Dr JOHN GREEN FREng FRAeS reports on the Greener by Design Workshop on the possibilities of reducing the climate impact of non-CO2 emissions from civil aircraft by tactical flightpath adjustments. 

Can small adjustments in flightpaths prevent the build-up of contrails?

It was not so long ago that any discussion of climate change in the press or on the radio started from the assumption that air travel was the chief culprit. That has changed slightly. Among others, IATA (the International Air Transport Association) has consistently pointed out that air travel accounts for only 2% of global man-made CO2 emissions and the press appears to have taken this on board.

However, while ‘everyone knows’ that CO2 is the greenhouse gas that causes climate change, and this fits with the IATA story, it is not the full picture. There are other important greenhouse gases. The IPCC (Intergovernmental Panel on Climate Change) attributes only 70% of man-made greenhouse-gas global warming to CO2 emissions. Much of the remaining 30% comes from methane (CH4) and nitrous oxide (N2O) emissions, which are both powerful greenhouse gases.

Aircraft do not emit either methane or nitrous oxide but they do emit other oxides of nitrogen (NO and NO2, collectively termed NOX). These are short-lived. They are not significant greenhouse gases in themselves but, via a sequence of chemical reactions, they have the effect at altitude of increasing ozone (O3, a powerful but short-lived greenhouse gas) and reducing methane. Aircraft also form contrails, which, in some climate conditions persist and develop into cirrus cloud. Both contrails and cirrus have a powerful effect on the Earth’s energy balance. The net effect of these and other, less significant non-CO2 emissions from aircraft is to increase the climate impact of air travel substantially. When the non-CO2 emissions are taken into account, air travel is estimated to account for about 5% of man-made global warming – two and a half times its proportional contribution to man-made CO2.

Scientific evidence

 Global radiative forcing from contrail cirrus and optical depth at 250 hPa for the year 2002. (Burkhardt & Kärcher, Nature)

It is these non-CO2 emissions that the Greener by Design workshop addressed. The idea of ‘smart flying’ to reduce climate impact by slight changes in flight altitude to avoid contrail formation has been around for a decade. Any move to try it for real has been inhibited, however, by scientific uncertainty in the climate impact of contrails and, in particular, of the cirrus cloud formed from contrails. In addition, it has to be established that any such changes in routing do not lead to extra CO2 emissions that offset any gain from contrail avoidance. Research at DLR (Deutches Zentrum für Luft- und Raumfahrt) provided a physics-based estimate of the impact of contrail-cirrus in 2011 and subsequent work has consolidated and refined the estimate. With this development, and the completion of the extensive REACT4C European study of reducing climate impact by re-routing, the time was ripe to gather together all the interested parties, scientists, operators, air-traffic managers and policy makers, to consider the future.

The morning was devoted to the atmospheric science, the potential for reducing climate impact by re-routing air traffic and the appropriate metrics for trading climate benefit against the inevitable increase in operating costs. The afternoon considered the potential impact on air-traffic management and on the airlines and also the future questions for policy makers.

In the morning, Prof Keith Shine of Reading University led off with an overview of all the significant non-CO2 emissions from aircraft. This was followed by Dr Klaus Gierens of DLR focusing on the current understanding of the physics of contrail-cirrus and Prof Dr Ulrich Schumann of DLR on our current ability to predict the climate impact for any specific flight. Prof Steven Barrett of MIT discussed the effect of biofuels on contrails and contrail-cirrus and finally Prof Dr Volker Grewe reported on studies at DLR on mitigating the climate impact of air travel, with emphasis on the comprehensive European REACT4C study, which considered impacts of contrails and NOX emissions. The morning ended with a panel discussion seeking a consensus on what was secure and what was still uncertain.

Smart flying

To reduce their impact on the atmosphere, aircraft could alter altitude, change routes or land more. (Volker Grewe, DLR) 

In the context of smart flying, arguably the most important point that emerged was that the climate impact of contrail-cirrus varies strongly with atmospheric conditions, time of day, latitude and what is below − cloud, land or sea. The net effect is a warming, the result of warming by trapping infra-red radiation, particularly at night, offset by cooling as a result of incoming sunlight being reflected or scattered back into space in daylight hours. Because of the large variability, a significant reduction in climate impact can be achieved by avoiding contrails on only a small proportion of flights – perhaps only 1%.

