Easy does it for greener skies

Dr JOHN GREEN FRAeS reports from the RAeS 'Mitigating the climate impact of non-CO₂ – Aviation’s low-hanging fruit' virtual conference in March 2021.

On 23-24 March 2021 the Royal Aeronautical Society hosted a joint conference shared between its Greener by Design group and the Institute of Atmospheric Physics of DLR, the German Aerospace Centre at Oberpfaffenhofen. The event was organised by the Contrail Avoidance Group, an informal body formed after the GBD conference in 2015 by members of Greener by Design with members from DLR, NATS and UK universities. The rationale for the conference was that within the scientific community there was now fairly solid agreement that the non-CO2 effects of aviation are responsible for two thirds of its impact on climate. The strongest impact is from contrails and contrail cirrus, with the impact of NOX emissions at altitude also contributing. For both these impacts, the prospects of substantial reductions are real and potentially realisable within a far shorter timescale than the options for reducing CO2 emissions. Hence the 'low hanging fruit' in the title of the conference.

The conference was a virtual, on-line event divided between two afternoons. The presentations were grouped under the headings 'The Science Base' on the first day and 'Mitigation Possibilities' on the second. The event was chaired by Geoff Maynard, chairman of Greener by Design. In his opening remarks, he emphasised that, while the subject of aviation’s non-CO2 effects was potentially wide-ranging, he wanted the conference to concentrate on its theme of mitigating the climate impact of these effects.

Day 1: The science base

Contrails over the North Sea (ESA)

The meeting opened with a keynote address by Robert Sausen, Head of the Institute of Atmospheric Physics at DLR. The talk, entitled 'Climate impact of aviation’s non-CO2 emissions - An overview', covered all aspects of aviation’s climate impact but then focused on the non-CO2 impacts and the options for mitigating them.

For CO2 emissions, he showed that, even with carbon-neutral growth after 2020 as projected by ICAO, the impact of aviation’s CO2 on temperature will continue to grow for the rest of the century. By 2100 the projected temperature increase will be more than three times the increase in 2020 There is quite a spread in the projections to 2100, depending on the assumed IPCC RCP (representative concentration pathway), but the importance of CO2 emissions from aviation is starkly clear.

He presented the most recent estimate by Lee et al(1) of the impact of the various contributors to warming, expressed (Fig 1) as the now-preferred effective radiative forcing (ERF). Alongside the figure he made three points:

1. The non-CO2 effects contribute at least 2/3 of the total aviation ERF.
2. Non-CO2 effects also occur if alternative fuels are used, in particular H2.
3. The magnitude of the non-CO2 effects depends on location and time of the emissions.

 

Figure 1. Current estimate of contributions from aviation to effective radiative forcing. (Lee et al, 2021)

Among the non-CO2 impacts, he highlighted the evolution of the perceived climate impact of NOX. This is shown in Fig 1 as a net positive contribution, the result of the effect of the increase in ozone being greater than that of the reduction in methane, both greenhouse gases influenced by NOx emissions. After a re-evaluation that increased some of the negative methane effects, the NOx contribution in 2005 had fallen from an estimated 13.8mW/m2 in 2009 to a value of 4.0mW/m2. This effectively demoted NOx as a climate issue, leading to the conclusion that, besides CO2, only contrails play a significant role in aviation’s contribution to climate change. However, in 2019 Volker Grewe and his colleagues at DLR published a paper(2) drawing attention to two methodological flaws in the analysis underlying this conclusion. Their analysis increased the estimated positive contribution of ozone and reduced the negative impact of the methane reduction, leading to an increase in the NOX RF in 2005 to 26.7mWm2, very similar to that of CO2. This is discussed further below.

He discussed various mitigation measures, including the concepts for Europe set out in the EASA report of 2020(3) demonstration of contrail avoidance by ATM and eco-efficient flight trajectories. All of these were presented by other speakers and are covered later in this report.

In his closing remarks he re-emphasised the importance for the aviation sector of its non-CO2 effects, the need for suitable metrics to support strategic decisions on mitigation measures and the range of mitigation measures available.

Oxides of nitrogen

Emissions from aircraft have twice the effect of those on the ground. (Met Office)

The next two presentations addressed different aspects of the climate impact of NOx. This is the mixture of NO and NO2 that is produced in the high pressure and temperature conditions in an engine combustion chamber by the combination of atmospheric nitrogen and oxygen. They are not a greenhouse gases but, at altitude, enter into complex chemical reactions that result in an increase in ozone concentration and a reduction in methane concentration, both strong greenhouse gases.

Agnieska Skowron of Manchester Metropolitan University discussed the effect of future atmospheric background composition on NOx climate impact in the longer term. Emissions from aircraft and from sources on the ground both influence the background level of NOx at cruise altitude, the emissions from aircraft having about twice the effect of those on the ground. The NOx emissions at ground level are expected to reduce by 2050 leading to a cleaner upper atmosphere, which will reduce the climate impact of NOx but this will be offset if NOx emissions in cruise continue to increase.

Higher background NOx causes increased RF from ozone but also reduced RF from methane, this reduction being greater when the most recent modelling of the effect of methane is adopted. The projected background concentration of NOx in 2050 is rather uncertain and Agnieska’s analysis indicates great uncertainty in the RF due to aviation NOx. Because of the predicted stronger negative effect from methane, the net RF might even be negative in 2050. The paper notes that measures to reduce engine NOx emissions increase fuel burn and CO2 emissions, which undoubtedly have an adverse climate impact. Combined with the above uncertainly in the NOx RF in 2050, the paper argues that the priority for industry should be to reduce CO2 emissions rather than NOx.

