Are contrails the overlooked culprit in aviation emissions? By Aviraj Dastidar

If you look up at a plane flying in the air, you might notice cloud-like streaks trailing behind it. These are contrails, short for condensation trails, and they form when water vapor in the air condenses around the particulate matter that aircraft leave behind in their wakes. Because the primary component of contrails is water, which has potent greenhouse gas properties due to its ability to absorb heat, discussion over contrail regulation has increasingly gained prominence in the eyes of American lawmakers and the general public alike. While carbon dioxide emissions from aircraft in the United States are already subject to significant regulation due to their contribution to the global climate crisis, many activists argue that contrails deserve similar scrutiny.

However, a closer examination of how contrails form suggests that blanket regulations may not be as effective as the broad-scale carbon emission regulations currently in place in the United States. This is because all flights produce carbon emissions, and the quantity of those emissions is largely dependent on the aircraft type and duration of the flight, which means that carbon output is more predictable and thus easier to regulate. As a result, fuel efficiency standards, carbon offset measures, and other traditional means of regulating greenhouse gas emissions can be applied uniformly to aircraft to generate a meaningful reduction in greenhouse gas emissions.

However, the same cannot be said of contrails, which are only formed by under 20% of all flights. Contrail formation depends on a narrow set of atmospheric conditions: altitudes between 32,000 and 42,000 feet, temperatures below -40ºC (when rates of atmospheric supercooling become greatly accelerated), and relative humidity levels of 100% or greater. These conditions are most commonly met during commercial airline operations in colder, more humid parts of the world.

It is also worth noting that contrails themselves have less of an environmental impact than the contrail cirrus clouds they can evolve into. These clouds, due to their radiative forcing effects, can have a greater warming impact on the Earth than even carbon dioxide. This effect is particularly pronounced at night, when the contrail clouds trap heat that would otherwise radiate into space.

The unique combination of factors required to form contrails and the extra requirements for those contrails to cause significant warming effects means that only a small percentage of flights produce these problematic contrails. Research has shown that the worst offenders are flights that fail to reach their aircraft’s maximum service ceiling. In fact, some analyses suggest that up to 65% of harmful contrails could be avoided simply by flying closer to the aircraft’s maximum altitude, thus without compromising safety.

Based on what we now understand about how contrails form and how they impact the environment, a more strategic regulatory approach can be constructed. Rather than imposing blanket regulations such as those used to regulate carbon emissions in the aviation industry, contrail mitigation efforts should focus on the minority of flights responsible for the vast majority of contrail-induced climate change effects. Specifically, regulations could seek to limit the number of flights that originate from or pass through cold and humid regions, and if possible, could encourage airlines to ensure flights that pass through these contrail-conducive regions occur during the daytime, where the warming effect is mitigated.

Similar to how carbon emissions are regulated, contrail mitigation could be enforced by adopting a combination of positive incentives and penalties. If successfully implemented, this specialized set of rules would have the potential to significantly reduce the contribution of contrail formation to the climate crisis, without placing unnecessary burdens on the wider aviation industry

At EcoAero, we recognize that not all climate challenges in aviation require a one-size-fits-all solution. Contrails, while less visible in policy discussions than carbon emissions, are an important frontier in the pursuit of sustainable flight. By focusing on science-based strategies that target the flight's most harmful contrail formation, we can take meaningful steps towards minimizing the environmental impact the aviation industry has. We believe that precision, not broad restriction, is the path forward, and with continued research and innovation, the skies can remain both accessible and environmentally responsible for generations to come.

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