As calls for action increase over the climate impacts of aircraft contrails, airborne research is intensifying with the expansion of two key projects in Europe and the US. In Germany, Lufthansa has started converting an Airbus A350-900 for use as a flying research laboratory as part of IAGOS-CARIBIC, a long-running European programme that uses commercial airliners to gather data on air quality for scientific evaluation. The A350 will be equipped with around 20 measuring instruments for a multi-year project beginning late next year to measure trace gases, aerosol and cloud parameters on selected long-haul flights. In the US, engine manufacturer GE Aerospace and NASA have just completed two flights with a Boeing 747 testbed aircraft to record three-dimensional imaging of contrails to better understand how they form and behave in certain conditions. The 747’s emissions were scanned using Light Detection and Ranging (LiDAR) technology installed on a following aircraft.

IAGOS (In-Service Aircraft for a Global Observing System) is a European programme established in 1994 to assess atmospheric composition, air quality and climate, using commercial jets as research platforms. There are currently 10 widebody Airbus A330 and A340 family jets from seven airlines deployed in the project, with another 10 having previously been used, again from seven participating airlines.  

The Lufthansa A350 selected for the IAGOS programme, registered D-AIXJ, is almost seven years old, and will add a new-generation airliner to the research fleet.

A measuring laboratory weighing two tonnes is being developed for the next phase of the programme, and once fitted with some 20 instruments, will be installed in the cargo hold of the A350 on selected scheduled flights from late 2025.

The onboard laboratory will be connected to the air intake system on the jet’s outer fuselage through permanently installed pipes, and used to measure over 100 trace gases, aerosol and cloud parameters from the ground up to the tropopause atmospheric region, at altitudes between nine and 13 kilometres.

The IAGOS programme is led by the Jülich Research Centre, one of Europe’s largest research organisations, which combines the expertise of global partners in weather services, airlines and the broader aviation sector.

The programme combines the complementary concepts of two research projects: MOZAIC (Measurement of Ozone, Water Vapour, Carbon Monoxide and Nitrous Oxides by Airbus In-Service Aircraft), which was funded by the European Commission between 1993 and 2004, and CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container).  

Lufthansa, which with Air France was an IAGOS launch partner in 1994, has gathered climate-related data for research on more than 35,000 of its passenger flights over the three decades.

Together with the Jülich Research Centre and Karlsruhe Institute of Technology, the Lufthansa Group has fitted a total of six Airbus aircraft with measuring equipment since the programme was inaugurated to collect information about atmospheric conditions during scheduled flights.

Lufthansa currently has two aircraft, an A330 and an A340, deployed in the programme, as well as another A330 from sibling airline Eurowings Discover.

On December 9, the Eurowings jet was used to gather climate data during a 10-hour, 45-minute flight from Frankfurt to Orlando, Florida. The information was collected continuously while the aircraft flew at an altitude of more than 10,000 metres (33,000 feet) over a distance of 7,600 km.

Other airlines currently participating in the programme are Taipei-based China Airlines, with two A330s, and Air Canada, Air France, Cathay Pacific, Hawaiian and Iberia, each with one A330.

After each flight, climate information gathered by the aircraft is sent automatically to the database of Centre National de la Recherche Scientifique in Toulouse, France, from where it is accessible for global research.

The data is currently used by about 300 organisations worldwide to provide fresh insights into climate development and atmospheric composition and help refine climate models and improve weather forecasting.

“We are proud to have been able to make a significant contribution to climate research for 30 years,” said Lufthansa Group’s Chief Technology Officer, Grazia Vittadini. “Through our commitment, we are helping to sustainably improve climate models and weather forecasts. Scientifically-sound findings are the basis for targeted measures on the path for more sustainable aviation.”

GE/NASA contrail research flights

In the US, GE Aerospace and NASA have built upon a 50-year collaboration by performing two research flights for their Contrail Optical Depth Experiment (CODEX) in which three-dimensional imaging was generated of contrails created by GE’s Boeing 747-400 Flying Test Bed aircraft.

The 747 was trailed by a G-111 aircraft from NASA’s Langley Research Centre in Virginia, which deployed Light Detection and Ranging (LiDAR) technology to scan the wake of the larger jet, enabling researchers to use new imaging to better understand how contrails form and behave.

During the flight tests, 3D images were generated of contrails from all four CF6 engines on the 747. GE Aerospace was also able to isolate the contrails from a single engine on the test jet.     

The flights expanded the company’s capabilities ahead of flight tests it is planning during this decade to assess the performance of new commercial aircraft engine technologies, including Open Fan, advanced combustion designs and other propulsion systems.  

“Understanding how contrails act in flight with the latest detection technology is how we move innovation forward,” said Arjan Hegeman, GE Aerospace GM of Future Flight Technology. “These tests will provide critical insight to advance next generation aircraft engine technologies for a step change in efficiency and emissions.”

Dr Rich Wahls, manager of NASA’s Sustainable Flight National Partnership, welcomed participation “on this first-of-its-kind flight experiment” in helping to reduce the impact of contrails.

“NASA is advancing the scientific understanding of contrails to improve our confidence in future operational contrail management decisions that consider overall climate impact and economic trades,” he said.

NASA, German Aerospace Centre (DLR) and contrail forecasting and management company SATAVIA, are also working together on atmospheric forecasting to identify the best conditions for studying the formation of contrails. SATAVIA was recently acquired by Aerospace Carbon Solutions, a division of GE Aerospace.

In this collaboration, DLR will help to identify the altitude and dimensions of contrail-forming regions, so that flight tests can be conducted using the LiDAR technology to improve contrail prediction, while SATAVIA will use the flight test results to validate and improve its numerical weather prediction capability, used to forecast contrail formation conditions.

At this year’s Farnborough Airshow, the chief technology officers of GE Aerospace, Boeing, Airbus, Dassault, Rolls-Royce, RTX and Safran called for government support to expand research that enhances scientific understanding of aviation non-CO2 effects such as contrails, nitrogen oxides, sulphur, aerosols and soot.

Top photo: The first IAGOS system has been in use on a Lufthansa Airbus A340-300 since 2011

Bottom photo: GE Aerospace’s 747 Flying Test Bed

Tony Harrington

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A global coalition of around 50 scientists specialising in climate and aviation-linked disciplines has signed an open letter warning of the warming impacts of aircraft contrails and calling on global decision-makers to urgently address the problem. Among the signatories is Jean-Pascal van Ypersele, a Belgian climatologist and former vice chair of the UN’s Intergovernmental Panel on Climate Change (IPCC). Contrails are the most significant of aviation’s non-CO2 effects, said the scientists in their letter. The communiqué was published by Brussels-based climate advocacy group Transport & Environment (T&E), which has also just released a report recommending deviating flight paths that it claims would halve the number of warming contrails by 2040, with a climate benefit substantially larger than the impact of the CO2 emissions caused by the extra fuel burn.

