CJI RESEARCH
CONTRAILS | NEW RESEARCH
Combatting Contrails
New research reveals the hidden threat of contrails but also shows how to mitigate their impact. Fayaz Hussain reports
CONTRAILS | NEW RESEARCH
Combatting Contrails
New research reveals the hidden threat of contrails but also shows how to mitigate their impact. Fayaz Hussain reports
CJI RESEARCH
4Air research on contrails – at a glance
- After analysing over 16,000 flights, if 0.3%, or 50 flights, had minimised contrails, their impact would have been cut by up to 51%.
- Minimising contrails from 0.73% (123 flights) would have reduced their impact by up to 75%.
- 23 flights with the highest contrail impact accounted for 35% of the total impact. 65% of the flights could have avoided or minimised impact by flying higher.
- About 18% of flights created contrails and 69% had warming impacts.
- 31% of contrails estimated to have cooling effects, but only offset the total warming impact by 13%.
- The average contrail was estimated to persist for 2.5 hours with the more impact coming from contrails that persisted for 6.5 hours.
HIGH ABOVE our heads, ghostly white streaks paint the sky, tracing the paths of jets as they crisscross the globe. These contrails – condensation trails – have long been viewed as harmless byproducts of flight (except by CIA conspiracy theorists). But a study by 4AIR has pulled back the curtain, revealing a hidden climate culprit that rivals CO2 in its impact. But there’s also good news about how to mitigate the threat.
After analysing more than 16,000 business aviation flights, researchers at 4AIR have uncovered new information about what are, in effect, artificial clouds. Far from being mere vapour trails, all aviation contrails are now emerging as a significant contributor to climate change. It’s one that poses unique challenges – but, thankfully, solutions are in sight.
The study found that between 1940 and 2018, all aviation contrails, along with the high-altitude emission of nitrous oxides (NOx), accounted for the majority of aviation's climate impact. The industry’s CO2 footprint, often the focus of environmental concerns, is estimated to have accounted for about one-third of climate impact during this period. The formation of contrails is not a constant phenomenon. 4AIR researchers found that, on average, aircraft spent only about 1.1% of their flight time in contrail-forming regions. However, this varied significantly based on geographic location, with European-based aircraft spending more time (2.1% of flight time) in these regions compared with their US-based counterparts (0.7%).
The primary concern about contrails lies in their potential to contribute to global warming. When persistent, these artificial clouds can trap heat in the Earth's atmosphere, leading to a net warming effect.
“Re-routing flights to avoid contrails could in theory reduce the climate impact of aviation…”
Nicolas Bellouin, professor in Climate Processes, University of Reading
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The 4AIR researchers noted: “Of the 16,888 flights, 17.9% of flights created a contrail, with 12.3% of flights creating a warming contrail and 5.6% of flights creating a cooling contrail. This means that of the contrails created, 68.6% or about two-thirds had a net warming impact and 31.4% or approximately one-third had a net cooling effect.”
The most striking revelation from the study is the disproportionate impact of a small number of flights. Of the total business aviation flights observed, just 0.73% of flights (about 123 out of 16,888) accounted for 75% of the total contrail impact. A mere 0.1% of flights (17 in total) were responsible for over 26% of the entire fleet's contrail footprint for a full year. This concentration of impact in a small number of “big hit” flights presents both a significant challenge and a unique opportunity for targeted mitigation efforts.
The study also revealed contrails were more likely to form during winter months, with less flight time spent in contrail-forming regions during summer, particularly in July and August. The altitude at which aircraft fly also plays a crucial role, with contrail-forming regions typically emerging between 33,000ft and 43,000ft. Flights that reached cruise altitudes within this range correlated with higher time spent in contrail-forming regions.
Further evidence of the contrail threat comes from Nicolas Bellouin, professor in Climate Processes, University of Reading. His study published in September analysed nearly half a million flights. “When a persistent warming contrail is formed, this contrail is often orders of magnitude more warming than the potential additional emission of CO2 to avoid its formation,” he concludes.
Edward Gryspeerdt, research fellow, Department of Physics at Imperial College London has also researched the physics and properties of clouds and their role in the Earth's climate system. “This is particularly severe [challenge] for business jets, which fly at higher altitudes and so form contrails … more than the most efficient commercial aircraft,” he tells CJI. “Formed at cold temperatures, with more ice crystals and potential longer lifetimes, this gives them a larger climate impact per passenger than their greenhouse gas emissions suggest.”
Meanwhile, the good news emerging from 4AIR study is that there are several potential strategies for mitigating contrail formation. These strategies, if implemented effectively, could significantly reduce the business aviation industry's non-CO2 climate footprint.
