Contrail Mitigation: A Collaborative Approach in the Face of Uncertainty

To measure and address the climate challenge posed by aviation contrails, RMI is working with aviation industry leaders, the tech sector, and the academic community in the newly launched Contrail Impact Task Force.

It’s a bird. It’s a plane. No, it’s a plane creating a persistent trail of condensation that might evolve into a contrail-cirrus cloud. Today, we do not know the precise warming magnitude of aviation-induced cloudiness, but we see a growing scientific consensus that the impact on climate may be comparable to aviation CO₂ emissions. Building off alliances and partnerships to decarbonize aviation, RMI is bringing together a cross-sector task force of aviation industry, tech sector, and academic leaders to explore opportunities to address the warming impact of certain contrails.

Aviation stakeholders recognize the importance of determining the precise impact of aviation-induced cloudiness and exploring possible solutions. While potential solutions have been largely unavailable to the industry, new prediction and verification tools to help mitigate warming from aviation-induced cloudiness are in development today. There may be no silver bullet solution, but through collaboration, tool developers and aviation stakeholders can work to better understand the science of contrails, explore potential tools to avoid their formation, and identify possible mitigation opportunities to reduce their impact.

What Are Contrails?

When you look up into the sky, on some days you may notice white, linear trails behind some aircraft. These are known as condensation trails, commonly referred to as contrails.

Aircraft can produce contrails at cruise altitudes — typically between 32,000 and 42,000 feet above the Earth in the upper troposphere — in regions of the atmosphere where the humidity is high enough and the temperature is cold enough for water to condense. Under these conditions, small particles such as soot exhausted by aircraft engines serve as condensation nuclei suitable for water droplet formation. As the particles cool and mix with the surrounding atmosphere, the water droplets freeze into ice crystals, creating contrails.

The Climate Impact of Contrails

The aviation industry contributes approximately 2.4 percent of global CO₂ emissions; however, non-CO₂ factors — including contrails — also contribute to atmospheric warming. While their exact impact remains uncertain, a median estimate averaged over all flights finds that contrails may cause a warming effect comparable to an additional 61 percent of total aviation CO₂ emissions. (See the final section of this article for more detail on this estimate.)

Depending on the atmospheric conditions, contrails can either dissipate or become persistent and evolve into contrail-cirrus clouds. These formations can last for hours. During daytime hours, persistent contrail-cirrus clouds help reflect some incoming solar radiation, potentially resulting in slight cooling. However, if persistent contrails form during or extend into nighttime hours, they can trap outgoing thermal radiation from the Earth’s surface, resulting in potentially significant warming.

Not all aircraft produce contrails. Studies to date have found that less than 10 percent of flights may be responsible for 80 percent of contrail warming. Through focused, collaborative effort, the aviation industry can better understand the impact of contrails and explore potential solutions.

Collaboration For Contrail Mitigation

In response to this area of interest, RMI and Breakthrough Energy are assembling the Contrail Impact Task Force in collaboration with Alaska Airlines, American Airlines, Southwest Airlines, United Airlines, and Virgin Atlantic, as well as Airbus, Boeing, Flightkeys, Google Research, and Imperial College London. The task force aims to reduce the climate impact from aviation contrails by:

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

Various contrail prediction and verification tools are in development. For example, models using weather forecast data can estimate contrail risk areas while flight planning is under way. By collaborating with each other, participants in the Contrail Impact Task Force can work to integrate identified contrail risk areas into flight plans, just like turbulence, icing, or inclement weather are incorporated into flight planning today. Leveraging existing flight planning routines, aircraft may be able to avoid generating contrails with adjustments to planned cruising altitudes and flight paths. Additionally, verification tools such as geostationary satellite imagery can be integrated with other in-situ and ground-based data to generate contrail “nowcasts” for real-time tactical avoidance. Once contrail prediction models are integrated into flight planning systems and routes are flown, satellite verification tools can identify actual contrails created and use that data to calibrate the predictive models.

Determining what it will take to integrate contrail prediction tools into existing flight planning platforms — and verifying contrail formation or avoidance — is only part of the challenge. For potential contrail mitigation solutions to be viable at scale, their impact on existing factors needs to be well understood. First and foremost, solutions must maintain the highest level of safety that exists in aviation today. Airspace is a limited resource, and ensuring appropriate separation between aircraft is critical. With vertical and lateral route adjustments being the primary means of contrail mitigation, potential solutions will need to be evaluated to determine their compatibility with existing procedures and ensure their application does not constrain airspace or impact safety.

In addition to safety and airspace considerations, another factor to evaluate is the potential impact on flight planning, pilot, and air traffic management workload. For example, to accommodate an aircraft seeking to avoid contrail formation, air traffic management may need to reroute another aircraft as well. Finally, both vertical and lateral route adjustments to avoid contrail formation will also affect fuel consumption, creating a trade-off between direct and indirect climate impacts. This relationship and potential trade-offs between CO₂ and non-CO₂ emissions will need to be well understood to ensure that flights applying prospective contrail mitigation solutions achieve the best climate outcome possible.

Supplementary to addressing the challenges identified above and determining the feasibility of applying contrail mitigation solutions, the Contrail Impact Task Force also has the opportunity to explore the benefits of sustainable aviation fuel (SAF) in mitigating contrails. SAF is critical to decarbonizing aviation, with the potential to reduce CO₂ emissions by 80 percent on a life-cycle basis, according to BloombergNEF. Additionally, the hydrocarbon compositions of SAF significantly reduce the release of particulate matter that enables contrail formation.

The creation of the Contrail Impact 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. 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.

Note: Estimating the Climate Impact of Contrails

In the Climate Impact of Contrails section above, emissions are converted to CO₂ equivalent (CO₂e) based on global warming potentials over a 100-year horizon (GWP100). Contrails are estimated to contribute the equivalent of 629 (median) megatons of CO₂e per year, or 61 percent of the CO₂ emitted by aircraft fuel consumption per year (1,034 megatons). 95th percentile confidence is 186 megatons of CO₂e per year (18 percent of CO₂) from contrails, and 5th percentile confidence is 1,075 megatons of CO₂e per year (104 percent of CO₂).

Source: Google Research, Climate, 2022. Calculations adapted from D. S. Lee et al., Atmospheric Environment 244 (2021), 117834; and G. Myhre et al. in AR5, 2013