Contrails are formed when vapour and particles of soot emitted from aircraft engines freeze into ice crystals in certain conditions and regions to create clouds that can trap heat.

“Global aviation traffic doubled between 2005 and 2019, and its CO2 emissions grew by 40%,” said the scientists in their letter, “and that just covers part of the problem. Planes cause much more warming than just through their CO2 emissions. Non-CO2 effects of aviation, such as nitrogen oxides and contrails, warm the planet at least as much as aviation’s CO2.

“Most contrails dissolve within a few minutes, but in certain conditions they can persist in the atmosphere, spread out and become artificial cirrus clouds with a net warming effect. The climate impact of these effects has been known for more than 25 years.”

But despite this knowledge, said the scientists, little has been done to address the problem.

“We, aviation and climate scientists, call upon global decision-makers to implement solutions to tackle non-CO2 effects of aviation on top of decarbonisation efforts,” they said.

“This starts by better awareness-raising of the general public. Airline passengers should be informed of the full climate impact of flying when booking a flight and companies performing business flights should include non-CO2 in their corporate reporting.”

As well, said the scientists, large-scale tests should be performed of contrail avoidance measures, supported by research and backed by government policies and monitoring designed to reduce warming contrails and other non-CO2 impacts of aircraft.

“This is a ‘no regrets’ approach that will help to slow climate change by a significant margin,” the scientists said. “Delaying action would be a critical error.”

Their letter closely follows the release of a Transport & Environment report, which states that by “tweaking the flight plans of only a few flights”, contrail-induced warming could be halved by 2040.  

The report says the net warming effect of aircraft contrails was “at least as important” as the damage caused by CO2 emissions from aircraft and that just 3% of flights produce 80% of contrail warming. It argues contrail reductions could be achieved simply by changing flight plans slightly to avoid conditions known to cause the phenomenon.

Such a strategy would marginally increase fuel consumption, said T&E, but would still deliver greater environmental benefits by preventing contrail formation.

“Changing flight paths to avoid contrails will only happen on a very small number of flights, and only for a small part of the journey,” says the report, adding that the climate benefits of from avoiding the most warming contrails would exceed the CO2 penalty of rerouting the plane by a factor between 15 and 40 times.

“The extra fuel burnt to avoid contrails would be less than 0.5% on the whole fleet over a year. And on those few flights where rerouting will happen, 80% of the contrail warming of the flight can be avoided with an extra fuel burn of 5% or less.”

The study conservatively estimates that slightly changing the flight path of a Paris – New York flight to avoid contrail formation would cost less than €4 per ticket, covering extra fuel used for the diversion and the use of associated technologies including humidity sensors and satellites.

Commenting on the report, T&E’s aviation technical manager, Carlos Lopez de la Osa, said some in aviation “overstate the scientific uncertainty of warming contrails – but the climate benefits of contrail avoidance are huge, and solutions are improving by the day.

“The aviation industry is being offered a simple and cheap way to reduce its climate impact. By identifying the very few flights that cause warming contrails and then tweaking their flight paths, we can have an immediate effect on contrails warming. So let’s no longer discuss whether we have to do it, but how to do it.”

Photo (T&E): Contrails, short for condensation trails, are created by aircraft flying through cold and humid areas of the atmosphere

Tony Harrington

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The results of a European study into the impact of a widebody jet flown with 100% sustainable aviation fuel show a substantial 56% reduction in the number of contrail ice crystals produced by unblended SAF compared to Jet A-1 fuel. The project, involving Airbus, Rolls-Royce, the German Aerospace Centre (DLR) and SAF producer Neste, was the third stage of the Emission and Climate Impact of Alternative Fuels (ECLIF) programme, with in-flight and ground emissions tests taking place in 2021. They worked together to quantify the reduction in engine soot particles produced by a Rolls-Royce powered Airbus A350 fuelled by Neste and trailed by a DLR research aircraft. In certain weather conditions, airborne vapour freezes around soot particles emitted by aircraft engines, creating cloudy canopies of ice crystals which can trap heat in the atmosphere.

Prior to the A350 test flights using 100% SAF – a campaign designated as ECLIF3 – airborne research was conducted in 2015 (ECLIF1) to characterise the emissions of synthetic fuels, followed in 2018 by flight tests with NASA (ECLIF2) to demonstrate that 50/50 blends of conventional jet fuel and SAF could reduce the climate damage caused by aircraft condensation trails.  

The ECLIF3 tests were conducted over the Mediterranean and southern France using the first Airbus A350 aircraft built by the airframer and powered by two Rolls-Royce XWB-84 engines. Conventional Jet A-1 was used as the reference fuel, while the comparative SAF was a mix of hydro-processed esters and fatty acids and synthetic paraffinic kerosene (HEFA-SPK).

In all conventional jet fuels, naturally-occurring aromatics serve as a vital sealant within the aircraft engine to help prevent fuel leaks. But this compound is slow-burning and emits soot particles which contribute to contrail crystals that can remain for several hours in cold, humid conditions at altitudes of eight to 12 kilometres, and can have a local warming or cooling impact depending on the position of the sun and underlying surface, with a warming effect predominating globally. Many types of SAF are free of aromatic compounds.

During the ECLIF3 programme, which involved multiple flights, the A350 testbed departed Toulouse Blagnac airport in France while DLR’s research jet, a Falcon 20-E, flew from the agency’s base in Oberpfaffenhofen, Germany, meeting at multiple points over the Mediterranean and southern France. The research aircraft was equipped with instruments to assess exhaust gases, volatile and non-volatile aerosol particles, and contrail ice particles produced by the A350, which it followed at various distances to capture data on emissions and condensation trails produced by both conventional Jet A-1 and Neste’s SAF.  

DLR used global climate model simulations to estimate how contrails could change the energy balance in the earth’s atmosphere, an effect known as radiative forcing. The tests revealed a 26% reduction in the overall climate impact of contrails when 100% SAF was used.

“The results from the ECLIF3 flight experiments show how the use of 100% SAF can help us to significantly reduce the climate-warming effect of contrails, in addition to lowering the carbon footprint of flying – a clear sign of the effectiveness of SAF towards climate-compatible aviation,” explained Markus Fischer, DLR Divisional Board Member for Aeronautics.

“We already knew that sustainable aviation fuels could reduce the carbon footprint of aviation,” added Mark Bentall, head of Research and Technology Programme, Airbus. “Thanks to the ECLIF studies, we now know that SAF can also reduce soot emissions and ice particulate formation that we see as contrails. This is a very encouraging result, based on science, which shows just how crucial sustainable aviation fuels are for decarbonising air transport.”

Alexander Kueper, Neste’s VP, Renewable Aviation Business, said SAF was recognised widely as a crucial means of mitigating the climate impacts of aviation. “The results from the ECLIF3 study confirm a significantly lower climate impact when using 100% SAF due to the lack of aromatics in Neste’s SAF used, and provide additional scientific data to support the use of SAF at higher concentrations than the currently approved 50%.”

