19 October 2021

GreenAir News

Reporting on aviation and the environment

COMMENTARY: Navigational avoidance of contrails to mitigate aviation’s climate impact – now is the time for action

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 CO2. 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.

Figure 1. Contrail avoidance by a small change in flight level (after Mannstein)

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. Distribution of contrail-cirrus energy forcing in Japanese airspace (Stettler et al)

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.