Contrails: an opportunity in plane sight

In the race to net zero, few sectors have a tougher challenge than aviation, which accounts for around 2.5% of global CO2 emissions from human activity, but around 4% of total anthropogenic warming impact. The difference between these two figures reflects the extraordinary complexity of the sector’s challenge.

Already dealing with their fair share of other challenges, airlines face a mammoth task to achieve their net zero targets, and need all solutions on the table. Talk to date has focused mostly on dealing with in-flight CO2 emissions via Sustainable Aviation Fuels as well as niche applications for electric, and hydrogen technologies.

The challenge is that each of these solutions are decades away from scaling. But forgotten in many of the (CO2 ) net zero conversations is a non-CO2 challenge, albeit a challenge that offers far more immediate upside opportunity.

In this paper we focus on contrail management – an overlooked and potentially cost-negative opportunity for sustainability that can be delivered with today’s technology.

Not only is the contrail impact material under the more conservative assumption, but the most warming of contrails could be solved for now. Unlike in-flight emissions from fossil fuels, which require massive evolution in replacement technologies such as hydrogen, electric, and scaling of energy-intensive Sustainable Aviation Fuels, contrails can be addressed with today’s technology, offering the sector a golden opportunity for genuine green proactivity that goes far beyond the tokenism it is often accused of.

And with the EU ETS requiring non-CO2 impact reporting from 2025, airlines with EU route exposure have every reason to grasp the nettle. Mitigating contrail impact is relatively straightforward, relative that is, compared to the other tools at aviation’s disposal like air space redesign, new propulsion certification, or scaling new fuels supply.

Since contrails are far more likely to form under particular atmospheric conditions (namely pockets of higher levels of moisture and lower temperatures), small adjustments in flight altitudes can avoid such conditions.

This need not be as massive an undertaking as it sounds, since research suggests that ~80% of the total contrail warming impact is caused by only ~2% of flights, with longhaul and night flights being the main concern.⁶

Whilst flights targeted for mitigation may face a fuel burn penalty for amending their flight routes, it is likely to be trivial – 2% according to a recent trial by Google and American Airlines,⁷ and no more than 0.5%, according to Ian Poll, Royal Aeronautical Society Past President and Emeritus Professor of Aerospace Engineering at Cranfield University, a figure corroborated by real-world adjustments recommended by contrail modelling specialist SATAVIA in trials backed by the European Space Agency with over 10 airlines, which has found on average that a fuel burn difference equating to ~100kg CO2 is necessary to mitigate 52T CO2e.

Keep in mind this is a per adjusted flight route – since even many long-haul flights will not create contrails on a given day, an airline-wide fuel burn penalty is likely to be more in the region of 0.01% of airline fuel total.⁸

At this level, any such penalty would be significantly less than the typical fuel excess already burned routinely by airlines pursuing maximally profitable operations, which depend on a range of factors including flight time, air traffic management charges, crew hours, aircraft rotation requirements, air traffic congestion time slots, etc. 

Estuaire is a software platform dedicated to monitoring and mitigating aviation’s climate impact. It adopts a detailed flight-by-flight analysis considering actual aircraft trajectories. This approach computes the climate impact of each flight, encompassing CO2 emissions and other factors like contrail radiative forcing.

Additional emissions such as nitrous oxides, soot, sulfur, and water vapor are categorized as “Other Greenhouse Gases.” Examining flight trajectories allows for a more precise understanding of contrail formation.

Multiple variables like flight timing, location, seasonal variations, weather, aircraft, and engine types significantly influence contrail formation. With this comprehensive dataset, non-CO2 radiative forcing effects are assessed more accurately.

Understanding the non-CO2 impacts of different aircraft types is crucial for informing future fleet strategies. An analysis was conducted comparing various aircraft types using data from around 20 individual aircraft (identified by tail numbers) for each type.

The analysis leveraged detailed information including aircraft and engine models, specific flight routes, weather conditions, and a contrail model called CoCiP. This comprehensive approach resulted in an overall climate impact measured in grams of CO2 equivalent per available seat kilometer.

SATAVIA , based in the UK’s Cambridge, has pioneered commercially attractive contrail management, citing the need to address a climate challenge accounting for up to 2.4 billion tonnes of carbon dioxide equivalent impact each year.

To this end, the company has developed a methodology for airlines to earn credits for mitigating climate warming caused by aircraft-generated clouds, a kind of short-lived climate forcer (SLCF). The principles are simple, even if the technology is not.