And Rolls-Royce’s Director of Research and Technology, Alan Newby, said the use of SAF at higher blend ratios would be a key factor in enabling aviation to achieve its target of net zero CO2 emissions by 2050. “Not only did these tests show that our Trent XWB-84 engine can run on 100% SAF,” he said. “The results also show how additional value can be unlocked from SAF through reducing non-CO2 climate effects as well.”

The ECLIF3 research team says its programme is “the first in-situ evidence of the climate impact mitigation potential of using pure, 100% SAF on a commercial aircraft,” and has reported the findings of its tests in the Copernicus journal Atmospheric Chemistry and Physics (ACP) as part of a peer-reviewed scientific process. The ECLIF3 programme also includes members of the National Research Council of Canada and the University of Manchester.

Last year, Boeing, NASA and United Airlines conducted contrail research in the US using a Boeing 737 MAX twinjet with one engine powered by 100% SAF and the other by conventional jet fuel, and Virgin Atlantic partnered with Rolls Royce, Boeing, Air BP and Virent to perform a trans-Atlantic crossing using a Boeing 787 powered purely by SAF, with a blend of 88% recycled waste fats and oils and 12% sustainable aromatic kerosene. UK-based technology group SATAVIA also worked with 12 airlines over 65 flights to test its DECISIONX route optimisation software, which uses atmospheric modelling data to assist carriers in planning flight paths which avoid conditions in which contrails can form. The company said its tests avoided more than 2,200 tonnes of carbon dioxide equivalent (CO2e), or an average of more than 40 tonnes per flight, with little impact on aircraft fuel burn or flight distances.

Photo (S. Ramadier): Airbus A350 fuelled with 100% Neste SAF trailed by DLR’s Falcon 20-E research jet

Tony Harrington

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IATA has revealed it is developing a SAF registry that will enable airlines worldwide to buy sustainable aviation fuel regardless of where it is produced and have certainty they can claim the environmental benefits of the SAF they have purchased for regulatory compliance purposes. Due to be rolled out in the first quarter of 2025, the development of the registry is being supported in a pilot phase by 17 major airlines and airline group IAG, as well as the national authorities of six countries, aerospace manufacturers Airbus, Boeing and GE Aerospace, and fuel producer World Energy. IATA said the registry would help promote a global SAF market and accelerate uptake, which is required to meet a 1,000-fold increase in SAF production by 2050. It estimates SAF output of 0.5 million tonnes (Mt) in 2023, or a 0.2% share of global jet fuel consumption, which it forecasts will triple to 1.5 Mt (a 0.53% share) this year.

SAF is expected to account for up to 65% of the total carbon mitigation needed to achieve the sector’s net zero emissions by 2050 goal and the registry would help meet the needs of all stakeholders as part of the global ramp-up of SAF production, commented IATA’s Director General, Willie Walsh.

“Governments need a trusted system to track the quality and quantities of SAF used. SAF producers need to accurately account for what has been delivered and effectively decarbonised. Corporate customers must be able to transparently account for their Scope 3 emissions. And airlines must have certainty that they can claim the environmental benefits of the SAF they purchased,” he said.

Key and unique among the stakeholders involved with the project, points out the airline trade body, is the participation of the government civil aviation authorities of Brazil, Japan, Kazakhstan, Malaysia, Singapore and Switzerland. “Relevant authorities can swiftly validate and approve claims, update national emission inventories and align their actions with international standards, such as those set by ICAO,” it explained.

IATA assures its SAF Registry will be neutral with respect to regulations, types of SAF and any other specificities under relevant jurisdictions and frameworks, and, as the initiator, it is working with certification organisations and fuel producers to standardise data for efficient processing.

The registry will help airlines meet regulations such as ICAO’s CORSIA scheme and the EU ETS, as well as ensuring compliance with SAF mandates and providing transparency to authorities regarding emissions reductions. IATA said it would also ensure the sector’s agreed SAF accounting and reporting principles were adhered to, fully in alignment with international protocols and industry best practices, and provide safeguards against double counting and double claiming.

Marie Owens Thomsen, IATA’s SVP Sustainability and Chief Economist, said the registry would be run under independent governance to ensure its impartiality and robustness, and participation would be on a non-profit, cost-recovery basis. “We all understand that SAF is going to be more expensive than jet fuel so it is imperative we stop adding layers to existing costs,” she said. “We also want all airlines to be able to participate in this global market, without geographical restriction, and equally important, we want all SAF producers to be able to sell to all airlines.”

IATA has identified 140 publicly-announced SAF projects now in progress in 31 countries and involving over 100 producers, with a cumulative renewable fuel capacity of 51 Mt. Around 70% of the projects are in Europe and the Americas, reflecting conducive government policies to promote SAF. Although HEFA will continue to dominate SAF production, Europe is now driving diversification, acknowledges IATA, with more individual projects across broader technologies.

But, said Hemant Mistry, IATA’s Director of Energy Transition: “We are far from where we need to be and not enough progress is being made on feedstocks.”

He said there were ‘low hanging fruit’ solutions though to accelerate access to critical SAF quantities, at least in the near- to mid-term. SAF production in existing refineries could be swiftly expanded through co-processing, which is ASTM approved but currently has a blend limit of 5%, although there are moves to increase the limit to 30%. However, he added, co-processing should be seen as a transition facilitator and not as a goal in itself.

Another quick win would be to focus policy incentives on switching renewable diesel production to SAF as road transport demand reduces and shifts to alternative energies such as electrification.

“Policy is the make or break to ensure we will have adequate supplies of SAF,” Mistry told a briefing to journalists at the IATA AGM in Dubai.

IATA is forecasting global jet fuel consumption of 285 Mt in 2024, just below the 2019 pre-pandemic level of 288 Mt, generating CO2 emissions of around 900 Mt by the sector.

Following its announcement last year of a methodology to track industry progress towards its net zero emissions by 2050 target, IATA confirmed it is anticipating the launch of its TrackZero reporting platform in Q4 2024. IATA will aggregate and report annually inputs from its member airlines on an industry basis. Non-IATA member airlines are also encouraged to contribute data and participate in the reporting. Individual airlines may use the aggregate data of the TrackZero report to benchmark their progress towards decarbonisation.

Acting on non-CO2

With the spotlight increasingly focused on aviation’s non-CO2 climate impacts, in particular from contrails, IATA recently issued a report on the issue. While accepting the scientific consensus that contrails have an aggregated, averaged warming effect on the climate, it points out there is a low level of confidence on quantifying how warming they are.

“However, this is not an excuse not to act,” Alejandro Block, Manager, New Energy and Technologies, told the briefing. “We want to help this quantification and we have spent a year speaking to meteorologists, climate scientists, researchers and aircraft manufacturers. Everyone told us that more data was required, particularly around humidity.”