One point that may be noted in the presentation but was not discussed at the meeting is that the baseline adopted in the modelling gave a net NOx RF of around 4. This is the modelling that, as noted above by Robert Sausen, had been challenged by Grewe et al in 2019. By correcting for what they argued were two methodological flaws, they had proposed that NOx RF in 2005 should be put at 26.7 rather than 4.0. This discrepancy needs investigation and resolution. It could have an important effect on the paper’s main conclusion.

The next presentation was by Volker Grewe of DLR, who addressed the current assessment of NOX impact and its dependence on time and place of emissions. The talk was divided into two parts, the first discussing the important effect of weather and the second revisiting the diagnostics to calculate NOx RF.

The importance of weather was illustrated by observing the climate impacts of NOX emissions at two locations on the same flight path on the same day. The first location was in a branch of the jet-stream that was diverted northwards by a blocking region of high pressure, the second location was in the centre of the high-pressure region, from which air eventually moved southwards towards the topics. Over a lifetime of 90 days, the different histories of ozone formation and methane reduction along the trajectories of the packets of NOx emitted at the two locations was striking.

The observation of this strong dependence on weather at the point of NOX emission led to the development of the concepts of climate change function and of climate optimised routeing. The substantial body of work supporting these concepts was summarised in the presentation. It has been followed by an exploratory 'proof of concept' investigation, comparing cost-optimised routes with climate optimised routes which direct traffic around 'climate sensitive' regions. The preliminary indications are that a 2% reduction in NOx RF can be achieved in this way. This topic of climate optimised routeing will be studied further at DLR in the CLIMOP and ACACIA projects and also in the European FlyATM4E programme.

The second part of the presentation considered the way that the contributions of NOx emissions to ozone and methane are diagnosed. Differences in approach can lead to very different results. A key issue is the evolution of methane, which is a relatively long lived gas with the steady state perturbation from current emissions not reaching an equilibrium level for more than 30 years. He compared the radiative forcing calculated for methane in 2018 on the basis of the NOx emission levels in 2005 taken as a reference for three different models. The first was with NOx emissions held steady at their level in 2005, the second was for their growth being linear through the 2005 level from zero in 1940 and the third was for their real growth since 1940 which by 2018 have reached about twice the level given by linear growth. The difference in the calculated methane negative RF between steady state and linear growth in emissions by 2018 was a reduction of around 21%. The linear transient growth was the basis of the modelling in Ref 1. The difference between steady state and real growth by 2018 was a difference of 42%.

A further point in the presentation was the difference in the predicted ozone impacts between the 'perturbation' method and the 'tagging' or 'contribution' method. This was illustrated by modelling the hypothetical case of eliminating the ozone emissions from road traffic, which account for about 12% of the contribution to the atmospheric ozone column. The effect of removing road traffic as a source was a compensating response in which the efficiency of ozone creation by other sources increased. The result, applying the contribution method, was that the reduction in the ozone column was 2% rather than the 12% given by the perturbation method. At present only one study of aviation’s climate impact has been published that has used the ozone contribution method, The results were noted in Ref 2, cited already by Sausen and set out in the table below.

 

Volker went on to note that what he asserts are the correction of flaws shown in this table would, if applied to the results shown in Fig 1, double the ERF of NOx, from 17.5mW/m2 to 35.2 mW/m2. This would make the ERF from NOx slightly greater than the 34.3mW/m2 attributed in Fig 1 to CO2. He did not say so but, if accepted, this would substantially change views on the priority to be given to reducing NOx emissions at cruise. He ended with a proposal that there should be further research into the methods of diagnosis used to assess the impact of NOx emissions. The present divergence needs to be resolved.

Contrails and contrail cirrus

Data has been collected using handful of instrumented airliners. (IAGOS)

The next three papers considered current evidence on persistent contrail occurrence, beginning with a presentation by Klaus Gierens of DLR on 'Contrail Statistics, Big Hits and Predictability'. He began by noting the 58% downward adjustment in Ref 1 from radiative forcing (RF) to effective radiative forcing (ERF) for contrail cirrus, an adjustment that does not apply to the RF from CO2. With this adjustment, the global values given in Ref 1 for 2018 are 34.3mW/m2 for CO2 and 57.4mW/m2 for contrail cirrus. Contrail cirrus provides by far the largest contributor to aviation’s ERF in 2018 which, in total, Ref 1 puts at 100mW/m2. He highlighted the units here – aviation’s total global ERF is measured in mW/m2.

For individual contrails, he went on to show that the net RF can be two to three orders of magnitude greater than this, in the range 10 to 100 W/m2. This includes both strong cooling cases in the daytime and strong warming at night. These strong effects arise only from persistent contrails, with lifetimes of the order of several hours, which form only in ice supersaturated regions (ISSRs). About 15% of all flight distances are in ISSRs but a minimally invasive strategy would be to avoid only those contrails with the strongest warming effect. This idea, to avoid only the 'Big Hits', emerged from a conversation with a DLR colleague, the late Hermann Mannstein. Big Hits comprise only 1-2% of all flown flight distances and offer a powerful handle for reducing climate impact. To pursue a strategy of avoiding them requires the ability to:

• predict contrail formation (Schmidt-Appleman criterion);
• predict contrail persistence (ice supersaturation);
• predict individual contrail forcing.

The first two of these can be provided by the ECMWF. The DLR CoCiP (contrail cirrus prediction) code provides the latter.

