In March the Greener by Design Group of the Royal Aeronautical Society, together with the Institute of Atmospheric Physics at the German aerospace research organisation DLR, jointly organised a virtual conference entitled ‘Mitigating the climate impact of non-CO₂ – Aviation’s low-hanging fruit’. The conference provoked a Commentary article in GreenAir in July from Professors Keith Shine and David Lee arguing that “low-hanging fruit may be superficially attractive, but only if that fruit is ripe. It is our contention that in reality, many years’ research is needed to establish whether it is viable. Rather than decreasing aviation’s climate impact, premature implementation of the strategy risks increasing it.”  As joint chair of the conference programme committee, and on behalf of the Greener by Design Contrail Avoidance Group, John Green writes to strongly challenge this contention.  The points made in the Commentary were once valid but take no account of the recent advances presented at the conference, he argues.

The conference was very well received by the delegates. One respondent said it was “Probably the most important aviation conference of the year”. A report on the event is available and there was an accurate follow-up article by GreenAir correspondent Susan van Dyk in April.

It was a significant milestone along the path that began in earnest in 2015 at a Royal Aeronautical Society conference entitled ‘Contrail-cirrus, other non-CO₂ effects and smart flying’ at which Professors Shine and Lee both gave invited papers. At the conclusion of that conference, the broad consensus of the round-table discussion was that the science was by then sufficiently mature to move towards action to reduce persistent contrails.  As a follow-up, Greener by Design invited colleagues from DLR, the UK air navigation service provider NATS and some UK universities to form an informal Contrail Avoidance Group. This development and the conference that prompted it were both covered in a report. Over the past five years the Contrail Avoidance Group has stimulated a body of research that has moved understanding forward appreciably. Much of the new information generated by participants in the Group was presented at the March conference.

The envisaged end point of the path we are following is the worldwide adoption of contrail avoidance by operational measures. We well understand that this is likely to be a long and arduous journey. At the 2015 conference, the final consensus was that the quickest route to this goal is likely to be by adopting a regional approach, with Europe taking the first steps. If appropriate ATM practices are adopted in Europe and can be applied to transatlantic air traffic, and if a reduced climate impact can be convincingly demonstrated, application of the practice in the American continent may well follow; eventually, the rest of the world will join in and ICAO will pass the appropriate regulations. Against this grand vision, the Contrail Avoidance Group has set itself the modest goal of achieving a real-world demonstration of contrail avoidance by ATM in the Shanwick Oceanic Control Area (OCA) of the North Atlantic. That goal is now in sight.

One compelling reason for pursuing contrail reduction is that, as recognised in the Shine-Lee Commentary, the present-day climate impact of contrail cirrus is estimated to be 65-70% greater than that of CO_2. What makes this an economically achievable aim is, as Dr Klaus Gierens pointed out at the 2015 conference, the climate impact of contrail-cirrus varies strongly with atmospheric conditions, time of day and other factors. Because of this large variability, a significant reduction in climate impact can be achieved by avoiding contrails on only a small proportion of flights. Accordingly, in his presentation to the March conference, Gierens described a minimally invasive strategy in which only those contrails with the strongest warming effect are avoided. These ‘Big Hits’, with Effective Radiative Forcing (ERF) in the range 10 to 100 W/m² (ie 2 to 3 orders of magnitude greater than the global ERF of contrail-cirrus currently estimated as 57.4 mW/m²) comprise only 1-2% of all flown distances. They offer a powerful handle for reducing climate impact. The basic concept for avoiding contrails, originally proposed by the late Hermann Mannstein of DLR, is illustrated in Figure 1.

The key fact is that the ice-supersaturated regions (ISSRs) required to form persistent contrails and contrail-cirrus are shallow. Consequently, as shown by Professor Ian Poll at the March conference, the fuel burn penalty of diverting to fly above or below them is very small. It is considerably less than 1% even if the aircraft is flying at the conditions for minimum fuel consumption. The study of contrail avoidance in Japanese airspace, covering all traffic in the defined region over six periods of one month spread over the year, as reported by Stettler et al. at the March conference, brings this home forcefully. 