He said most scientific literature agreed that contrail predicting models were most sensitive to humidity, but the lack of data lowers the confidence on the accuracy of these models. IATA is now focusing on how to get more humidity data, he said, although only a very few aircraft were currently fitted with humidity sensors. This was not stopping airlines from acting, he pointed out, and a number were already involved in trials to understand contrail avoidance and running validation exercises to check the accuracy of contrail predictions.

“We believe that post 2030, we will be able to more reliably predict individual contrails and their warming impact,” said Block. “In the future, contrail management could also be further understood and integrated to include fuels like SAF and new energies.”

Data collection is also at the centre of a new EU regulation under the EU ETS that will require all airlines to participate in the monitoring, reporting and verification (MRV) of non-CO2 emissions from January 2025. While not opposing the MRV, IATA says the industry has concerns over the move and has made recommendations.

“Firstly, the MRV is not like the MRV for CO2 where you report fuel burn and this allows you to estimate CO2 emissions with high confidence,” said Block. “For non-CO2, airlines will be asked to report a lot of data, which will be put into a black box coupled with weather models and a non-CO2 effect will be estimated. This estimate will be wrong. How can we then use that to reduce emissions?

“Secondly, all scientific papers support more humidity data and also note the sensitivity of particulate matter emissions from engines. But no papers suggest that airlines reporting all the data required will actually result in more accurate non-CO2 baselines or mitigation.

“We are concerned that the focus could be driven away from CO2, where we have high confidence, and onto non-CO2, where we have low confidence. This is dangerous and could have severe climate consequences.”

Above: IATA’s Marie Owens Thomsen, Hemant Mistry and Alejandro Block

Christopher Surgenor

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Global airline body IATA has called for “urgent action” on better understanding of the climate effects of non-CO2 emissions from aircraft at high altitudes, in particular cirrus cloud formation from persistent contrails. Scientists have long warned that contrails have a significant net warming impact on the climate but a new report by IATA says significant knowledge gaps remain in the complexity of contrail science and calls for more data and research. However, it is pushing against a move by the EU to introduce from January 2025 mandatory monitoring, reporting and verification (MRV) of non-CO2 emissions from all flights departing, arriving as well as within the European Economic Area (EEA). An alliance of European low-cost airlines – Ryanair, easyjet and Wizz Air – together with NGOs led by Transport & Environment have now written to urge the European Commission to resist IATA’s lobbying to restrict the scope to intra-EEA flights.

All sides accept that aviation’s climate impact extends beyond CO2 emissions, with non-CO2 effects such as contrails and nitrogen oxides also contributing to global warming. However, with uncertainties over the scale of the problem and how to tackle it, action by industry and policymakers has so far been lacking, although that is beginning to change. A number of airlines are taking part in contrail avoidance trials, while others have installed monitoring equipment on their aircraft. The EU, on the other hand, now wants access to extensive non-CO2 emissions data from flights before implementing a policy decision that could eventually result in airlines paying for their non-CO2 as well as CO2 emissions under the EU Emissions Trading System (EU ETS). The UK government has indicated it intends to carry out a policy consultation on the non-CO2 issue sometime this year.

Following its newly-released report ‘Aviation Contrails Climate Effect: Tackling Uncertainties & Enabling Solutions’, IATA is calling for “a strengthening of collaboration between research and technological innovation, coupled with policy frameworks to address aviation’s non-CO2 emissions through more atmospheric data.” The lack of high-resolution, real-time data on atmospheric conditions – particularly humidity and temperature at cruising altitudes – hinders precise contrail forecasting, argues the report.

“The industry and its stakeholders are working to address the impact of non-CO2 emissions on climate change, particularly contrails,” said IATA’s Director General, Willie Walsh. “To ensure that this effort is effective and without adverse effects, we must better understand how and where contrails form, and shrink the uncertainties related to their climate impact.”

This requires more trials, collection of more data and improvement of climate models, alongside developing mature technologies and operations, he said.

“Formulating and implementing regulations based on insufficient data and limited scientific understanding is foolish and could lead to adverse impacts on the climate,” he warned. “That is why the most important conclusion from this report is to urge all stakeholders to work together to resolve current gaps in the science so that we can take effective actions.”

The study, conducted with a number of industry organisations and research institutions, recommends a course of action over the period to 2050. In the immediate term, until 2030, the priority should be on mitigating CO2 emissions, while increasing airline participating in sensor programmes, continuing scientific research and improving humidity and climate models for the purposes of contrail mitigation, it says. In the mid-term (2030-2040), action should be taken for data transmission, continuous validation of models and encouraging aircraft manufacturers to include provisions for meteorological observations, as well as selected avoidance.

Over the longer term (2040-2050), the report expects aircraft to be continuously supplying data, with models and infrastructure in place to provide reliable results. By then, there should also be a more complete understanding of the non-CO2 effects from alternative fuels, it forecasts.

“These action items collectively aim to mitigate the climate impact of aviation while advancing scientific understanding and technological capabilities,” it says.

However, IATA is dismissive of the EU’s plans to collect non-CO2 data from flights as of next January and for the European Commission to come up with a legislative proposal by the beginning of 2028 to expand the scope of the EU ETS to include non-CO2 aviation effects.

The proposal could serve as “a first-stage experiment that attempts to achieve a baseline estimation of the non-CO2 effects of aviation,” says IATA. “However, it is currently not feasible to validate the output from the experiment to ensure that it accurately represents reality.

“Studies have shown that estimating the formation of individual contrails using past weather and trajectory data could lead to incorrect results 50-80% of the time. An MRV system for non-CO2 emissions today could support further research thanks to additional data but the science is not mature enough to allow confidence in its implementation at a policy level.

“It is conceivable that by attempting to avoid the formation of contrails and reduce reported non-CO2 emissions, operators could inadvertently increase their CO2 emissions. The complex and likely trade-offs amongst different non-CO2 emissions, and between these and CO2 emissions are still poorly understood.

“Considering all the challenges and uncertainties, introducing an MRV system as early as January 2025 would not serve to mitigate aviation’s non-CO2 effects under the EU ETS.”

The decision to include non-CO2 MRV was agreed by the European Parliament and member states last year and incorporated as an amendment to the EU ETS Directive 2003/87/EC. While MRV under the EU ETS in respect of CO2 emissions is restricted to intra-EEA flights, the new non-CO2 rule also applies to all flights that depart or arrive at an EEA airport.

IATA recommends airline participation in the MRV framework should be voluntary, given its “experimental nature”, and  its application scope should be “strictly intra-EU” to mirror that for CO2.

“Any intention to expand beyond the current EU ETS application scope for aviation would imply a legal risk of extraterritorial impact and would work counter to any MRV implementation,” it argues. “Furthermore, the probability of contrail formation is highly dependent on the region: mid-latitudes have a higher probability of contrail formation than the tropics or the equator, so contrails affect different regions differently.”

