Aviation’s path to net zero emissions is often focused solely on CO2 reduction. However, contrails – those white lines we see trailing behind aircraft – may have a warming effect comparable to aviation’s CO2 emissions. In this episode of our ‘Sustainability in the Air’ podcast, Dr Florian Allroggen speaks with SimpliFlying’s CEO Shashank Nigam about the science behind contrails, their climate impact, and practical solutions for mitigation.
As Executive Director, Aerospace Climate & Sustainability, and a Research Scientist in MIT’s Department of Aeronautics and Astronautics, Allroggen brings unique insights into both the scientific understanding of contrails and the practical challenges of implementing solutions, with a focus on both environmental impact analysis and developing actionable solutions for the aviation industry.
Here are the key highlights of the conversation:
Understanding contrail formation and impact (5:20)
Comparing contrail warming to CO2 effects (9:40)
Strategic vs tactical approaches to contrail avoidance (19:45)
Real-time detection and mitigation tools (29:10)
Implementation challenges and future research (38:00)
Rapid fire! (44:15)
Keep reading for a detailed overview of the episode.
Why contrail mitigation matters
The aviation industry’s focus on CO2 emissions, while crucial, only addresses part of the climate impact challenge. Contrails represent a significant but often overlooked contributor to aviation’s warming effect, requiring immediate attention alongside long-term CO2 reduction strategies. As Allroggen emphasises,
“When we talk about net zero for aviation, there is a world beyond contrails... Equally important, if not more important, is that we get our transition to low carbon energy carriers right... We need both.”
This dual challenge requires a holistic approach to aviation sustainability. While CO2 emissions have long-lasting effects that accumulate over centuries, contrails present an immediate but shorter-term impact that could potentially be mitigated through operational changes.
The industry needs to develop practical solutions while acknowledging scientific uncertainties and operational complexities, particularly in the context of maintaining safety standards and managing air traffic flow. Understanding and addressing both CO2 and non-CO2 impacts is essential for achieving meaningful progress toward aviation’s sustainability goals, argues Allroggen.
5 key elements of contrail mitigation
1. Understanding contrail formation
The science behind contrail formation involves a complex interaction between aircraft exhaust and atmospheric conditions. Allroggen provides a clear explanation:
What’s in the exhaust of an airplane is actually water vapour, about 28% by mass... when [an airplane] cruises at like 9 to 12 kilometres or 30 to 40,000 feet, it’s actually one of the coldest spots that we have in the atmosphere.”
These conditions create ice clouds that can persist for hours, leading to two distinct effects: during the day, contrails can both reflect sunlight back into space (cooling) and trap heat from Earth (warming).
However, nighttime contrails only contribute to warming. This understanding is crucial because, as Allroggen notes, “Every contrail actually, it’s net effect... it’s the balance of this cooling and this warming.” While some contrails may have a cooling effect, research indicates that the average contrail contributes to warming, making this a critical area for climate impact mitigation.
2. Temporal impact assessment
One of the most complex aspects of contrail impact is its relationship with time horizons. Allroggen explains:
“If you actually only care for the near future for the next 5-10 years or something, then the contrail effect is relatively large compared to CO2. Why? Because you ignore all the future warming that is still to come from the CO2...
If you care for longer time horizons, well then suddenly all the future CO2 counts, your CO2 impact is actually relatively larger to the contrail.”
This temporal dimension is crucial for policy-making and mitigation strategies. While CO2 remains in the atmosphere for centuries, gradually accumulating its warming effect, contrails’ impact is immediate but short-lived, lasting only hours.
This difference creates a unique challenge in how we evaluate and prioritise mitigation efforts, requiring careful consideration of both short-term and long-term climate impacts in aviation policy and operational decisions.
3. Practical mitigation strategies
The most promising near-term solution for contrail mitigation involves strategic flight path adjustments. Allroggen uses an analogy to explain contrails:
“These contrail forming regions, they’re a little like pancakes. They’re actually horizontally very large, but they’re vertically very thin. So if you want to fly around them in many situations, you would fly large detours, which could delay you and it’s lots of additional fuel burn.”
Instead of horizontal detours, altitude adjustments are a more optimal solution to contrail mitigation. Airlines can implement this through two approaches:
strategic or pre-flight planning based on weather forecasts, or
tactical or real-time adjustments during flight.
Each approach has its advantages and challenges, with strategic planning aligning better with current flight operations but requiring accurate forecasting of contrail-forming conditions. The tactical approach offers more flexibility but needs real-time detection capabilities and air traffic control coordination.
4. Real-time detection tools
MIT has developed innovative technology for contrail detection and avoidance. As Allroggen describes,
“We started with a key technology, which is based on a NASA satellite that gives us a picture of the Americas every five minutes. And in this imagery… we can actually see contrails.”
The system uses machine learning to identify contrails and determine their altitude, enabling real-time decision-making for flight path adjustments.
Currently being tested with Delta Air Lines, this technology represents a significant step forward in practical contrail mitigation. The system provides information to dispatchers and pilots, who can then make informed decisions about altitude changes based on real-time conditions and operational constraints.
5. Implementation challenges
The implementation of contrail mitigation strategies faces several complex operational and technical hurdles that must be carefully considered.
The primary challenge lies in air traffic control integration, particularly in congested airspace. As Allroggen explains, “If you want to do this at large scale, you will basically also... start to block off large chunks of airspace. This is something that air traffic control will need to be aware of.” While such adjustments might be feasible in oceanic airspace, they present significant challenges in busy areas like the US East Coast, where airspace management is already complex, adds Allroggen.
Moreover, implementing these measures at scale presents another significant challenge. As Allroggen notes, “There’s no way that you can integrate [contrail mitigation tools into flight planning] at large scale with a non-automated process.” This necessitates developing standardised tools and protocols, while ensuring they meet aviation’s strict safety requirements.
“The number one paradigm in the aviation industry today is safety... we’re not going to sacrifice the safety paradigm of aviation for contrail avoidance,” he emphasises.
Before large-scale implementation can begin, extensive experimentation and what Allroggen terms “failure mode analysis” must be conducted. This methodical approach, while potentially slower, ensures that contrail mitigation strategies can be implemented effectively while maintaining aviation’s high safety standards.
‘Sustainability in the Air’ is the world’s leading podcast dedicated to sustainable aviation. Through in-depth conversations with top aviation leaders, we break through the clutter and provide a clear roadmap for a net-zero future.
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