The presentation reported a recent assessment of the accuracy of the ECMWF forecasts. Relative humidity data collected in flight on instrumented civil aircraft in the IAGOS programme were compared with predictions for the same time and place drawn from the ERA-5 (EMCWF re-analysis) data set. Predictions of the Schmidt-Appleman criterion were found to be reasonably good but not predictions of ice supersaturation. The conclusion was that the current ability to make on-the-point and bang-on-time prediction of ISSR location needs to be improved. Further assessment led to a more positive conclusion. Regional contrail forecasts by ECMWF, seeking regions where persistent contrails might appear, are satisfactory for planning research flight campaigns, Further analysis showed that forecasting accuracy was improved by using a threshold of less than 100% for RHi (relative humidity with respect to ice). The basic forecasting capabilities are sufficient for giving ATM instructions on contrail avoidance but further work to improve ISSR forecasting is needed. Further work is also needed to build a statistically robust characterisation of Big Hit situations. However, even at this stage, the avoidance of night-time contrails in the winter months, when most of the impact of Big Hits occurs, appears to be practicable with present weather forecasting capability.

The presentation which followed, by Ulrich Schumann of DLR, reviewed the evidence on contrails that has been provided in 2020 following the drastic reduction in air traffic under the Covid-19 restrictions. He began by emphasising the importance of observation.in developing an understanding of the radiative effects of contrail cirrus. This was strikingly illustrated by the demonstration, from satellite data, of a double hump in the graph against time of cirrus formation and outgoing long wave radiation over the Atlantic. The double hump was shown to correlate with the two peaks, morning and evening, in transatlantic air traffic. The peak in the contrails lagged about three hours behind the peak in the traffic, corresponding to the time needed for the contrails to grow to be observable by satellite.

The Covid shutdown caused a reduction of 92% in air traffic over Europe in April 2020 and a reduction of 72% over the six-month period from March to August, an unprecedented event. Contrail coverage over Europe with an optical depth greater than 0.1 decreased from 4.6% in 2019 to 1.4% in 2020. The reduced contrail coverage caused 70% less longwave, 73% less shortwave and 54% less net RF. Demonstrating the impact of the shutdown was complicated, however, by the very different European weather in 2019 and 2020, which made it difficult to quantify the difference in the contrail impact between the two years. The study confronted this challenge by using two tools to evaluate contrail thickness and outgoing radiation. One used the DLR CoCiP model together with weather data from the IFS (Integrated Forecasting System) of the ECMWF, the other used the Meteosat-SEVERI observation results. Comparisons between the two methods were shown for six month averages, for 2019 and for the differences between 2020 and 2019, of contrail optical thickness (OT), outgoing long-wave radiation (OLR) and reflected solar radiation (RSR). The positive effect of introducing the CoCiP model is clear from the table below, which shows the statistics of the difference between the satellite observations and predictions by the IFS, with or without CoCiP. The tables show that, particularly for optical thickness and outgoing long-wave radiation, the correlation coefficient r, normalised mean bias and root mean square error are all considerably better for the predictions which include CoCiP.

 

 

The air traffic reduction during the COVID-19 pandemic has provided a test case for aviation RF that will generate further studies, including a planned assessment of the 2019-2020 contrail differences over a 12 month period. Results to date are encouraging, however, showing that contrail signatures are identifiable despite strongly different atmosphere and surface conditions in 2019 and 2020. The implications of the study are:

• Contrails RF is highly variable, locally far larger than any other aviation RF;
• Contrails RF can be predicted for given traffic for the next days with IFS/CoCiP;
• Contrail cirrus cover/irradiance predictions can be validated with METEOSAT CiPS/RRUMS.

The overriding message is that the mitigation potential is large and the methods now available are capable of testing it.
The third presentation on contrails was by Luca Bugliaro of DLR, whose subject was contrail and contrail cirrus observations by satellite. The presentation was in two parts, the first on the potential and challenges of contrail evaluation tools and the second on the properties and radiative effect of a contrail cluster during its life cycle.

The first part, on contrail avoidance strategy, reviewed practical requirements discussed by earlier speakers but then focussed on the challenges of identifying, measuring and monitoring the development of contrails by satellite. The basic tool is the automated contrail detection algorithm (CDA) developed by Mannstein at DLR in 1999 and since adapted to Meteosat Second Generation and MODIS and used internationally in a wide range of studies. There is a difference in resolution between images from orbiting and geostationary satellites. The first pass over an area four times a day and have higher spatial resolution. The latter produce images every five minutes of the same area of land and have high temporal resolution, enabling close tracking of contrail development and movement. Sample images showed that this process on its own is not wholly reliable, missing some contrails and in other cases producing false alarms. It can be improved by using air traffic data and also combining weather data with the contrail prediction tool CoCiP, which helps to account for the spatial and temporal shift with respect to air traffic and also helps to reduce false alarms. Even so, Bugliaro’s current view is that evaluating contrail avoidance measures in ISSRs, is a challenging task that requires much effort and large statistics. It cannot be taken for granted.

The second part of the presentation was on a case study of a contrail outbreak on a particular day, which was explored by the DLR HALO research aircraft and by a range of satellite-based methods. The satellite observations provided information on contrail properties and radiation through the day, from 500 to 1800, supplemented by observations by the HALO aircraft in mid-afternoon. The contrail cluster formed in an initially clear sky, its optical thickness predicted by CoCiP increased steadily until about 1500 and then declined to about 1/3 of its maximum value by 1800. In parallel there was a growth in natural cirrus from zero at 1100 to a significant plateau by 1300. The results for the radiative effect for the cluster was that the outgoing long-wave radiation stayed at 20W/m2 between 900 and 1700. The reflected solar radiation fell below -20W/m2 between 1300 and 1600 but then reduced in magnitude rapidly in the evening. The net radiative effect was thus a warming of around 10W/m2 in mid-morning, falling to a slightly negative value in early afternoon and then rising again to around 10W/m2 by 1700. While listing areas in which further research was needed, he echoed the point made by Klaus Gierens and Ulrich Schuman that the observed outgoing radiation, at 20W/m2, is about 200 times the total RF attributed to aviation.