However, a policy paper by European NGO Transport & Environment (T&E) responds that full geographic scope is essential to ensure the credibility of the scheme. “It allows a better understanding of the impacts of long-haul flights, which research shows to cause more warming and present more promising mitigation opportunity,” it says. “A reduction in the scope would significantly limit the amount of data and the opportunities to mitigate non-CO2 effects beyond intra-EEA flights.”

The paper notes that shipping companies are now required to monitor maritime non-CO2 emissions for voyages to, from and within the EU. “Aviation cannot seek another exemption while other sectors are required to do more.”

It adds: “As non-CO2 emissions account for two thirds of aviation’s climate impact and adversely affect human health, the aviation industry must no longer avoid its responsibility but instead take decisive action to confront its complete environmental impact. The MRV scheme is a necessary first step aimed at better understanding these effects with a view to explore mitigation pathways. Any divergence from the original full scope would only lead to a large part of aviation emissions remaining hidden from regulators and consumers alike.”

Commented T&E Aviation Policy Manager Krisztina Toth: “Non-CO2 emissions were recognised as a climate problem 25 years ago. A monitoring tool offers a much needed first step that will help bring more understanding of the full climate impact of aviation. But some legacy carriers are lobbying to weaken the proposal, using uncertainty as an excuse.”

Full scope alliances

T&E has formed an coalition on the issue with low-cost carrier easyJet, as well as industry actors and other NGOs to urge the Commission to maintain the full scope of the non-CO2 MRV.

In a letter to the director-generals responsible for climate and transport, as well as the climate minister for Belgium, which currently holds the EU presidency, the group said: “It is critical that the full geographic scope is retained, as it is the only scientifically sound basis to understand the impact of aircraft types and geographies, and allow a better understanding of the impacts of long-haul flights, which research shows to cause more warming and present larger mitigation opportunities. It is vital that activity in areas such as the North Atlantic region, with a high concentration of contrail formation, are monitored and understood.

“Given the volume of their contribution to this issue, any deviation to exclude long-haul routes from the scope would be a significant missed opportunity which would empty the MRV of most of its meaning from a climate impact mitigation perspective, and undermine the scientific basis for future action. It would also go against the original agreement between the co-legislators.”

In a separate letter to the two Commission director-generals, easyJet has teamed with rival low-cost carriers Ryanair and Wizz Air to present a similar joint position on maintaining the full scope of non-CO2 MRV. “We call on the Commission to reject IATA’s attempts to restrict monitoring of non-CO2 effects to intra-European flights,” they said in a statement.

“We do not understand the intent of this effort to undermine the MRV scheme and why significant parts of the industry do not want to further the understanding of the science of non-CO2 effects,” says the letter.

“The purpose of the scheme is to support the development of a robust scientific evidence base. There is currently simply too much uncertainty around non-CO2 effects to drive policy development or to even reach a coherent understanding of the impact of non-CO2 on warming. This is precisely why we need such a system and why it must not be limited to intra-EU flights.

“Without robust science, it will not be possible to develop a policy measure to address non-CO2 effects, so if the EU chooses to restrict the MRV it is by default choosing to remove the option of a future policy instrument.”

IATA’s concern over the extraterritorial application of the non-CO2 MRV to airlines from third countries arriving at and departing from EEA airports has justification. The EU ETS in its original scope was similar but it faced considerable international opposition from countries such as China, India, Russia and the United States. China threatened to withdraw a sizeable order of Airbus aircraft for one of its carriers. The US passed legislation, which is still in force, that provides powers to prohibit its airlines from complying with EU ETS regulations. In the face of such opposition, and with the expectation that ICAO would come up with a market-based global scheme to address carbon emissions from international flights (ICAO member states later agreed the CORSIA carbon offsetting scheme), the EU backed down and restricted the scope of the EU ETS to intra-EEA flights on a time-limited basis.

The letter from the three LCCs acknowledges “there are reasonable concerns” around the scale of MRV data required from airlines and third country airline involvement. However, they say: “We think these can be resolved through a pragmatic approach to the implementation of the MRV.

“The concern that foreign governments and their carriers might in future object to having to report non-CO2 emissions is also not a reason to restrict the measure to intra-EU flights. These carriers are already subject to reporting requirements under CORSIA and will be subject to reporting requirements under ReFuelEU Aviation. There is no technical reason why extra-EU flights should be exempted from reporting their non-CO2 emissions.

“Options to tackle non-EU country objections, should they arise, could involve limiting or delaying the enforcement for non-EU carriers, or other options involving EU funding to address any imbalance in costs. The scope restriction to intra-EU flights is not a necessary outcome.”

Article updated May 10 to include a link to the full letter from the three LCCs.

Photo: DLR

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SATAVIA has claimed significant reductions in climate-warming contrails from a range of commercial jet types during a 10-month test programme conducted last year on 65 flights operated by 12 airlines. The UK-based company said route optimisation developed using its DECISIONX contrail software had collectively avoided over 2,200 tonnes of carbon dioxide equivalent (CO2e), or an average of more than 40 tonnes per flight, with minimal impact on aircraft fuel consumption or flight distances. Contrails are formed in specific meteorological conditions which cause vapour to freeze around soot particles expelled from aircraft engines, creating cloudy canopies of ice crystals which can trap heat in the air and worsen aviation’s climate warming effects. SATAVIA’s software uses extensive atmospheric modelling data to help flight planners identify and avoid conditions in which contrails can form. Among the airlines participating in the tests were Icelandair, Kenya Airways, Condor and SunExpress, performing a mix of short, medium and long haul flights.

Aircraft contrails can either cool or warm the atmosphere by day, depending on conditions, while all contrails created at night cause warming. SATAVIA’s CEO, Dr Adam Durant, said identifying and avoiding conditions which enable formation of the most warming contrails provided a proactive and immediate mitigation strategy to help manage the non-CO2 climate impacts of aviation.

“We’re not reducing emissions, we’re avoiding the warming caused by the clouds aircraft can make,” he said, emphasising that contrail management was not a carbon offsetting measure. “We’re addressing what is known as a short-lived climate forcer, calculating the energy we’ve avoided and converting that to a CO2-equivalent mitigation.”

The  SATAVIA test programme, financially backed by the UK and European space agencies, enabled the company and the participating airlines to assess the contrail avoidance software in a wide range of routine operating environments. 

The company said the initial test results also highlighted a significant opportunity for a new voluntary carbon market segment as the aviation industry continued to seek ways to address its environmental impacts.

Additionally, said SATAVIA, the tests progressed its plans to secure endorsement from Gold Standard, the Swiss-based climate accreditation platform, ahead of an EU requirement from next year that airlines must monitor, report and verify their non-CO2 impacts as part of the European Union Emission Trading System (EU ETS).

Achieving accreditation from Gold Standard is a high priority for SATAVIA, which wants its contrail avoidance technology externally validated, and non-CO2 mitigation credits recognised and introduced for the aviation sector.

