot particles to contrail lifetime is still evolving.

“How long is it going to take to scale SAF production – 10, 20, 30 years, or even longer?” he questions. “Contrails management can be done at full scale within the next two years with adequate support. It is important to separate the two issues as SAF is not going to solve contrails.”

Photo: DLR

Europe’s aviation regulatory authority EASA has for the first time certified an aircraft – the Airbus A330-900 – to ICAO’s new CO2 emissions standard, which was adopted by the ICAO Council in 2017 and implemented into the EASA Basic Regulation in July 2018. The certification process provides an assessment of an aircraft’s fuel efficiency and therefore of the CO2 it emits while in operation. Under a complex formula, the fuel efficiency in cruise flight is certified, which is influenced by the engines as well as the aircraft’s aerodynamic characteristics and weight. The standard, which is to be adopted by certifying airworthiness authorities worldwide, complements existing aircraft noise and engine emissions standards and will be applicable both to new type-certified aircraft and to in-production aircraft, with all aircraft being produced needing to comply by January 2028. The A330neo – comprising the A330-800 and the extended fuselage A330-900 and incorporates the latest generation Rolls-Royce Trent 7000 engine – was launched in 2014 and received its type certification from EASA in 2018. Airbus said its other airliners will receive CO2 certification in due course and conform with the 2028 deadline requirement.

“Airbus is proud to be the first commercial aircraft manufacturer to receive EASA certification for ICAO’s new CO2 requirement,” commented Simone Rauer, Airbus’ Head of Environmental Roadmap. “ICAO standards are important elements of the global ICAO action plan to regulate emissions from aircraft and engines, and to help ensure a level-playing field in the industry. For the A330neo in particular, the award demonstrates this aircraft meets ICAO’s environmental regulations beyond 2028.”

The aircraft manufacturer said it would learn from being the first to apply for the certification requirement and that the standard was “technology-encouraging” as it was based on demonstrated and proven technologies which could be inserted into the next generation of aircraft. It added the CO2 standard was an “important new brick” to complement a range of existing ICAO measures that help to control and curb emissions attributed to civil aviation.

The ICAO CO2 standard defines the certification of a CO2 emissions Metric Value that comprises two main components: a fuel burn component (called Specific Air Range, or SAR) measured at three different masses during the cruise and then averaged, and a non-dimensional component (called Reference Geometric Factor, or RGF) representing the cabin ‘useful’ surface from the cockpit door to the aft end of the pressurised area. The resulting CO2 emissions Metric Value is expressed as kilogrammes of fuel per kilometre.

The complexity of the metric is because of the way in which an aircraft is operated, said EASA. Fuel consumption depends not only on speed but also on the flight altitude and on the weight of the aircraft, which is higher in the beginning of the cruise phase than towards the end of it, as fuel is burned during the flight, it explained.

EASA described the new certification as a key milestone on its roadmap to establish an environmental label for aviation by 2022. Amongst other values, the label will use CO2 emissions data to provide a comprehensive assessment of the environmental performance of an aircraft, it said.

“This is a new and important factor for environmental certification in light of the global efforts to decarbonise the aviation industry,” said EASA Executive Director Patrick Ky. “There is still a long way to go to reach this goal but every step is important in demonstrating that aviation is moving determinedly towards that objective.”

Airbus voluntarily applied for the CO2 certification of the A330-900 following a call from EASA in late 2019. EASA said that based on various expressions of interest to date, it anticipated more manufacturers will be seeking early CO2 certification in the immediate future.

While the ICAO CO2 standard was implemented into the EASA Basic Regulation nearly three years ago, the United States aligned its own regulations with ICAO’s only last December when the Environmental Protection Agency (EPA) finalised greenhouse gas emissions standards for airplanes used in commercial aviation and large business jets. Adoption of the standard faced strong criticism from US environmental groups, which perceive it to be ineffective and not designed to result in reductions of aviation emissions. In January, groups represented by Earthjustice plus a number of states started legal proceedings against the outgoing Trump administration challenging the adoption of the standard. The EPA and industry argued that without adoption, the US FAA would be unable to certify an aircraft, placing US aircraft manufacturers at a competitive disadvantage (see article).