The afternoon session concluded with a round table discussion chaired by Professor Helen Rogers of NMITE and involving all the day’s speakers except Agnieska Skowron. It was a lively and wide-ranging discussion which ended with a fair degree of consensus. In order to avoid repetition, the outcomes of the discussion are reported in the presentation by Helen Rogers which opened proceedings on the second day.

Day 2: Mitigation possibilities

Understanding of contrails has improved in recent years.

To provide a background to the day’s discussion of mitigation, the day began with a summary by Helen Rogers entitled 'What have we learnt during Day 1: The Science Base?'. This was a review of the perceptions agreed in the round table discussion that closed proceedings on Day 1. Her conclusions were:

• Major improvements in the science understanding have been made;
• Non-CO2 effects are very large for aviation and depend on location and time;
• Our understanding of contrail cirrus has improved significantly, as evidenced using data from the COVID-19 pandemic lockdown;
• We now have the first calculations of the indirect effect of aerosols on cirrus although the range is very large;
• Air traffic is not always fuel-optimised, so mitigation is possible without increased CO2 emissions, although this still requires validation.

What still remains:

1. We need to define an appropriate optimal metric(s) depending on our endpoint goals;
2. Additional/better relative humidity measurements in the UTLS region are required if we really want to focus on accurately determining ice super saturation and testing model predictions;
3. Weather predictions of relative humidity in the UTLS need further validation;
4. Important to consider emission contribution when modelling aviation’s impact;
5. Aviation needs a step change approach to reduce its climate impact significantly.

What can we do to address these issues?

1. Focused science research on optimal metrics;
2. Aviation can help improve observations by in-situ measuring during operations, as well as supporting other measurement initiatives;
3. Aviation should call for validation of RH predictions from NWP providers;
4. Perturbation and contribution methods are both of value and can help answer different questions;
5. Industry should prepare for SAF.

Possible first steps:

• Minimising contrail RF by route changes may be an option for future sustainable air transport but industry needs to demonstrate the route optimisation can be implemented operationally;
• Further research is required to determine appropriate, reliable metrics and estimates of aviation’s climate impact, together with accurate predictions of regional and temporal distributions.

In the discussion that followed it was agreed that demonstrating the practicality of contrail avoidance with minimum fuel burn penalty was a top priority, along with the need for improved relative humidity forecasting in the UTLS and the development of appropriate metrics from the climate impact of short-lived emissions.

Emissions reduction by engine technology

Rolls-Royce's (Advanced Low Emissions Combustion System) ALECSys will be flown on a 747 testbed this year. (Rolls-Royce)

The next presentation, with the above title, was by Paul Madden of Rolls-Royce. He reviewed the company’s wide range of activities to reduce emissions. For smaller aircraft it is exploring more electric pathways with emphasis on battery technology. It is also investigating hydrogen-powered gas turbines for regional aircraft. For larger aircraft, which will rely on kerosene powered gas turbines for the foreseeable future, it is working to clear the way for the use of 100% SAF, as against the current 50% limit.

As regards the non-CO2 emissions at cruise, which are determined primarily by the design of the combustor, the one with greatest climate impact is NOx. Emissions of NOx in the landing and take-off area (LTO) are constrained by ICAO regulation. The regulation applies only to LTO NOx and, although there is some link between this and NOx at cruise, there is currently no proposal to limit its level at cruise. ICAO has now also established limits on nvPM (non-volatile Particulate Matter) in the LTO area, again as a health measure. Whether or not this limit has any bearing on contrail formation is unclear, but LTO NOx and nvPM are now two emissions that are regulated by ICAO and potentially subject to increased stringency in future.

The presentation briefly described the technology of the RQL (Rich Burn Quick Quench Lean Burn) combustor, which is the low NOX combustor in the current family of large Rolls-Royce engines. This was a significant advance on previous designs but has reached its limits. The paper went on to describe in detail the lean burn combustor developed in the ALECSys (Advanced Low Emissions Combustion System) programme. This combustor has fuel staging, with a rich burning pilot at its core for low power stability, switching to the lean burning main burner for low NOX and near zero nvPM at high powers. An important part of the ALECSys programme was the development of the control laws and control system to ensure that the cross-over from pilot to main occurs at the right power level. This is dictated by efficiency, operability and nvPM emissions. The near zero nvPM emissions at cruise power should reduce contrail formation, but this needs further study to confirm.

Current in-production engines are being improved to reduce the LTO nvPM at the expense of some increase in LTO NOx. What effect if any this has on cruise NOX was not stated. For the ALECSys engines, however, the reduction in NOx emissions at cruise will be appreciable. The ALECSys combustor system has been through an extensive ground testing programme on two Trent 1000 engines, which included fully satisfactory tests on a 100% HEFA SAF (Sustainable Aviation Fuel derived from Hydro processed Esters and Fatty Acids). Flight testing of the two ALECSys demo engines on the Rolls-Royce Boeing 747 flying test bed is planned in the coming months.

Mitigating contrail impact

The ECLIF3 project, begun in January 2021 saw the first in-flight monitoring of SAF emissions. (Airbus)

The first of four presentations addressing this subject was given by Christiane Voigt of DLR. Her title was “Reducing the climate impact of contrails by SAF and contrail avoidance”. Her first theme was the beneficial effect of substituting a sustainable fuel for Jet A1. She showed results from the international multi-agency ECLIF2/NDMAX campaign in 2018 which included flight tests in which an instrumented DC-8 captured data in the wake of DLR’s ATRA A320 aircraft. These were the first experiments to provide in-flight measurements of soot and ice particle numbers for a range of alternative SAFs. They showed a linear reduction in soot particle number and a steeper reduction in ice particle number with reducing % volume naphthalene content of the fuel. When translated into impact on contrail formation, however, the effect is found to be non-linear, a 50-70% reduction in ice numbers leading only to a 20-40 reduction in contrail RF. Key messages from this work are: substitution of biofuels can provide a reduction in contrail RF without any question of it being offset by increased CO2 emission; fast implementation in the fuelling system is possible; non-linearity requires stronger reductions in aromatics, hence regulations to lower the maximum levels of aromatics in fuels, with a certification target of 100% SAF.