Figure 2 taken from this study shows 80% of the total energy forcing (EF) by contrails over this period was by 2% of flights, confirming the assessment by Gierens of the frequency of the Big Hits. Stettler at al investigated the effect of an increase or decrease of 2,000ft in cruise altitude for contrail forming flights. They used the DLR CoCiP code to predict the change in total energy forcing, integrating the Global Warming Potential of the change in CO₂ over a time horizon of 100 years. Overall, they found diverting 1.7% of flights reduced contrail-cirrus EF by 59% and total (contrail + CO₂) EF by 36%. The average fuel burn and CO₂ penalty per diverted flight was 0.27% and for the fleet overall it was 0.014%.

These are striking results and, taken together with the other results presented at the March conference, overturn the previously asserted view that contrail avoidance by ATM measures will increase fuel burn, CO₂ emissions and airline costs unacceptably. It is time for the airline industry to recognise this and lend its support to a campaign to implement contrail reduction.

The main obstacle to the successful implementation of a contrail avoidance strategy is the present limitation in the ability of meteorological organisations to predict the location of ISSRs with sufficient accuracy. Improved ISSR prediction was agreed at the March conference to be a priority area for future advance. The study of forecasting accuracy presented by Gierens concluded that present methods are sufficient to predict the region and approximate time period for contrail-cirrus formation but more work is needed. It is a field in which computing power has recently been increased substantially and in which Gierens sees the future need now as an improved representation of the microphysics of ice clouds within their ambient humidity (and supersaturation) field. It is envisaged that this would lead to improved predictions of both cirrus properties and ISSRs. A future GreenAir Commentary is expected by Durant et al. from SATAVIA reporting important advances in ISSR prediction that have recently been made.

The declared aim of the Greener by Design Contrail Avoidance Group is to achieve a successful demonstration of contrail avoidance by tactical use of ATM in the Shanwick OCA. The Aerospace Technology Institute is working to facilitate this, although progress has been delayed by Covid. In addition, as reported at the March conference, DLR and Eurocontrol are jointly engaged in a year-long investigation of tactical contrail avoidance in the crowded airspace of the Maastricht Upper Area Control (MUAC) region of Northern Europe. Both these trials will be significant steps along the road to embedding contrail avoidance as part of ATM procedure in Europe and across the Atlantic, with the eventual roll out of the practice worldwide. As admitted above, this is likely to be a long and arduous road. There will be much work needed to answer all the questions and gain the confidence of the operating and regulating community that the time is ripe to take the next step.

Professors Shine and Lee assert that the time is not ripe. Greener by Design strenuously challenges this. Their Commentary consists of generalised statements and makes no reference to the papers presented at the March conference. Indeed, we might be forgiven for wondering if either of them is aware of the material presented. And we consider it ingenuous to imply that unless their advice is heeded, contrail avoidance will be introduced imminently. There is plainly no possibility of that. Contrary to their view, we see now as time for action – urgent action.

Contrail avoidance has the potential to reduce aviation’s climate impact by at least a half. It can be applied to the entire world fleet, whatever its level of its technology. It is not just the ‘low-hanging fruit’, it is the lowest hanging fruit available to the aviation industry. It will take some time to roll out but could be expected to be effective worldwide many years before the yet-to-be developed new technologies (such as synthetic fuels) have penetrated a significant proportion of the fleet. It is important that policymakers appreciate the potential reduction in climate impact from aviation that can be achieved by contrail avoidance. Supporting a flight demonstration of its practicality is a key first step which should be taken as a matter of urgency.

Top photo: Contrails over the North Sea (credit: European Space Agency)

About the author

Dr John Green (greensinwoburn@gmail.com) spent his early life as an aerodynamicist researching turbulent boundary layers at Cambridge and the then Royal Aircraft Establishment (RAE), becoming successively Head of Subsonic and Supersonic Wind Tunnels, Propulsion and Noise Divisions before becoming Head of Aerodynamics Department. After time in MOD HQ as Director, Project Time and Cost Analysis and in the Embassy in Washington as Deputy Head of British Defence Staff he returned to RAE as Deputy Director Aircraft before resigning from government service to become Chief Executive of the Aircraft Research Association. He was President of the Royal Aeronautical Society in 1996-97 and of the International Council of the Aeronautical Sciences in 1996-98. He has been a member of the Executive Committee of Greener by Design since its formation in 2000, chaired its Technology Sub Group that produced two substantial reports on environmental impact mitigation in 2001 and 2005 and since 2016 has chaired its informal Contrail Avoidance Group.    

Views expressed in Commentary op-ed articles do not necessarily represent those of GreenAir.

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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