AEROSPACE Climate change conference report

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.

ESA

On 23-24 March 2021 the Royal Aeronautical Society hosted a joint conference shared between its Greener by Design (GbD) group and the Institute of Atmospheric Physics of DLR, the German Aerospace Centre at Oberpfaffenhofen. The event was organised by the Contrail Avoidance Group (CAG), 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 X 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, online 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.

Day 1: The science base

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 Intergovernmental Panel On Climate Change (IPCC) RCP (representative concentration pathway), but the importance of CO2 emissions from aviation is starkly clear.

Oxides of nitrogen

The next two presentations addressed different aspects of the climate impact of NOX X. 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 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.

Data has been collected using a handful of instrumented airliners flying normal operations under the In-service Aircraft for a Global Observing System (IAGOS) initiative. IAGOS IAGOS

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

The next presentation was by Volker Grewe of DLR, who addressed the current assessment of NOX X impact and its dependence on time and place of emissions.

Contrails and contrail cirrus

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’. 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. The third presentation on contrails was by Luca Bugliaro of DLR, whose subject was contrail and contrail cirrus observations by satellite.

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 Upper Troposphere/Lower Stratosphere (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. It is 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 radiation forcing (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 shortlived emissions.

Emissions reduction by engine technology

The next presentation was by Paul Madden of RollsRoyce. 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. The company 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, Rolls-Royce is working to clear the way for the use of 100% sustainable aviation fuel (SAF), as against the current 50% limit. 

The presentation briefly described the technology of the RQL (rich burn quick quench lean burn) combustor, which is the low NOX X 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 X and near zero nvPM at high powers.

Mitigating contrail impact

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 Advanced Technology Research Aircraft (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.

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

Meanwhile, Marc Stettler of Imperial College, London 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 longwave 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.

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 24,500ft over Belgium, Luxembourg, the Netherlands and north-west Germany. It is the third busiest upper area control centre in Europe.

Ian Poll of Cranfield University concluded this session 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.

Regulation

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. 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.

Geoengineering – could contrails cool the planet?

This presentation was followed by one from Robert Whitfield of GbD, who chairs the Governance SubGroup of the GbD Contrail Avoidance Group. When the CAG first met, two options for reducing climate impact were on the table. The first was changing cruise altitude to avoid forming evening and night-time warming contrails. 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.

GbD CAG objectives

The final presentation was given by myself of GbD. Its subject was the Contrail Avoidance Group (CAG) linked to GbD, how it came to be, what it had achieved and what it aims to achieve. Summing up – the CAG take-home messages were:

  • 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 (Oceanic Area Control Centre) OACC, managed by NATS, would be an excellent starting point. It is time to take the next step.

Day 2: Roundtable and summary​

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 X 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.

Rolls-Royce’s advanced low emissions combustion system (ALECSys) will be flown on a 747 test bed this year. Rolls-Royce

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 taskforce 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 the author, 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. COP26 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.

Read an extended technical report from this conference on the AEROSPACE Insight blog

Cutting Aviation’s Climate Change Impact, RAeS Conference, 19/20 October 202​

 

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