A powerful early advocate of smart flying was the late Hermann Mannstein of DLR. In 2005 he showed that a substantial fraction of contrails and contrail-induced cirrus can be avoided by a relatively small change in flight level, due to the shallowness of the ice-super-saturated layers necessary for the formation of persistent contrails. His colleague Ulrich Schumann, in reporting on the prediction capabilities of the DLR CoCiP (Contrail Cirrus Prediction tool), illustrated Mannstein’s concept with two diagrams. The first illustrates route optimisation by changing flight level to avoid forming contrails. The second shows the next step, when conditions are propitious, of changing flight level in order to create contrails that will reflect sunlight and have a net cooling effect. If the necessary meteorological information is available, the CoCiP programme can estimate the outcomes of both kinds of action.

Like contrail-cirrus, the other powerful non-CO2 emission, NOX, has both a warming effect, through the creation of ozone, and a cooling effect through the destruction of methane. On average, the climate impact of NOX increases strongly with altitude of emission and Volker Grewe described studies by DLR of reducing net climate impact by reducing cruise altitude and Mach number. For an existing aircraft, flying lower and slower reduced climate impact by 30% from a cost penalty of around 5%. If the same aircraft were re-winged to be optimised to fly lower and slower, the 30% reduction could be achieved without a cash cost penalty (but a DOC penalty due to reduced cruise Mach number and hence reduced utilisation and reduced return on investment).

He then described the eight-partner REACT4C programme which investigated the potential for climate-optimised flight routing across the north Atlantic involving both changes in track and in altitude. One illustrative example showed a striking difference in the impacts of NOX released on a particular winter’s day at two points (A and B in the synoptic chart) which were three hours flying time apart on a transatlantic flight. One packet of NOX was carried northward and had a life of three weeks; the ozone it created meandered around the northern hemisphere for the next two months. The other was carried southward into the more chemically reactive tropics, had a life of one week but created more than three times as much ozone which travelled around the tropics and southern hemisphere for the next two months. With strong sunlight in the tropics and weak sunlight and shorter days in the northern winter, the warming from the second packet was an order of magnitude greater than that from the first, released three hours later on the same flight.

The overall conclusion from REACT4C is that, applied to all transatlantic flights for a full year, climate-optimised routing could reduce climate impact by more than 20% at a cost penalty of around 7%. However, the low-hanging fruit are more easily gathered. Valuable reductions in climate impact can be had at a cost penalty of around 0.5% by small changes in flight level to reduce contrail formation.

The paper by Steven Barrett concluded that the use of biofuels would probably increase net warming from contrails while cleaner burning engines should reduce it; two theoretical ideas involving operational variations in combustor characteristics to change contrail thickness were also mentioned.

Cloud control

Smart flying would only affect a small porportion of flights.

In the panel discussion at the end of the morning, it was agreed that the large variability in contrail-cirrus thickness and climate impact holds out the promise of significant benefit by adjusting the routes of only a small proportion of flights. It was suggested that deliberately creating contrails in daytime could be regarded as geo-engineering and raised ethical questions that would have to be addressed. Finally, it was thought that average temperature response (ATR) over a defined time horizon is an appropriate metric for assessing the impact of the non-CO2 emissions. The time horizon is more a matter for policy makers than scientists but in due course it should be possible to arrive at a consensus.

In the afternoon there were papers by Dr Jarlath Molloy of NATS, Captain Hugh Dibley of the RAeS and Prof David Lee of Manchester Metropolitan University, followed by a panel discussion of all the issues.

The NATS perspective

Jarlath Molloy outlined prerequisites of a contrail avoidance system and suggested the Shanwick Oceanic Area Control Centre, covering a large area to the west of Ireland, as a suitable basis for a trial of contrail avoidance. The control centre is at Prestwick and handles approximately 80% of transatlantic traffic. It has the attraction that most incoming flights from America pass through the region at night, when contrail-cirrus has its greatest warming effect. He expressed reservations about seeking positively to form contrails in the day, which he too saw as geo-engineering, and listed the challenges, technical and non-technical, that would have to faced. Despite these challenges, he concluded that a contrail avoidance system is now technically possible and that a trial of it would be most likely to succeed in oceanic airspace. The implementation of satellite surveillance (ADS-B) in 2018 may allow more flexibility than the current system in planning and adapting routes to avoid contrail formation.