In support of this aim, a joint programme between Airbus, Rolls-Royce, DLR and the SAF supplier NESTE began in January 2021. The programme, ECLIF3, is based at Toulouse and involves ground and flight trials with an Airbus A350-900 as the test vehicle and the DLR Falcon 20-E as the chase plane. It will be the first in-flight study of 100% sustainable aviation fuel on a commercial passenger jet and will investigate all aspects of emissions, including those relevant to contrail formation. First in-flight measurements are planned for April 2021.

The effect of the lockdown in 2020 was studied by a multi-agency project in BLUESKY, using the DLR HALO and Falcon aircraft to collect atmospheric data. The data relevant to contrail formation were for one day only and are consistent with the data for a longer period reported by Schumann above. Future DLR flight programmes include the multi-institute CIRRUS-HL mission on the HALO aircraft which will investigate contrail cirrus particle microphysics and shape, and soot cirrus. There will also be day-night flights to investigate cirrus radiative effects and it is intended to have a dry run of a contrail avoidance test, In addition, data will be gathered to extend previous DLR work to assess the quality of weather forecasting in the UTLS, which is believed to need improvement, particularly for relative humidity. The presentation concluded with an outline of the studies of cloud physics currently planned in five flight programmes involving DLR. These are ECLIF 3 and CIRRUS-HL described above; ECO2FLY investigating emissions from lean combustors on a bizjet, due to begin in October 2021; H2CONTRAIL, an investigation from 2021 to 2025 of contrails from hydrogen; and, from 2022 to 2026, Project WandeLH2B, a further investigation of hydrogen propulsion.

Mitigating aviation contrails was the title of the following presentation, by Marc Stettler of Imperial College, London. He outlined the physics behind the formation of persistent contrails and contrasted the effects of daytime and night-time contrails. Daytime contrails both cool the earth by reflecting incoming sunlight and warm it by trapping outgoing long-wave radiation. After sunset, night-time contrails only trap the long-wave radiation. Not all contrails are created equal. Their impact on climate depends on meteorological conditions, engine particle emissions, diurnal and seasonal cycles and surface albedo.

Options for mitigation include alternative fuels to reduce particle emissions, cleaner engines for the same reason, and flight diversion to avoid ice supersaturated regions (ISSRs). Only the latter two options were addressed in this presentation. Figure 2 below illustrates the idea of diverting to avoid an ISSR, as proposed by the late Hermann Mannstein of DLR

Figure 2. Contrail avoidance concept proposed by Mannstein

The paper(4) on which the presentation is based describes a study of this kind of diversion, applied to six one-week periods of air traffic between May 2012 and March 2013 in Japanese airspace. The climate impact of the contrails created by this air traffic was calculated using the DLR CoCiP code. The metric for the impact was the energy forcing EF of an individual contrail, determined by the integration with time over its life of the product of its length and width and its local RF. The energy forcing from the CO2 emitted during the flight is determined from the global warming potential of the mass of CO2 emitted during the flight integrated over time horizons of 20, 100 and 1,000 years The main comparisons in the paper adopt a time horizon of 100 years.

The paper showed the fleet-level results in terms of the distributions in time of flights forming contrails and of their contrail EF. Overall, only 17.8% of flights produced contrails. A more striking finding, however, was the concentration of the energy forcing into a still smaller proportion of flights. Fig 3 shows the results of ranking contrails in terms of their energy forcing EF, with cumulative EF plotted against the proportion of total flights. The chart shows that 2.2% of flights account for 80% of the total contrail EF. This aligns with the Big Hits concept advocated on Day 1 by Gierens and leads to the conclusion that a fleetwide diversion strategy is not necessary.

 

Figure 3. Distribution of cumulative contrail energy forcing across fleet

The study found that the contrails producing the largest EF were produced typically between 15.00 and 06.00. A small-scale diversion approach was adopted. For every flight which produced a high EF contrail, the EF was calculated for diversions of ± 2,000 ft and the trajectories with the lowest EF selected. Up to 10% of all flights were allowed to divert at a time. Overall it was found that diverting up to 1.7% of flights could reduce the contrail EF by 59%. The average fuel burn penalty and additional CO2 emissions for each diverted flight was 0.27%. For the fleet overall, the fuel burn and CO2 penalty was 0.014%. The effect of introducing cleaner engines into the calculation was assessed by applying the diversion strategy to those aircraft with DAC (double annular combustor) engines, which reduce contrail EF by about 40%. The chart below compares the effects of diversion, DAC engines and a combination of the two. It is assumed there is no change in fuel efficiency for DAC engines.

The presentation which followed was by Rüdiger Ehrmanntraut of Eurocontrol. Its subject was the 2021 live trials for contrail prevention at Maastricht Upper Area Control (MUAC) This control centre handles all air traffic above 24,500 ft over Belgium, Luxembourg, The Netherlands and north-west Germany. It is the third busiest upper area control centre in Europe.

Airlines were informed by NOTAM that, from 18 January 2021 to 31 December 2021, between 1500 to 0500 UTC in winter and 1400 to 0400 UTC in summer, MUAC will be running a contrail prevention trial. During this period, flights may be tactically requested by the sector controller to deviate from the planned/requested flight level. The purpose of the project is to establish and test a procedure that avoids persistent contrails in the MUAC area of responsibility. The trial is being run jointly with DLR, using numerical predictions of relative humidity and temperature by DWD (Deutscher Wetterdienst – ICON).