Having last year received concept approval from Gold Standard, SATAVIA is now undergoing design certification of its patented methodology, which it says will provide commercial incentives for airlines to voluntarily reduce their contrails before the practice is mandated.

“This new voluntary carbon market will be worth billions of dollars globally, creating a bottom-line rationale for operators to cut their non-CO2 climate footprint,” explained Durant. “As a low-cost, easy-to-implement software solution, contrail management can help move aviation towards climate-neutral operation on near-term timescales.

“We’ve shown that aircraft operators can use DECISIONX to minimise the warming caused by aircraft contrails and reduce their climate footprint with minimal impact on day-to-day operations,” he said. “We also invested in extensive scientific validation to support post-flight verification of avoided climate impact.”

SATAVIA has previously simulated tests of its software with Dubai-based Emirates, the world’s largest operator of long-haul flights, followed by in-service assessments with the national carrier of the United Arab Emirates, Abu Dhabi-based Etihad, with which the company has an extensive partnership.   

The latest test programme was performed last year with 12 participating airlines including Icelandair, Kenya Airways, Lufthansa Group’s Condor and SunExpress, a joint venture between Lufthansa Group and Turkish Airlines.

The study included both day and night flights operated by aircraft types ranging from Embraer regional jets to narrowbody and widebody Airbus and Boeing equipment.

The flights assessed in the programme were divided between short-haul services of less than three hours, medium-haul flights of three to six hours, and journeys of more than six hours. They included both passenger and freight flights, all of which were powered by conventional jet fuels.

As well as further evaluating SATAVIA’s contrail-avoidance technology, which directs aircraft to climb above or descend below contrail-forming conditions, the latest study also identified operational challenges occurring in daily flight operations.

“The results from the trials with DECISIONX, which uses Earth Observation and other meteorological data to power and validate atmospheric modelling, demonstrate how fundamental the use of space is to this global ambition,” said Dr Craig Brown, Director of Investment at the UK Space Agency, which co-funded the 12-airline study. “SATAVIA’s technology could make a significant impact on the voluntary carbon market, boosting opportunities for aviation sector investment in the UK while supporting major industry initiatives against climate change.”

Arnaud Runge, technical officer for the European Space Agency, said his organisation’s co-funding of the programme had helped SATAVIA to engage a larger group of airlines, generating a greater range of data and highlighting how space technologies could assist the air transport industry in progressing its environmental agenda.

Sophia Dunning, Condor’s senior manager for sustainability, said contrail management “offers potential for a much more conscious approach to the impact of daily flight operations on the environment and the climate.

“We are very excited about future developments of the technology in this area.”

Photo: Satavia

Tony Harrington

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The UK aerospace sector has published a first-of-its-kind Non-CO2 Technologies Roadmap that sets out a collective ambition to improve understanding of non-CO2 aircraft emissions and the technology advancements needed to address their climate impact. Non-CO2 emissions from aviation are considered to contribute a climate impact in terms of global warming but there remains a high level of scientific uncertainty as to the quantified impacts. Developed with input from almost 100 leading experts from industry and academia, the roadmap aims to reduce this uncertainty and deliver technology solutions to mitigate or prevent non-CO2 emissions during flight. The roadmap will also inform the activities to be prioritised for funding under a new non-CO2 programme managed by the Aerospace Technology Institute, UK Department for Business and Trade and Innovate UK. Meanwhile, in the US, engine manufacturer Pratt & Whitney has joined a FAA ASCENT project to study non-CO2 emissions from ground testing of conventional jet fuel and 100% sustainable aviation fuel.

Collectively, non-CO2 emissions refers to the direct and indirect effects of combustion found in aircraft exhaust plumes, aside from carbon dioxide, which research suggests could be greater than the impact of CO2 alone. The climate impact from aviation emissions at high altitude, such as the formation of contrails, is highly complex and variable. In addition, there are complex interactions and interdependencies between emission types, as well as variation in the persistence and length of time that different emissions impact the climate. Contrails, for example, can have cooling as well as warming effects.

Aircraft emissions and their climate interactions (© ATI)

The new funding programme aims at accelerating the research and development of technologies related to reducing these broader emissions. The ATI Non-CO2 Programme and the funding will support the UK government’s Jet Zero Strategy and its commitment to tackle aviation’s non-CO2 emissions.

The Non-CO2 Technologies Roadmap becomes the fourth pillar of the UK Aerospace Technology Strategy, ‘Destination Zero’, which guides industry and government investment into aircraft technologies, with the aim of “growing the UK’s share of the global aerospace market, supporting high-skilled jobs and leading the way on more sustainable air travel.”

Delivered in partnership with the UK Department for Business and Trade and Innovate UK, the ATI Non-CO2 Programme will focus predominantly on industrial research and technology development or enabling-technology projects. It will bring academia, government institutions and industry together to translate theoretical concepts into viable technologies. A new Expert Advisory Group is being created to bring industrial and scientific expertise to the programme.

“While reducing and eliminating carbon emissions rightly remains a key focus for aviation globally, advanced technologies being developed today should also consider broader atmospheric emissions,” said Gary Elliott, CEO of the Aerospace Technology Institute.

“As understanding of aviation’s non-CO2 impact grows, so too does the market opportunity and the UK is ideally positioned to unlock the technologies that will maintain global connectivity while meeting environmental commitments and delivering economic benefit across the UK.”

The Non-CO2 Programme will be funded through the ATI Programme, which, since its launch in 2014, has invested £3.6 billion ($4.5bn) of joint government and industry funding in aerospace technology R&D. ATI says up to £17 million over four years will be allocated to successful projects. The grant funding can be applied for during a two-week application process starting 13 May 2024, with the outcome expected by 18 June. Potential applicants can also join a webinar to hear more about the roadmap and programme on 18 April.

UK Industry Minister Nusrat Ghani commented: “I am delighted to see our world-leading aerospace sector propelling research and development  to new heights of innovation in the pursuit of cleaner, greener air travel. This roadmap and programme will harness the world-class capabilities of UK industry and academia as they work together at the forefront of technological innovation to address globally significant challenges.”

(Clearer version of roadmap here)

In the United States, aerospace industry and academia are also working together to help understand and reduce the climate impact from aviation’s non-CO2 emissions. A project conducted under the FAA’s ASCENT programme by Pratt & Whitney, Missouri University of Science and Technology (Missouri S&T), Aerodyne Research and the Environmental Protection Agency will measure emissions from a Pratt &Whitney GTF engine combustor rig test stand using conventional Jet A kerosene and 100% sustainable aviation fuel.

The project will compare emissions from Jet A and SAF comprised of 100% HEFA-SPK supplied by World Energy. The rig tests will take place at Pratt & Whitney’s Middletown facility in Connecticut, using a Rich-Quench-Lean combustor. According to the engine manufacturer, the rig allows testing of the full range of combustor operating conditions, including at take-off, ground and cruise altitudes, to help better understand the environmental and emissions benefits of using SAF. The partners will collaborate on test design, execution and emissions data analysis.