Photo: Airbus A330neo

The aviation sector’s efforts to mitigate anthropogenic climate change have predominantly focused on CO2 emissions. However, the non-CO2 effects created by aircraft flying at cruise altitude have been found to be larger than the impact from CO2 alone, and estimated at two thirds of the net radiative forcing from aviation, but this has been largely unaddressed by the airline industry and policymakers. This is mainly due to uncertainties over the scientific understanding through a lack of reliable data and how best to overcome the effects through aircraft operations, as well the inability to agree on what policy measures should be introduced to deal with the issue. Susan van Dyk reports on a recent conference hosted by the Royal Aeronautical Society’s Greener by Design Specialist Group, in collaboration with German aviation research organisation DLR, which presented the latest scientific studies on non-CO2 impacts and potential strategies and policies for mitigation.

Non-CO2 climate effects come from NOx emissions, soot from fuel combustion, water vapour and formation of persistent contrails (contrail cirrus) which contribute to increased cloudiness. Unlike CO2 impacts, non-CO2 effects depend on the location and time of emissions and this plays a role in selecting suitable mitigation steps. The climate impact of aviation from non-CO2 effects can be mitigated through various measures, including new engine technologies to reduce emissions, use of sustainable aviation fuel and air traffic management (ATM) strategies.

Contrail cirrus formation has the largest positive net (warming) effective radiative forcing (ERF) effect, followed by CO2 and NOx emissions. Contrail cirrus alone contributes >50% of the net ERF impact and is therefore an important target for mitigation of non-CO2 effects. Contrails are formed by aircraft through condensation of the water vapour from fuel combustion. While most contrails disappear relatively quickly, persistent contrails form cirrus clouds under certain temperature and humidity conditions. The cirrus clouds have some cooling effect by reflecting the sunlight, but also has a warming effect by trapping heat radiating from the earth’s surface.

Mitigation through flight diversion aims at avoidance of ice supersaturated regions (ISSR) where contrail cirrus formation has the greatest impact. The net warming effect of contrail cirrus is also greatest between 3pm until 6am and flight diversion is focused on this time period. Contrail avoidance through diversion of flights and ATM measures could represent a fast way (hours or days) to reduce the impact of aviation on the earth radiation budget.

Flight diversion and contrail avoidance is the current focus of live trials in the Maastricht Upper Area Control (MUAC). Dr Rudiger Ehrmanntraut, Senior Project Manager at Eurocontrol, explained that vertical diversion, 2000 ft above the ISSR or 2000 ft below the ISSR during the time period of 4pm to 6am, is the main approach during this project to avoid persistent contrail formation. A crucial prerequisite, he said, is the accurate prediction of ISSRs and potential areas of persistent contrail formation using meteorological data.

Dr Marc Stettler from Imperial College London has been carrying out research on the extent of flight diversion that will be required. His work has demonstrated that only 2.2% of flights account for 80% of the total net energy forcing due to contrails. Therefore, total fleet diversion is not necessary to achieve a significant impact, he argued. By diversion of 1.7% of flights, contrail radiative forcing can be reduced by 59%. If flight diversion is used simultaneously with cleaner engines, the contrail radiative forcing can be reduced by 92% (which amounts to 57% of the total radiative forcing).

However, flight diversion increases fuel consumption and CO2 emissions. Professor Ian Poll from Cranfield University quantified this fuel penalty for avoiding ISSRs and concluded the overall extra fuel burn was insignificant. If an aircraft is diverted 2000 ft above the ISSR, there is a 1.5% increase in fuel, and 1.4% when diverted 2000 ft below. However, this fuel penalty is only for the duration of the diversion, not the whole flight. A reduction in altitude results in a reduction in NOx emissions as an added benefit.

A significant contributor to formation of persistent contrails is the creation of soot as a result of fuel combustion. Studies at DLR by Professor Christiane Voigt’s research group show that 80% of soot particles form ice particles, which leads to persistent contrails. Lower soot formation will impact contrails and contrail cirrus formation and reduce some of the climate impacts of non-CO2 emissions. Reduction of soot particles causes a non-linear decrease in radiative forcing and it has been demonstrated that a 50-70% reduction in ice particles leads to 20-40% reduction in contrail radiative forcing. According to Voigt’s research, the main cause of soot formation is the aromatics in jet fuel, so reducing soot emissions can be achieved by decreasing the aromatic content. She said an effective way of reducing the aromatic content is through the use of sustainable aviation fuel (SAF). Currently available SAF, based on hydrotreatment of fats, oils and greases (called HEFA), is almost entirely composed of paraffins and cycloparaffins with virtually zero aromatics.

