Hidden Contrails in Clouds Could Be Worsening Aviation’s Climate Impact

New research published in Nature Communications reveals that aircraft contrails, already known to contribute significantly to global warming, may have an even greater climate impact than previously estimated. The study highlights that many contrails form not in clear skies, but within existing cirrus clouds, where their effects have largely gone undetected and unaccounted for in climate models.

Contrails form when hot, humid aircraft exhaust mixes with cold air at cruising altitudes, producing ice crystals that can spread into contrail cirrus. These high-altitude clouds trap outgoing heat and warm the planet, with an overall climate impact comparable to aviation’s carbon dioxide emissions. Until now, most assessments assumed contrails form mainly in clear skies, overlooking those embedded within natural cirrus clouds.

Two complementary studies led by Petzold et al. and Seelig et al. challenge this assumption. Drawing on seven years of in-situ aircraft measurements, satellite observations, and meteorological data, the researchers show that contrail-favourable conditions frequently occur inside existing cirrus. In northern mid-latitudes, around half of these conditions arise within subvisible cirrus or near-clear skies, situations most likely to amplify warming, while the remainder occur within thicker cirrus clouds, where their climate effect is more complex.

Using high-resolution lidar data from the CALIPSO satellite, Seelig et al. were able to isolate and quantify the radiative forcing of more than 40,000 embedded contrails. Their analysis found that these “hidden” contrails exert a measurable warming effect, particularly at night, and could add roughly 10% to current estimates of contrail-related radiative forcing when scaled globally.

The findings suggest that embedded contrails represent a non-negligible and previously underestimated component of aviation’s overall climate footprint. They also underscore the challenge of distinguishing contrail-induced cirrus from natural clouds, a task that requires advanced satellite observations, aircraft trajectory data, and targeted atmospheric measurements.

As global temperatures continue to rise, the research reinforces the urgency of addressing aviation’s non-CO₂ climate effects. Improved contrail detection and prediction, operational strategies such as altitude adjustments to avoid contrail-prone regions, and integration of natural cloud effects into climate models could offer near-term, cost-effective pathways to reduce aviation-related warming while longer-term solutions, including sustainable aviation fuels, continue to develop.