“As the aviation industry targets a goal of net zero CO2 emissions by 2050, we continue to pay close attention to addressing the environmental impact of other emissions, including cruise non-volatile particulate matter and NOx,” said Sean Bradshaw, senior technical fellow of sustainable propulsion at Pratt & Whitney. “Combustor rig tests with 100% SAF provide a controlled environment for generating valuable baseline data, which will support future studies using full-scale engines on-wing at ground and flight test conditions.”

Added Dr Philip Whitefield of Missouri S&T: “SAF containing low sulphur and aromatic hydrocarbon concentrations could contribute to reduced sulphur dioxide and non-volatile particulate emissions, which are associated with contrail formation and the impact to global warming.”

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A new assessment by a group of UK climate scientists into potential solutions to limit non-CO2 emissions produced by aircraft concludes there is no ‘silver bullet’ and concerted efforts and commitments are needed to better understand the complex chemistry trade-offs involved before making policy decisions. The findings, published in the Royal Society for Chemistry’s journal, Environmental Science: Atmospheres, are the result of a two-year study by Manchester Metropolitan University (MMU), Imperial College London and the universities of Oxford and Reading. Aviation is responsible for around 2.5% of global CO2 emissions caused by human activity but due to the amount of non-CO2 emissions it produces, the sector is responsible for around 4% of the increase in global mean temperatures. The effects have been known for many years but the science remains inconclusive, although recent trials claim contrail avoidance technology and route planning, plus the use of sustainable aviation fuels, may provide a partial solution.

However, the new paper’s lead author David Lee, Professor in Atmospheric Science at MMU and a lead author on landmark papers on aviation and the climate, cautions that reducing the impact of non-CO2 emissions on the climate is not straightforward. “Practically all routes forward with conventional liquid hydrocarbon fuels involve trade-offs, mostly at the expense of emitting more CO2, whether it be technological or operational efforts,” he said.

“These trade-offs and uncertainties mean that there are no simple silver bullets or low-hanging fruit to solve the problem. What is often forgotten is that while the non-CO2 climate impacts of, for example, an individual flight are short lived, a substantial proportion of the emitted CO2 persists for a very long time, literally tens of millennia. This means it is a difficult balancing act if reducing non-CO2 emissions leads to an increase in CO2 emissions.”

Added co-author Keith Shine, Regius Professor of Meteorology and Climate Science at the University of Reading: “Given the many uncertainties in the size of aviation non-CO2 climate effects, it is premature to adopt any strategy that aims to decrease non-CO2 climate effects but, at the same time, risks increasing CO2 emissions. We must be mindful that aviation affects local air quality as well as climate. Sometimes measures that improve one will be to the detriment of the other.”

Aviation non-CO2 emissions that affect climate include nitrogen oxides (NOx), aerosol particles (soot and sulphur-based) and water vapour. Aerosols and water vapour have small direct radiative effects but are also involved in the formation of contrails and contrail cirrus, currently the largest non-CO2 effect on  climate, but which comes with large uncertainties. According to the paper, these non-CO2 effects on climate are quantified by scientists with low confidence, compared to that of CO2, which is quantified with high confidence.

Reducing the occurrence of persistent contrails through navigational avoidance of cold ice-supersaturated regions has currently very low confidence as a mitigation measure, says the paper, because of the challenges in making accurate meteorological forecasts on the time and space scales required for operational implementation on an individual flight basis. “Robust statistics are needed to assess whether diversions would have avoided contrails, and these are not yet available,” it adds. The evidence also remains weak for the use of SAF in reducing contrail cirrus forcing, says the paper.

“It is clear that there is an appetite amongst some stakeholders for non-CO2 mitigation of aviation effects on climate, but we have serious reservations over recommending definitive courses of action until there is better quantification of the actual effects, and further studies of the trade-offs between non-CO2 reductions versus potential CO2 increases,” conclude the authors. “It is realised that this represents a serious barrier to technology development and policy making but there are no short cuts, and the underlying danger is of either nugatory and expensive efforts which are not easily reversed or making matters worse in terms of the total climate effect of aviation.”

The researchers argument for more work to be performed on the complex trade-offs “in order to urgently search for solutions” has been recognised by the UK government, which recently announced, through the Natural Environment Research Council, a £10 million ($12.5m) research programme to help inform policy decisions in this area.

“This is a very welcome and much needed development by the government,” said Professor Lee. “Some of our previous research that was used by the IPCC (Intergovernmental Panel on Climate Change) has vitally informed the government on the scale of the problem. We have endeavoured to keep the UK Department for Transport informed of our work while we prepared this assessment, to inform the shape of future research needed.”

Photo: University of Reading

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While mitigating CO2 emissions from flights through the sale and purchase of carbon offsets or credits has been around for many years, no such market mechanism exists to address the non-CO2 climate impacts from flights, although some carbon offset providers will apply a simple multiplier to the CO2 emissions to account for them. Contrail prevention technology company SATAVIA is now working to develop a methodology concept that would deliver credits for mitigating climate warming caused by aircraft-induced clouds. The methodology has just been approved for progression by carbon and sustainable development standards body Gold Standard, which could pave the way for the future issuance of non-CO2 Certified Mitigation Outcome Units (CMOUs) to provide a credit-based incentive for aircraft operators implementing contrail management.

Aircraft-generated contrails are short-lived climate forcers (SLFCs) that cause surface warming, which may account for over 60% of aviation’s climate impact, around double that of direct CO2 emissions from aircraft, says SATAVIA.

“SLFCs are an important and growing field of climate impact mitigation, pioneered by Gold Standard in 2015,” said Owen Hewlett, Chief Technical Officer at Gold Standard. “It is clear that in order to combat the climate crisis, we need to couple a serious conversation about the volume and necessity of flights with a significant reduction in the impact of flying.”

Cambridge, UK-based SATAVIA has been working with airlines including Etihad to develop a contrail management platform called DECISION:NETZERO that optimises flight plans for contrail prevention, incorporating contrail likelihood forecasts based on atmospheric and climate science. The platform is designed to validate contrail prevention and achieved climate benefit, using atmospheric modelling and observational data. The company is seeking to build a methodology to convert the achieved climate benefit into future carbon equivalent units that can be traded on voluntary carbon exchange platforms.

“SATAVIA’s methodology is an example of a novel activity addressing SLFCs and will provide aircraft operators with a clear incentive to implement contrail management in day-to-day activity, making a tangible reduction of aviation’s climate impact,” said Hewlett.

Approved for progression, SATAVIA is now aiming to get approval from Gold Standard’s Technical Advisory Committee and has ambitions to achieve Design Certification later this year or early 2024. According to SATAVIA, this will then enable the issuance of contrail CMOUs against declared aviation non-CO2 climate impact inventories, so ensuring their use in support of contrail management activity. It stresses CMOUs will be subject to sector-specific rules and restrictions, so limiting their use to in-sector stakeholders, and in parallel, Gold Standard will also develop registry functionality to allow CMOU trading.