ASTM jet fuel specifications require aromatics at a minimum of 8.5% by volume and a maximum of 25%. A minimum aromatic content is considered essential to maintain seal integrity in the engine and aircraft and prevent seals from leaking. Current certification under ASTM D7566 only permits a 50% blend of HEFA with conventional fossil jet fuel and the blend must contain at least 8.5% aromatic content by volume. An earlier study, published in Nature in 2017, was carried out with a 50% blend of HEFA and demonstrated a reduction in the particle number and mass emissions immediately behind the aircraft by 50-70%. It concluded the effect was mainly due to reduced aromatics content and reduced soot particle formation.

Recent research as part of the ECLIF3/ND-MAX campaign, a collaboration between DLR and NASA, has also confirmed a strong reduction in soot emission indices by using low aromatic fuels. According to Voigt, her research has shown naphthalenes are more efficient soot precursors compared to monocyclic aromatics and decreased naphthalene content decreased the soot formation. She emphasised that stronger reductions in aromatics in jet fuels are needed to reduce persistent contrails and this will require changes in certification standards to permit fuel with lower than 8.5% aromatics. In addition, certification should also permit the use of 100% alternative jet fuels rather than the current maximum of 50%.

Rolls-Royce and Airbus recently announced what they say is the world’s first in-flight emissions study using 100% sustainable aviation fuel (see article). The ongoing Emission and Climate Impact of Alternative Fuels (ECLIF) project studies the impact of 100% SAF on aircraft emissions and performance. Paul Madden, Energy Emissions Expert from Rolls-Royce, told the conference there had been no operability issues or seal leakage problems as a result of using 100% SAF in the aero engine manufacturer’s engines, although he conceded this may not be the case in older engines.

Madden also explained some of the modified engine operations that Rolls-Royce is implementing, such as the lean burn engine, which can improve NOx emissions and soot formation. NOx emissions are not reduced through the use of SAF, and cleaner burning engines will be the main approach for mitigation.

Stephen Arrowsmith, Chief Expert on Environmental Protection at the European Aviation Safety Agency (EASA), highlighted the potential policy measures that could be implemented to address non-CO2 impacts as published in a recent EASA report (see article). The EU Emissions Trading System (EU ETS) currently only addresses CO2 emissions through policy.

The report divided policy options into three categories: financial/market-related, fuel and ATM. Financial measures suggested include a monetary charge levied on aircraft NOx emissions on one side and/or the inclusion of such emissions under the EU ETS on the other. Potential fuel-related measures could encompass the reduction of aromatics within fuel – leading to cleaner fuel burn and reduced nvPM emissions – and the mandatory use SAF. Both measures would target soot emissions and associated formation of contrail cirrus clouds.

Proposed measures in the ATM category are avoidance of ice-supersaturated areas and a climate charge. Flight diversions to avoid ice supersaturated areas would reduce the formation of contrail-cirrus clouds, while a climate charge would address all non-CO2 effects (NOx, water vapour, soot, sulphates, contrails). Several research issues would have to be addressed before these measures could be implemented, said Arrowsmith. Fuel-related measures could potentially be implemented in the short term (2-5 years), while other measures will likely take longer to implement (5-8 years) due to outstanding research questions, he believes.

The conference succeeded in highlighting the important, but somewhat overlooked, climate impact of non-CO2 effects from aviation. It demonstrated this impact can be significantly mitigated through modified air traffic management and use of sustainable aviation fuels with low aromatic content.