Smart flying challenges

Would smart flying involve extra training or equipment?

Hugh Dibley discussed the challenge smart flying would present to the airlines. Besides the additional fuel used in re-routing there was potential disruption to schedules, crew scheduling and aircraft rotation. Aircraft would need additional on-board instrumentation (he listed the equipment given in a US patent application by Mannstein and Schumann), possibly requiring additional maintenance. There would need to be some additional crew training. All these carried costs for the airlines. In the first instance, he suggested the provision of funds for airlines to carry out trials on suitable routes.

Regulations timelag

David Lee drew on his long experience as a scientific adviser in policy discussions within the EU and ICAO of CO2 regulation. The timescales, from first action to final implementation were exceedingly long, and CO2 was technically fairly straightforward. He foresaw difficulties in seeking international agreement on any form of regulation covering the non-CO2 emissions and highlighted the problems of measurement, verification and transparency that had to underlie any regulation. He posed a number of questions and left the meeting to reflect on them.

The workshop concluded with a lively panel discussion chaired by Prof Ian Poll of Cranfield. The panel members were Andrew Booth, flight operations specialist at Rolls-Royce, Prof Brian Collins of University College, former Chief Scientific Adviser to both DfT and BIS, Prof Volker Grewe of DLR, Dr Jarlath Molloy Environmental Affairs Manager at NATS and Dr Steve Smith, Head of Climate Science for the Committee on Climate Change. The audience joined in the discussion. The panel was invited to consider five questions though, in the event, one was dropped for lack of time.

What are the main obstacles to implementing smart flying? The perceived complexity of what is involved, the reluctance of policy makers to embrace something which entails uncertainty and the lack of interest by the industry were all cited as potential obstacles, though none should be a fundamental stopper. The need for a high degree of agreement in the scientific community about the appropriate trade-off between the effects of the short-lived and long-lived contributors was agreed, but the climate scientists believed that we were now at a point where this is not a fundamental obstacle. Cost needs to be addressed but, again, this is not seen as a fundamental obstacle.

What regulatory measures or incentives can be envisaged? This came down to the question of how to change behaviour. It was thought all stakeholders would need to be involved in the debate but there was no clear view as to whether it could best be done by regulation or by an incentive similar to inclusion in a carbon trading scheme.

Practical steps

What would be the next steps?

Would it be best introduced regionally? Although it was recognised that eventually smart flying should be adopted worldwide, the final consensus was that it would be best to begin regionally; the north Atlantic, with Europe, taking the initiative, was argued to be the most promising starting point. The exceedingly slow progress of ICAO in developing regulations to limit CO2 emissions was seen as a powerful argument against any attempt to take smart flying forward internationally though ICAO. Further, it was argued that some kind of demonstration would be needed before any general introduction could be contemplated. Again, this argued for the process to begin in the north Atlantic, probably in a single Oceanic Area Control Centre, such as Shanwick.

Finally, what actions and by whom, are needed to move towards implementation? The proposition was made for a top-down approach, with the Presidents of the appropriate learned bodies jointly convening a meeting of stakeholders to consider the way forward. Counter to this, some were not convinced that the scientific evidence was yet robust enough to justify any action other than continued research. The consensus among the scientists, however, was that the scientific understanding and forecasting ability did indeed justify moving forward towards some kind of practical demonstration of the practicability of smart flying.

At the very end of the discussion, the summary conclusion was that the time was ripe to move towards such a demonstration. Initially, the case for such action needed to be made, the readiness of the science base needed to be made clear and the form of the demonstration had to be determined. Someone had to start the process and Greener by Design undertook to do so. At the next meeting of its Executive Committee, Greener by Design will develop an outline plan to engage the key stakeholders in planning a way forward. The aim would be to generate the case for, and the route map to, the creation of a practical demonstration of the practicality and challenges of reducing climate impact by smart flying.  


12 February 2016