One of the most promising findings is that 70% of the worst contrail-producing flights could have reduced their impact with a relatively small change in cruise altitude. In many cases, even more encouraging is the fact that flying higher could reduce both contrail formation and CO2 emissions, offering a win-win scenario for environmental mitigation.
“65% of these flights [16,888 studied as part of the study] could have avoided or minimised their contrails by only flying higher, with a 2,000 – 4,000 foot cruise altitude increase. An additional 9% of flights could have avoided or minimised their contrails with a greater than 6,000 foot cruise altitude increase,” according to the 4AIR reasearch. However, challenges from a duration as well as service ceiling perspective remain.
Given 4AIR’s findings that a small number of flights contribute disproportionately to the overall contrail impact, focusing mitigation efforts on these “big hits” could yield substantial results.
Some business aircraft, such as the Pilatus PC-12, are likely to have “reduced contrail footprints”, says 4AIR.
The study suggests that if mitigation efforts had been successfully applied to just 50 flights over the course of the year, it would have avoided 51% of the entire sample’s contrail impact. This targeted approach could offer a highly efficient way to reduce business aviation's climate impact. The report emphasises the importance of “including contrail forecasting on every flight,” stating it “will be an important action to reduce aviation’s non-CO2 footprint.”
The study highlights that the same contrail emitted during the middle of the day would have a very different effect if it had been overnight. This temporal aspect of contrail impact suggests that adjusting flight times could be another tool in the mitigation toolbox, although this would need to be balanced against operational and economic considerations.
While the complete elimination of contrails may not be feasible given current business aviation technology and operational realities, the study suggests that their impact can be significantly reduced through targeted mitigation efforts. By focusing on the small percentage of flights responsible for the majority of the contrail impact, the aviation industry could make substantial progress in reducing its non-CO2 climate footprint.
However, it's important to note that contrail mitigation efforts must be balanced against other operational and environmental considerations. For instance, while flying at higher altitudes might reduce contrail formation in some cases, it could potentially increase fuel consumption and CO2 emissions in others.
Gryspeerdt, from Imperial College London, tells us: “These potential trade-offs between the climate effect of contrails and aircraft fuel efficiency should be carefully considered in order to meet future aviation climate targets.”
Additionally, air traffic control constraints and safety considerations will always play a primary role in determining flight paths and altitudes.
Bellouin, at Reading University, added: “Re-routing flights to avoid contrails could in theory reduce the climate impact of aviation and make air travel more sustainable.”
Given the current trajectory of the aviation industry and climate change mitigation efforts, it's likely that contrails will remain in skies years from now. However, with increased awareness, improved forecasting models, and targeted mitigation strategies, their frequency and impact could be significantly reduced.
The 4AIR study calls for further research in several areas to refine understanding and mitigation of contrails by expanding datasets to improve understanding of business aviation’s contrail impact. This would provide a more comprehensive picture of global contrail impacts and help identify any regional or aircraft-specific patterns.
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“To better understand business aviation’s contrail footprint, expanding our dataset to a broader range of aircraft types and geographies would help to better understand the global impact of contrails from business aviation as well as factor in additional popular models, like the PC-12, which likely have reduced contrail footprints,” reports 4AIR.
While the impact of contrails is more significant than many realised, the concentrated nature of this impact – with a small number of flights responsible for a large portion of the effect – suggests that targeted mitigation strategies could yield substantial benefits.
The answer to the challenge of combatting contrails lies in a multi-faceted approach, combining improved weather forecasting, smart flight planning, and potentially new technologies to minimise contrail formation.
As the aviation industry continues to invest in cleaner fuels, more efficient engines and revolutionary aircraft designs, combatting contrails offers another route to meeting business aviation’s environmental responsibilities.
“This is particularly severe [challenge] for business jets, which fly at higher altitudes and so form contrails…”
Edward Gryspeerdt, Department of Physics at Imperial College London
‘CHALLENGES & OPPORTUNITIES’
THE latest research on contrails from 4Air reveals the extent of the problem they create and how it can be mitigated, according to Kennedy Ricci, (pictured), president of the organisation.
“The results of this study demonstrate both the challenges and opportunities with reducing aviation’s footprint from contrails,” said Ricci. “Effectively reducing our contrail warming impact requires considering contrails on every flight, but successfully avoiding contrails on just a handful of flights would have a major impact, potentially without CO2 trade-offs.”
Ricci added: “The analysis by 4AIR reveals that altitude adjustments on just 50 flights out of 16,888 could reduce the contrail impact by more than 50% overnight.”
The report also lays the groundwork for 4AIR’s new contrail monitoring and reporting service. The research is published ahead of the 2025 regulation requiring non-CO2 monitoring under the EU Emissions Trading System (EU ETS).