Photo: NASA’s ‘airborne laboratory’ flies close behind the DLR A320 Advanced Technology Research Aircraft (ATRA), flying through the Airbus’ exhaust plume. On board, scientists measure the composition of the exhaust stream and analyse the effects of biofuels like HEFA on the formation of soot particles and ice crystals (credit: DLR/NASA/Friz)

About the author

Susan van Dyk

A Research Associate at the University of British Columbia in Canada, Susan is a scientist and independent researcher specialising in biofuels and renewable energy, with a particular focus on sustainable aviation fuels. She has published numerous peer-reviewed papers and been the author of multiple reports on drop-in biofuels and SAF. She has worked on a number of Canadian biojet initiatives. Susan can be contacted through her website https://www.svdconsulting.ca/

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Tasked by the European Commission to conduct an update on the non-CO2 effects of aviation on climate change, Europe’s regulatory agency EASA has issued a report that highlights the latest understanding of the science and suggests technological, operational, policy and financial tools to address the issue. In addition to CO2, aircraft emit a wide variety of gases and aerosols at cruising altitude that influence climate directly and indirectly. The analysis confirms their significance is at least as important as those of CO2 alone, although the complexity of measuring non-CO2 climate impacts, together with the uncertainty regarding trade-offs between the various impacts, makes targeted policy development in this area challenging, say the authors. However, potential policies suggested include a levy on aircraft NOx emissions and/or the inclusion of such emissions under the EU ETS, and mandatory use of cleaner burning sustainable aviation fuels.

Steve Arrowsmith, Chief Expert for Environmental Protection at EASA, who led the project, told a webinar organised by NGOs Carbon Market Watch and Transport & Environment there had been past studies of the topic in 2006 and 2008 that primarily focused on NOx emissions but it was considered the scientific understanding was not sufficiently mature to propose policies to address non-CO2 impacts. He said the understanding had evolved considerably over the past decade, including on some new effects, although there remained significant uncertainties with regard to the magnitude of these impacts.

The report was compiled by renowned climate science, technology, ATM and policy experts from the EU, Norway and the UK. The non-CO2 climate impacts assessed arise from aircraft engine emissions of oxides of nitrogen (NOx), soot particles, oxidised sulphur species and water vapour. The chemical and physical processes can lead to contrail and contrail cirrus impacts in particular local atmospheric conditions, and complex impacts arising from NOx and particulate matter (PM) emissions during cruise.

The net impact of aviation non-CO2 emissions is a positive radiative forcing (warming), although there are a number of individual positive and negative (cooling) forcings, for which large uncertainties remain. The largest aviation non-CO2 impacts that can be calculated with best estimates are those from net-NOx (NOx is not a climate warming agent per se but its emission results in changes in the chemical balance of the atmosphere to ozone and methane which have radiative impacts, quantified as a net-NOx effect) and contrail cirrus, both of which have significant uncertainties in their magnitude, particularly contrail cirrus. Contrails predominantly cool if the sun is close to the horizon and warm if the sun is high in the sky. However, they exclusively warm at night, thereby resulting in a net positive (warming) radiative forcing.

The scientific community has adopted Effective Radiative Forcing (ERF) as a better metric of an absolute impact when compared to Radiative Forcing (RF) as it shows better proportionality to changes in global mean surface temperature response, particularly for short-lived climate forcing agents such as clouds and aerosols. The usage of ERF rather than RF is potentially significant for aviation NOx and contrail cirrus impacts. Aviation ERFs are less well quantified than RFs for net-NOx impacts but better quantified for contrail cirrus forcing effects.

Research shows the ERF from the sum of non-CO2 impacts yields a net positive (warming) that accounts for more than half (66%) of the aviation net forcing in 2018. However, in the same year, the uncertainty distributions showed that non-CO2 forcing terms contributed about eight times more than CO2 to the overall uncertainty in aviation net forcing.

While the confidence level on the magnitude of the impact of NOx remains low, the current understanding is that NOx still has a net positive climate forcing effect. However, say the scientists, if surface emissions of tropospheric ozone precursors (NOx, CO, methane and non-methane hydrocarbons) decrease significantly in future and aviation emissions increase, it is possible that the net aviation NOx ERF will decrease, or even become negative (i.e. cooling), even with increasing total emissions of NOx.

“This highlights one of the problems of formulating NOx mitigation policy based on current emissions/conditions,” says the report.

Emissions multipliers

A major scientific and policy challenge also remains comparing long-lived aviation CO2 emissions with short-lived non-CO2 emissions and their impacts on a common scale. CO2 has multiple lifetimes in the atmosphere because of different timescales but a significant fraction – around 20% – accumulates and remains in the atmosphere for millennia.