SATAVIA uses granular climate modeling to predict specific areas in which persistent contrails are likely to be created. Based on this analysis, it recommends slight modifications to relevant flight trajectories, enabling aircraft to avoid such warming regions. It can then use post-flight data analysis to quantify achieved climate savings.

The methodology has received concept approval from Gold Standard, the World Wildlife Fund-founded global standards body, enabling future issuance of SATAVIA’s provisional Certified Mitigation Outcome Units (CMOUs) for non-CO2 aviation emissions avoided via contrail management activity.

This positions CMOUs as incentives for aircraft operators to undertake contrail management in advance of regulation and accelerate progress towards lower climate impact flying. The company is already working with Etihad on a commercial basis, alongside POCs with KLM and KLM Cityhopper and European Space Agency-funded trials with multiple other operators, saving thousands of CO2 equivalent tonnes to date.

That real world activity also provides evidence to counter claims that ‘more study is required before action is taken’. From the below data sample, it it is evident certain conditions (even under the most conservative of CO2 conversion assumptions, and real world fuel burn penalties) warrant action now.

For other more marginal cases, further research will indeed help clarify when interventions are and are not warranted. According to SATAVIA figures, assuming a value of £30 per ton of CO2e avoided, an airline can expect to generate a benefit of 5-7x its mitigation cost per modified flight. 

According to Professor Ian Poll: ‘if civil aircraft were to actively avoid all persistent contrail formation, aviation’s daily climate impact would be halved at a stroke.’ Longer term, Poll and others believe it may even be possible for aviation to produce a net cooling impact through the active management of daytime contrails and elimination of nighttime contrails.

However, contrail management remains largely ignored as governments and media focus on decarbonization in the literal, narrower sense, and ignore wider warming effects. The EU’s reporting requirement on non-CO2 from 2025 starts to shift this dynamic, and several national governments are now pursuing their own discovery phase.

Airlines, still recovering from the pandemic, will eventually be forced or incentivized to pick what the Royal Aeronautical Society describes as the ‘really low-hanging fruit of aviation’.

Whilst a ban on persistent contrails is one option, enforcement is an obvious issue and commercial incentives such as SATAVIA’s tradable credits represent a perhaps more pragmatic solution.

Among the greenwashing potential of claims around “energy efficient fleets” and the likely inability of many airlines to secure sufficient Sustainable Aviation Fuel to meet their own commitments, contrail mitigation also presents a genuine opportunity for the sector to show it doesn’t need to wait for the regulatory stick to take action.

• Airlines
  Almost all aircraft operators are likely to have some contrail climate impact, with the highest liabilities for long-haul flights, so avoid the temptation to kick this can down the road. 
  • Avoid having your PR team push a narrative that ‘the science isn’t there yet’, with partners to baseline your contrail-related warming impact and the resulting opportunity to improve. Instead, work with partners to baseline your contrail-related warming impact and the resulting opportunity to improve. 
  • Investigate available contrail avoidance technologies available to in-flight and ground crews, including weather forecasting, instrumentation, and bespoke predictive platforms. 
  • Begin discussions with solutions providers, including the business case for implementation costs, operational modifications and fuel penalty, with potential monetary value of credits and public relations opportunity of early adoption. 
• Policymakers 
  • Urgently and explicitly include non-CO2 warming effects in aviation industry climate policy, including impact reporting under national inventories/emissions trading schemes. Implement incentives for airlines to adopt contrail management as standard operating procedure (e.g. phased introduction of differentiated and dynamic airspace pricing).
  • Consider territory-wide contrail mitigation potential through the integration of mitigation techniques in your airspace, likely in collaboration with your national Air Navigation Service Provider or regulator. 
  • Consider also coordinated, international introduction of additional sensors to build up a more comprehensive, real-time data lake of relevant atmospheric conditions. 
  • Study long-term potential for net cooling through deliberate contrail management. 
• Developers and future operators of hydrogen flight
  Get comfort that engine design and flight altitudes will not inadvertently contribute to contrails. While this is more relevant for larger, higher altitude aircraft than turboprop retrofits, hydrogen’s byproduct of water vapor has the risk of undermining the environmental credentials of hydrogen flight. Unless, that is, combined with daytime-specific efforts to create net cooling contrails.
• Aviation regulators and Air Navigation Service Providers
  Explore the collaborative potential with contrail management service providers for territory-wide integration.