A new study alleviates concerns that rerouting flights to avoid forming contrails – high-altitude clouds known to contribute to climate warming – might inadvertently harm the climate more than it helps. Researchers from Sorbonne Université and the University of Reading found that, for most flights over the North Atlantic, the climate benefits of avoiding contrails outweigh the additional carbon dioxide (CO₂) emissions from altered flight paths.
The study, published in Atmospheric Chemistry and Physics, provides clarity on the complex balance between contrail formation and CO₂ emissions, which is crucial for mitigating aviation’s climate impact.
Previous concerns were centered on the uncertainty of how to calculate this balance, known as CO₂ equivalence. Critics feared that miscalculation could lead to misleading conclusions, inadvertently promoting contrail avoidance when it might harm the climate.
However, the research demonstrates that contrail avoidance, in the majority of cases, would benefit the climate, regardless of the method used to estimate CO₂ equivalence. The researchers analyzed nearly half a million flights over the North Atlantic from 2019, concluding that rerouting flights to avoid contrail formation typically reduces climate impact.
Contrail formation and climate impact
Contrails, or condensation trails, are the visible streaks left behind by planes at high altitudes. While they may appear harmless, contrails can trap heat in the Earth’s atmosphere, contributing to global warming.
Building on earlier research, the study suggests that avoiding contrail formation could help reduce aviation’s overall climate impact. But balancing the benefits of avoiding contrails with the additional fuel burn and CO₂ emissions from rerouting has been a challenge for researchers and policymakers alike.
“Rerouting flights to avoid contrails could in theory reduce the climate impact of aviation and make air travel more sustainable,” said Professor Nicolas Bellouin, co-author from the University of Reading. “Our findings lift a major obstacle against implementing contrail avoidance, but we now need better forecasting and real-world trials to make this work in practice.”
Contrail avoidance results in reduced warming
The researchers calculated how much warming the CO₂ emissions and contrails from the 2019 North Atlantic flights would cause over time. They estimate that without contrail avoidance, these flights would warm the climate by about 17 microKelvins (μK) by 2039 and 14 μK by 2119 (a microKelvin is a very small unit of temperature change).
However, when the researchers modeled rerouting flights to avoid contrails by using just 1% more fuel, they found that total warming would decrease significantly. By 2039, warming would be reduced by about 5 μK, or 29% less than without rerouting. By 2119, warming would be about 14% less.
This study utilized nine different methods to measure climate impact, and in most cases, all agreed that rerouting flights would be beneficial, as long as the rerouted flights successfully avoided forming contrails.
Uncertainties and next steps
Despite the positive findings, the study’s authors caution that challenges remain in forecasting precisely where contrails will form and how much warming they cause. They suggest focusing initial efforts on rerouting flights that create the most climate-warming contrails, where the benefits are clearest.
In the push for a more sustainable aviation industry, this study strengthens the case for contrail avoidance strategies, though real-world application will require improved forecasting capabilities and trials to ensure its effectiveness in practice.
Journal Reference:
Borella, A., Boucher, O., Shine, K. P., Stettler, M., Tanaka, K., Teoh, R., and Bellouin, N., ‘The importance of an informed choice of CO₂-equivalence metrics for contrail avoidance’, Atmospheric Chemistry and Physics (2024). DOI: 10.5194/acp-24-9401-2024
Article Source:
Press Release/Material by University of Reading
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