The CO2 equivalent emissions metric (CO2-e) that is currently widely used, including within the EU ETS for stationary installations, is the Global Warming Potential for a time-horizon of 100 years (GWP100). This metric estimates an overall CO2 multiplier of 1.7 to account for future impacts of aviation non-CO2 emissions. To address the challenge, the scientific community has proposed a number of alternatives to the GWP100, including the Global Temperature change Potential (GTP) metric, which estimates a 1.1 multiplier.

The report points out that there is no exclusively correct choice of an equivalent emissions metric as the choice depends on the policy (for example whether it is a temperature target or an emissions reduction target) and the subjective choice of the time horizon of interest.

The simple approach of applying a multiplier to account for the climate effects of non-CO2 emissions – for example a net GWP100-based multiplier – averaged across the aircraft fleet and all atmospheric conditions may not be appropriate, say the experts. Also, they argue, the use of the multiplier does not incentivise reductions of non-CO2 emissions independently of CO2 emissions, neither at the global/regional fleet level nor on an individual flight-by-flight basis.

Another option would be to calculate the total climate impact of individual flights and then determine the CO2 equivalent emissions on a flight-by-flight basis. Such equivalents could be used as the basis for a policy instrument but, says the report, once again the magnitude of the equivalency depends on the choice of metric and time horizon.

The report states that a relatively new application of the GWP, referred to as GWP*, produces a better temperature-based equivalence of short-lived non-CO2 climate forcers by equating an increase in the emission rate of a short-lived climate forcer (SLCF) with a one-off ‘pulse’ emission of CO2. The GWP* is an example of a flow-based method that represents both short-lived and long-lived climate forcers explicitly as ‘warming-equivalent’ emissions that have approximately the same impact on the global average surface temperature over multi-decade to century timescales. Based on this method, the indication is that aviation emissions are currently warming the climate at around three times the rate of that associated with aviation CO2 emissions alone.

“It could be argued that temperature-based metrics, and the GWP*, are potentially more useful for temperature-based policy objectives, such as the temperature targets of the Paris Agreement. They also provide a more physical basis of actual impacts than GWPs for SLCFs,” says the report.

However, it adds: “This report does not recommend one specific metric or choice of time horizon. These choices partly depend on the suitability of the metric to a particular mitigation strategy and partly upon the user’s choices, which may be influenced by socio-economic factors, such as equity valuation.”

Non-CO2 mitigation

Technological or operational measures to mitigate aviation’s non-CO2 impacts that involve a reduction of a SLCF, such as NOx or contrail cirrus, are covered by the report. Because they can result in increased CO2 emissions, however, measures need to be considered carefully to ensure the net impact is beneficial, it cautions. “The ratio between benefits and disbenefits will change with the time horizon being considered but a reduction in SLCFs might make it easier to achieve climate change targets in the next decades and up to a century.”

Avoiding contrail cirrus-forming in ice-supersaturated regions of the atmosphere is an example of an operational measure to reduce the climate impact of aviation. There is some evidence, say the experts, that most of the total forcing comes from a few events where formation is large and long-lasting – sometimes referred to as ‘big hits’. Flights impacting these events should be targeted for avoidance, rather than all flights, and research into reliably forecasting ‘big hits’ should be undertaken. Avoidance of ice-supersaturation regions requires accurate prediction at least 24 hours in advance and meteorological forecast modelling needs to be improved as the capability to forecast persistent contrails is limited, says the report. The potential impacts of trade-offs from increased CO2 emissions as result of flight re-routings also need to be more thoroughly understood to ensure ‘no regret’ policies, it adds.

Reducing contrail cirrus impact, as well as improving air quality, could also be met without modification of flight trajectories or incurring an additional fuel consumption/CO2 penalty by reducing soot particle emissions. This could be achieved by using low-carbon sustainable aviation fuels (SAF). The reduction in the use of aromatics in fuel is seen as an important mitigation measure to reduce non-CO2 aviation emissions. SAF has shown a reduction in non-volatile particulate matter (nvPM) emissions in landing and take-off (LTO) operations and cruise due to their lower aromatic and sulphur content.

The report says there is scope for improving emission characteristics through the hydrotreatment of conventional fossil fuels to reduce aromatics and sulphur although the extra costs and energy requirements would need to be examined in order to balance the differential environmental benefits.