SATAVIA says it will seek CMOU recognition from ICAO in respect of the UN agency’s CORSIA offsetting scheme for international aviation, “with the aim of bringing contrail management incentives to the widest possible market.”

Commented the company’s CEO, Dr Adam Durant: “Our practical approach to contrail mitigation offers an incentive for aircraft operators to immediately start cutting their non-CO2 climate footprint, which we hope will drive rapid adoption across the industry. As a software solution incorporating the excellent and decades-mature atmospheric science available to us, contrail management provides the airline sector with an immediate and tangible option to reduce the climate impact of flying.

“With the incentive provided by Gold Standard CMOUs, aviation could reduce its non-CO2 impact by perhaps 50% before 2030. All we need is a willingness to adopt this approach that, importantly, doesn’t require any changes to regulation and could be deployed at scale today.”

SATAVIA has so far worked with Etihad, KLM and KLM Cityhopper, and Emirates, in addition to multiple operator demonstrations supported by the UK-based European Space Agency through the Business Applications programme.

Since starting aircraft operator contrail management trials in 2021, SATAVIA said having now worked with nine airlines, results had shown on average around a 75-tonne CO2-equivalent saving per flight, with a fuel impact of 0.1% on modified flights only.

“This exciting development will supercharge industry progress towards greener flight operations,” said Mariam Musallam Al-Qubaisi, Head of Sustainability and Business Excellence at Etihad Airways. “By implementing small navigational changes to a minority of flights, Etihad can use SATAVIA technology to prevent contrails and generate CMOUs to support new and more sustainable business models.”

Professor Sir Iain Gray of Cranfield University, Chair of the UK’s Jet Zero Council Task & Finish Group on aviation’s non-CO2 effects, commented: “The aviation sector has, and remains, focused on finding ways to cut CO2 emissions but it is also widely understood that aviation has significant non-CO2 impacts, such as contrails. It’s great to see a UK-based SME leading on ways in which to address contrail management and help mitigate this climate-warming effect.”

EU lawmakers believe non-CO2 effects on climate from aviation are at least as important as the impact of CO2 alone. An agreement earlier this year between the European Council, representing EU member states, and the European Parliament on changes to the EU Emissions Trading System provides for the Commission to implement a monitoring, reporting and verification (MRV) system for non-CO2 effects in aviation from 2025. By 2027, the Commission is expected to submit a report based on the MRV and, by 2028, after an impact assessment, make a proposal to address non-CO2 effects.

Photo: SATAVIA/Gold Standard

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A study led by Google Research and Breakthrough Energy, together with support from American Airlines, has found that by using artificial intelligence (AI) based predictions to make small modifications to routes that were projected to create contrails, their formation could be reduced by 54%, although avoidance burned 2% additional fuel. Other recent studies have shown that only a small percentage of flights need to be adjusted to avoid the majority of contrail warming. Therefore, say the researchers, the total fuel impact could be as low as 0.3% across an airline’s flights, suggesting contrails could be avoided at scale for around $5-25/ton CO2e using the predictions, making it a cost-effective warming reduction measure that could be improved still further with enhancements. A small group of American pilots flew 70 test flights over six months, after which Google Research analysed satellite imagery.

Cirrus clouds created by aircraft contrails are said to account for around 35% of aviation’s global warming impact During the day, the clouds reflect back incoming sunlight and warmth but nighttime contrails trap heat and a net warming effect takes place. However, contrails only form in ice-supersaturated air below a critical temperature threshold, within areas tending to be tens to hundreds of kilometres in the horizontal and only a few hundred metres in the vertical extent.

“The challenge is knowing where these regions will occur,” said researchers from Google. “By combining massive quantities of weather data, satellite data and flight data, AI can create state-of-the-art predictions of when and where contrails are likely to be formed. Pilots and dispatchers can then use this information to adjust the altitudes of their flights.”

The research team developed an approach to leverage and analyse satellite imagery, and spent hundreds of hours labelling tens of thousands of contrail images collected by the geostationary satellite, GOES-16. Labelled data was used to train a computer vision model to detect contrails when they are formed. The detection model can identify contrails that are visible within GOES imagery in a matter of 30 minutes, say the researchers, who then combined the computer vision model with large-scale weather data, flight data and satellite images to develop a more accurate prediction model.

The contrail-likely zone predictions were integrated into the tablets American’s pilots used in flight, so they could make real-time adjustments in altitude to avoid creating contrails, just as they would do to avoid turbulence. The prediction model’s performance was then evaluated using satellite imagery to compare the number of contrails produced in flights where pilots used the predictions against where they were not.

“We now have the first proof point that commercial flights can use these predictions to avoid contrails, as verified in satellite imagery,” said Juliet Rothenberg, Head of Product for Climate AI at Google Research. “We’re grateful for our partnership with American Airlines and Breakthrough Energy – together we’ve taken a significant step towards understanding a high-potential climate solution.”

Google Research said it planned to extend its models to geostationary satellites over Europe, Africa, the Indian Ocean, East Asia and Western Australia.

“The results from this small-scale test are encouraging and, while there are more questions to answer about how to operationalise contrails avoidance across our industry, we’re excited to have played a role in establishing their first proof point,” said Jill Blickstein, VP Sustainability at American. “And we’re looking forward to sharing what we learned with stakeholders in the aviation industry and beyond.”          

Added Marc Shapiro, Director of Breakthrough Energy Contrails: “Avoiding contrails might be one of the best ways to limit aviation’s climate impact, and now we have a clear demonstration that it’s possible to do so. This study is a great example of what happens when creative, ambitious organisations work together to better understand and solve a tough problem.”

The study follows the setting up of a cross-sector task force last year of aviation industry, tech sector and academic leaders to explore opportunities to address the warming impact of certain contrails. Assembled by RMI and Breakthrough Energy, the Contrail Impact Task Force includes airlines Alaska, American, Southwest, United and Virgin Atlantic, as well as Airbus, Boeing, Flightkeys, Google Research and Imperial College London.

It aims to reduce the climate impact from contrails by:

• Sharing and expanding on the latest science on the climate impact of contrails;
• Developing actionable strategies to avoid warming contrails;
• Analysing the operational and financial challenges of implementing potential solutions; and
• Establishing a roadmap for implementation and validation of contrail mitigation tools.

“The creation of the task force is a significant step in fostering the industry’s climate action. As a cross-industry stakeholder group, it demonstrates leadership and continued commitment to reduce aviation’s climate impact,” said an RMI blog. “The exact contribution of contrails to climate warming may still be uncertain, but a collaborative approach can serve as a catalyst for developing a greater understanding of contrail science, prediction and verification tools, and mitigation opportunities.”

Top image: Google AI detecting contrails over the United States, based on satellite imagery

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