The trial aims to answer a number of questions:

• Can we organise air traffic such that areas which allow the formation of persistent contrails can be avoided?

• Can we predict the formation of contrails with a reasonable skill?

• Can we predict the formation of persistent contrails with a skill that is sufficient for deviating air traffic?

• Alternatively, can we detect ISSRs and avoid them in real time?

It is too early to have validated results (after six weeks of trial, every other day, in the winter season with low ISSR). Traffic downturn due to the pandemic allows for tuning systems and procedures whilst minimising impact for airline operators. The feasibility of adapting operational working processes needs vb to be assessed, while avoiding additional workload and impact on ATM capacity. The climate benefit of contrail avoidance must be validated against additional CO2 emissions and the costs of additional fuel burn plus the CO2 charging scheme on top.

A key issue is the accuracy of ISSR prediction. The MUAC concept is for tactical air traffic control with real-time online decision making. The preference, therefore, is to have a real-time online ISSR observation system in addition to a predictive system. The question is raised as to whether this can be achieved by current generation satellites and ground-based cameras, Looking forward, the MUAC perception is that operational decision making needs to be improved by moving from central supervisory towards local sector decisions. This requires a solid ISSR detection or prediction system and better integration into the controller working starts to build up again. Continued work to tune and validate ISSR predictions, seek alternative validation of ISSR predictions and work towards real-time online ISSR detection are all in the outlook.

The session on mitigating contrail impact was concluded by Ian Poll of Cranfield University with a paper on the accurate prediction of fuel burn and emissions. His motivation for the paper was to provide the atmospheric science community with a modern, accurate, open source and independently verifiable method for predicting fuel burn. He listed a number of desirable attributes which made the method preferable to the 'black box' methods that had been used in previous studies. Citing the motto of the Royal Society, Nullius in verba, take no one’s word, he asserted that science that depends upon 'black box' methods can never be considered to be sound. The new method he described was developed jointly with Ulrich Schumann of DLR and was fully documented in four peer reviewed articles in The Aeronautical Journal. Ref 5 sets out the fundamental aspects of the method.

The underlying principle is that, for a given fuel, the rate of fuel burn in straight and level flight at constant speed depends only on the mass of the aircraft and the product of its propulsion efficiency and its lift to drag ratio (𝜂o𝐿/𝐷). This key quantity is a function of the aircraft lift coefficient CL, the flight Mach number M∞ and the flight Reynolds number Rac. The simple idea underlying the method is that curves of (𝜂o𝐿/𝐷) against CL, when normalised with respect to the optimum, defined as the maximum value attainable of (𝜂o𝐿/𝐷), collapse fairly closely to a single curve, Fig 4.

Figure 4. Normalisation of against C_L through optimum values

The variation with Mach number in Fig 4 can be largely eliminated by expressing the ratio of the best values of (𝜂o𝐿/𝐷) and CL as two functions of the ratio of flight Mach number to optimum Reynolds number. These two functions, together with the function defined in Fig 4, are 'near universal”', ie they apply to all aircraft. For an individual aircraft, the problem is reduced to finding the values of (𝜂o𝐿/𝐷), CL and M∞ when (𝜂o𝐿/𝐷) has its absolute maximum, or optimum value. These optimum values have been found and published for 53 turbofan aircraft.

Applying the method to aircraft flying at the usual long-range cruise Mach number and lift coefficient, the fuel used per unit distance is approximately 1% higher than that at the optimum. The effect of a reduction in cruise altitude of 2,000ft to avoid an ISSR is an increase in fuel burn of 1.4%; for a reduction of 4,000ft, it is 5.5%. A reduction is preferable to an increase in cruise altitude because the effect on fuel burn is less and the descent educes the climate impact of NOX. The presentation also discussed the alternative to flying around rather than under or over an ISSR. Two alternative avoidance strategies were considered, one 'tactical', the other strategic, showing how a decision could be made on the contrail avoidance measure with the lowest fuel and time cost. Usually the lowest increase in fuel will be by flying under the ISSR but it was emphasised that the addition trip fuel usage of both the 'tactical' and 'strategic' diversion options are very small, typically less than 1%, adding less than five minutes to a trans-Atlantic flight. These conclusions are consistent with results reported by Stettler. They should be set against some of the more alarmist statements on fuel burn and schedule disruption that have been made in recent times.

Regulation

Boeing has committed to producing airliners capable of 100% SAF use by 2030. (Boeing)

The subject of the next presentation was the potential to reduce non-CO2 climate impact by regulation. To fulfil the requirement of Article 30(4) of the EU ETS Directive, a report was commissioned by the European Union Aviation Safety Agency (EASA) in 2019(3). This report was prepared by a 16 strong team of specialists from six institutions, supported by a stakeholder group of similar size. The presentation was given by Stephen Arrowsmith of EASA, the project team leader (three others from the team had given presentations at the conference on the previous day).

The terms of reference of the study set out three tasks: Task 1: Current status of science and remaining uncertainties on climate change effects of non-CO2 emissions; Task 2: Existing technological and operational options used to limit or reduce non-CO2 impacts from aviation and related trade-off issues; and Task 3: Potential policy action to reduce non-CO2 climate impacts, pros/cons and associated knowledge gaps. The topics of the first two tasks were well covered in the preceding presentations and the potential to reduce climate impact by regulation is the essence of Task 3. Six policy options were shortlisted to be considered in greater detail.