The global aircraft fleet NOx performance, in terms of certified data, is likely to improve as older high-NOx engine designs are replaced with new engine combustion technologies, with NOx emissions on a per passenger kilometre basis expected to show a reduction over time, although significant reductions may be limited. Levels of nvPM emissions are likely to improve as engines with technology designed for NOx control enter the fleet, although technologies to mitigate nvPM are less well understood than NOx. Beyond 2040-2050, hybrid-electric aircraft and revised configurations could offer significant reductions in NOx emissions.

Non-CO2 emissions charging

In the meantime, potential policy options to reduce non-CO2 climate impacts could include a NOx charge or the inclusion of aircraft NOx emissions in the EU ETS.

A charge on NOx would cover total NOx emissions over an entire flight and calculated using certified LTO NOx emissions data, the distance flown and a factor accounting for the relation between LTO and cruise emissions. The report cites a 2009 legal analysis that suggested neither ICAO’s Chicago Convention nor its recommended policies on taxes and charges prevented the implementation of such a measure. The charge would incentivise engine manufacturers to reduce LTO NOx emissions during their design process and airlines to minimise NOx emissions in operation, while taking into account associated trade-offs. Further research and monitoring is still needed on the climate impact of aviation NOx, caution the experts, but if there is the political will to take the option forward then they suggest the measure could potentially be implemented in five to eight years.

Incorporating aviation NOx emissions into the EU ETS would also take five to eight years and the same caveats over research and the uncertainty about the issue would apply, says the report, along with the incentive by manufacturers and airlines to reduce NOx. As existing EU ETS legislation uses the GWP100 metric to convert other greenhouse gases to CO2 equivalents for stationary installations, so it is assumed this would be the metric applied to the aviation sector. The measure could be implemented by adjusting the existing legislation and building on existing administrative processes and precedents, for example baseline, cap, auctioned allowances and MRV and accreditation. The same EU ETS geographical scope for aviation could be applied to NOx as that for CO2 emissions.

However, Arrowsmith told the webinar: “There is clearly uncertainty with regard to the climate impact and that was identified [by the experts] as a political risk in terms of the integrity of the EU ETS, recognising that as the science evolves, the intended effect of something that is put in place may change.”

Other policy measures could entail reducing the maximum volume concentration of aromatics within fuel uplifted at European airports and an EU blending mandate to boost the use of sustainable aviation fuels. If the political will was there to take these options forward, the aromatics measure could potentially be implemented in the five-to-eight-year period or perhaps longer, while a SAF blending mandate, already under consideration by the European Commission, could be achieved in a shorter timeframe, advises the report.

Another option, although more complicated and taking longer to implement, would be to levy a charge on the full climate impact of each individual flight, so having the broadest coverage of all the policy measures. The introduction of such a charge requires a good estimate of the climate costs at a flight level and, says the report, there is no scientific consensus on the methodology to calculate these costs.

“It could be argued that a levy that aims to internalise the external costs would be considered a charge and not a tax,” it says. “In this case, the charge would be related to recover the external costs of the climate impact of aviation.” Significant research is needed to develop and define this measure, it adds.

Arrowsmith said key messages to be taken from the report included a need to continuously review the latest scientific understanding on non-CO2 impacts and conduct further research, potentially through the EU’s planned Horizon Europe scientific research initiative, to increase certainty, consider different metrics and time horizons, enhance existing analytical methods to estimate aircraft non-CO2 emissions, and enhance capability to accurately predict the formation of persistent contrails. In addition, he said, there was a need to maintain and regularly review existing ICAO environmental certification standards on CO2, NOx and nvPM, and to incentivise the uptake of sustainable aviation fuels.

Responding to the report, Transport & Environment said the EU could not afford to wait the five to eight years proposed to implement aviation non-CO2 mitigation policies. It said the measures should be included in the Commission’s upcoming Sustainable and Smart Mobility Strategy that is due out shortly. Contrail avoidance should also be prioritised in the revision of the Single European Sky, it recommends, and pricing for non-CO2 emissions used to incentivise airlines to use eco-friendly flight paths.

“The European Commission was first tasked with addressing the non-CO2 emissions of flying in 2008. It shouldn’t waste any more time in implementing the solutions that are available today,” said Jo Dardenne, T&E’s Aviation Manager. “Contrails and other non-CO2 effects need to be urgently tackled to avert climate crisis.”

Photo: MIT