Quantification of the radiative forcing of contrails embedded in cirrus clouds

Aviation leads to the emission of CO₂ but also exerts non-CO₂ effects on climate (Lee et al., 2021). The latter include line-shaped condensation trails (contrails) and contrail cirrus that are known to cause warming. However, contrails can also form in already existing cirrus clouds. So far, such embedded contrails have received little attention and their climate impact is unknown. Here, we combine aircraft position data with height-resolved cloud observations from spaceborne lidar to obtain about 40,000 cases, in which aircraft are confirmed to have passed through cirrus less than 30 min before the observation. The data set is used to contrast the properties of perturbed from unperturbed cloud regions, and to infer the local net radiative forcing (RF) of embedded contrails. We find that cirrus with embedded contrails has an overwhelmingly warming effect (83% of cases) even though the majority (62%) of cases occurs during daytime when the addition of a contrail could potentially lead to cooling. The annual mean local net RF of individual embedded contrails ranges between -320 mW m⁻² (2020, COVID lockdown) and 160 mW m⁻². Considering the period from 2015 to 2021, we find an annual mean local warming effect of 60 mW m⁻². Expanding these findings to the global scale suggests an annual global mean net RF of embedded contrails on the order of 5 mW m⁻². This corresponds to around 10% of the current estimate of the climate impact of line-shaped contrails and, together with recent findings that conditions for contrail formation are found most often in already-existing cirrus (Petzold et al, 2025), suggests that embedded contrails are a non-negligible contributor to aviation’s impact on climate.

References:

Lee, D. S. et al. The contribution of global aviation to anthropogenic climate forcing from 2000 to 2018. Atmos. Environ. 244, 117834 (2021), https://doi.org/10.1016/j.atmosenv.2020.117834

Petzold, A. et al. Most long-lived contrails form within cirrus clouds with uncertain climate impact. Nat. Commun. 16, 9695 (2025), https://doi.org/10.1038/s41467-025-65532-2