Fuel-related measures included reductions in aromatics through fuel specification or SAF blending mandate. Key issues were: to reduce scientific uncertainty on climate impact from a reduction in persistent contrail cirrus formation as a result of cleaner fuels and lower aircraft nvPM emissions; a facilitation initiative to ensure uptake of SAF by the aviation sector; and a system to monitor fuels used and environmental benefits delivered.

ATM-related measures included avoidance of ice supersaturated regions and a climate charge. Key issues were: to reduce scientific uncertainty on climate benefit from optimisation of flight paths; enhanced meteorological forecast model capabilities needed to predict persistent contrails correctly in time and space; select appropriate CO2 equivalent emissions metric and time horizon to assess trade-offs; and agree on climate damage costs to determine level of charge.

A pilot project operating over the Atlantic is needed to assess the feasibility and costs/benefits. The feasibility is more limited within congested European continental airspace. Communication on benefits as well as incentives are needed to ensure buy-in.

In summary, regulators need regularly to review latest scientific understanding of non-CO2 impacts. They need to maintain and regularly review existing ICAO environmental standards (CO2, NOX, nvPM). The use of sustainable aviation fuels (SAF) has shown a reduction in both CO2 and non-CO2 emissions. The ReFuelEU initiative is currently considering policy options to incentivise the uptake of SAF. Further research should be pursued, potentially through Horizon Europe at EU level, to: increase certainty on climate impact from non-CO2 emissions; consider different metrics and time horizons that could be used to assess the impact of potential policy measures; enhance existing analytic methods to estimate aircraft non-CO2 emissions in cruise based on certified LTO emissions data; enhance capability to predict accurately the formation of persistent contrails.

Geoengineering - could contrails cool the planet?

Could contrails actually help cool the planet? (NASA)

This presentation was followed by one from Robert Whitfield of Greener by Design, who chairs the Governance Sub-Group of the Greener by Design Contrail Avoidance Group. When the Contrail Avoidance Group first met, two options for reducing climate impact were on the table. The first was changing cruise altitude to avoid forming evening and nighttime warming contrails, as in Fig 2. The second was, earlier in the day, deliberately flying into ISSRs in order to form contrails to reflect sunlight – ie cooling contrails. Both options had been proposed by Hermann Mannstein. It emerged that half the group thought that both options should be explored but half the group foresaw political reluctance to consider deliberately forming contrails on the grounds that it could be considered geoengineering. Consequently, a governance sub-group was formed to consider the sensitivities in this area.

Geoengineering is the deliberate large-scale intervention in the Earth’s natural systems to counteract climate change.

• The large-scale removal of carbon dioxide from the atmosphere (‘carbon dioxide removal’ – CDR);

• The reflection of more sunlight back into space to cool the planet (‘solar geoengineering, or solar radiation modification’ – SRM).

There are big questions about significant risks and potential trade-offs some of these approaches would bring and how they compare with the risks of a warming world.

Figure 5. Buying time with solar geoengineering.

Fig 5 is a schematic produced by the proponents of SRM. They argue that, despite aggressive measures to cut greenhouse gas emissions and a programme of CO₂ removal, SRM will need to be deployed for a period to keep climate impact within tolerable limits.

The presentation traced the evolution of thinking on geoengineering governance from the Royal Society report and the evolution of the Oxford Principles in 2009 through a range of successive stages: the Code of Conduct for Responsible Geoengineering Research (CCRGRin 2015/7; the Carnegie Climate Governance Initiative (C2G) et al in 2017; the 2018 IPCC report on 1.5 degrees; the UN Environmental Assembly (UNEA) of April 2019; and, most recently, SCoPEx, the Stratospheric Controlled Perturbation Experiment, led by the Keutsch Group at Harvard. Some of the principles developed in the above initiatives are now the subject of international agreement, others are voluntary.

The conclusion on governance in the context of this conference are:

• Creating persistent cooling contrails at scale may be considered as solar geoengineering but it could help to address temperature overshoot;

• Pending internationally negotiated governance for solar geoengineering, the SCoPEx project governance approach provides key learning;

• Avoiding persistent warming contrails at scale is not geoengineering but a significant mitigation opportunity;

• The motivation of governance measures should be to avoid doing harmful things;

• The SCoPEx project may have some suggestions for governance;

• The key watchword is transparency, transparency, transparency.

GBD CAG objectives

A trial project of contrail-reduction flying is underway at NATS. (NATS)

The final presentation was given by John Green of Greener by Design. Its subject was the Contrail Avoidance Group linked to Greener by Design, how it came to be, what it had achieved and what it aims to achieve.

Greener by Design (GBD) first came together in March 2000 as a co-operative group representing the civil aerospace community, under the title Air Travel – Greener by Design, with the aim of reducing air travel’s environmental impact. Its Technology Sub-Group produced a report in July 2001 addressing design and technology possibilities to reduce noise and air pollution around airports and also to reduce impact on climate. The last of these was taken by the Sub-Group as the most important. NOX was the only non-CO2 emission discussed in the report.

In 2004 the Aerospace Innovation and Growth Team (AEIGT), a UK government body, asked the GBD Technology Sub-Group, to address a list of environmental questions and make recommendations for future research. The Sub-Group report, issued in July 2005, contained many recommendations including one, prompted by research at DLR and Imperial College London, on contrail avoidance - 'The practicalities and difficulties of adapting the European air-traffic-control system to enable contrail formation to be reduced by denial of critical flight levels and by re-routing'. The recommendation met a general pushback. It would be too difficult and too costly for airlines and the science base was not secure enough.

In June 2011, at a workshop held at the Royal Society by the COSIC (Contrail Spreading Into Cirrus) project led by Leeds University, Ulrike Burghardt of DLR presented a prediction of radiative forcing by contrail cirrus. This, the first quantitative treatment of the problem, was generally recognised as a potential game changer.

Four years later GBD held a workshop/conference in London with the title 'Contrail-cirrus, other non-CO2 effects and smart flying'. In the round table discussion that concluded the meeting, the majority view was that the science base was now secure enough to support a policy of contrail reduction by 'smart flying'. It was agreed that the aim should be to begin regionally in the north Atlantic, with Europe taking the initiative. Someone had to start the process and GBD undertook to do so. This led to an informal meeting in April 2016 at Gatwick Airport of a group of key participants in the workshop and the formation of the Contrail Avoidance Group (CAG).

The founder members of the group were from the DLR Institute of Atmospheric Physics, the GBD Executive Committee, NATS and the University of Reading. The conclusions of the meeting were: (1) the science base is now good enough to justify proceeding; (2) the ultimate aim is to stage a flight demonstration of contrail reduction by ATM in the Shanwick OCA of the north Atlantic; (3) paper studies, a 'virtual demonstration', are needed to make the case for a real demonstration; (4) a sub-group will be formed to review broader ethical and governance issues, e.g. geoengineering.

CAG has met four times since then. There has been no external funding, all work has been done within existing research funds or pro bono, but the presentation listed a substantial volume of research and publications that has been stimulated by the CAG. The presentation noted two important recent developments, the EASA study(3) and the formation of a plan by the Aerospace Technology Institute at Cranfield to build a consortium to conduct the contrail avoidance trial that the CAG has envisaged.

The CAG take-home messages are:

• The non-CO2 climate impact of aviation is about twice as important as that of CO2 alone and the greater part of it is from contrails and contrail-cirrus;

• Contrails can be substantially reduced by a small alteration in cruise altitude or route with minimal effect on CO2 emission and airline costs;

• This change in operational procedure can be adopted by the world fleet in less time and at a fraction of the cost needed for equivalent improvements via new technology;

• The development of safe, worldwide ATM procedures for contrail avoidance will require enlightened leadership. A high profile, scientifically-driven demonstration in the Shanwick OACC, managed by NATS, would be an excellent starting point.

It is time to take the next step.

 

Day 2: Roundtable and summary

During the Covid-19 grounding of flights, DLR research aircraft gathered atmospheric data in the BLUESKY project. (DLR)

The conference concluded with a roundtable of the Day 2 speakers, under the chairmanship of Iain Gray, Director of Aerospace at Cranfield University.

There was a general sense that regulators were engaged but were not moving fast enough. Paul Madden argued that there was close contact with the regulators regarding NOx standards but he acknowledged that on SAF more could be done, a view shared by Christiane Voigt. Voigt stressed the need to put incentives in regulatory agreements. Stephen Arrrowsmith acknowledged that non-CO2 is back in the political spotlight but there is a need to maintain momentum. Regulators are looking for confidence that they have the appropriate context to measure (metrics, time horizon) and that the options that are being considered will have the environmental benefits attributed to them - a no regrets policy.

Turning to ATM capacity, it was agreed that a key learning point from a trial will be how ATM copes with an additional role. Rudiger Ehrmanntraut proposed a KPI on how well the centre can operate in avoiding contrails. Ian Poll noted that the new system capability across the North Atlantic should help improve capacity.

Regarding the science, Helen Rogers acknowledged that there were some known unknowns, particularly soot, but that there is a very good understanding of a lot of these effects. She did not see significant changes coming that would lead to incorrect policy decisions. She did, however, recommend that a task force be appointed to address the issue of metrics and time frames, possibly leading to a basket of metrics, some for regional impacts and others global. Marc Stettler broadly agreed, acknowledging the limitation of the detail and accuracy of the computer models and confirming the need to do more projects like the MUAC trial, expanding into larger regions with more flights to both improve the science and understand better how it can be operationalised.

For John Green, contrail avoidance is the really low hanging fruit of aviation. The bogeyman is that people still talk about it involving problems and costs for the airlines. People need to understand that recent work shows that you can achieve a substantial reduction in warming from air travel at very little cost to the airlines. The downside for the airlines is very small, which, on top of the positive impact of this change on the perception of aviation’s contribution to the world’s problems, should lead to the airlines getting behind this.

Iain Gray concluded that the science is sufficiently mature now to support moving to the operational side of things. COP 26 later this year invites an announcement around a physical demonstration of contrail avoidance. This is not a trade-off but something that we have to do. He vowed to never again attend a meeting on aviation’s emissions without taking the opportunity to introduce the subject and importance of the non-CO2 agenda. Furthermore, he committed himself to be a champion of the need to rapidly establish a big demonstration project that brings to life the issues discussed at the conference.

References

1. D.S. Lee et al, The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, Atmospheric Environment, 244 (2021) 117834.

2. V. Grewe et al, The contribution of aviation NO_X emissions to climate change: are we ignoring methodological flaws? Environmental Research Letters, Volume 14, Number 12, December 2019.

3. S. Arrowsmith et al, Updated analysis of the non-CO₂ climate impacts of aviation and potential policy measures pursuant to EU Emissions Trading System Directive Article 30(4), 2020, www.easa.europa.eu/document-library/research-reports/report-commission-european-parliament-and-council.

4. R. Teoh et al, Mitigating the climate forcing of aircraft contrails by small-scale diversions and technology adoption, Environmental Science and Technology, https://dx.doi.org/10.1021/acs.est.9b05608.

5. D.I.A. Poll and U. Schumann, An estimation method for the fuel burn and other performance characteristics of civil transport aircraft in the cruise. Part 1 Fundamental quantities and governing relations in a general atmosphere, The Aeronautical Journal, Vol. 125, No. 1284, pp 296-340, February 2021.

 

Dr John Green FRAeS
11 May 2021