Long-lived contrails in cirrus clouds underestimated with uncertain climate impact

Contrail-cirrus is considered the most important component of aviation-induced climate impact. However, a reliable assessment requires a better understanding of the environment in which they are formed, and the resulting radiative effects. One study has been recently published on the quantification of the radiative forcing of contrails embedded in cirrus clouds (Seelig et al., 2025) and found a contribution of around 10% of the current estimate of the climate impact of line-shaped contrails.Our study (Petzold et al., 2025) focuses on the occurrence of long-lived contrail-cirrus with natural cirrus clouds. To this end, we investigated the distribution of relative humidity with respect to ice (RHice) – the key parameter controlling the lifetime of contrails - in clear sky as well as inside optically thin and thick cirrus clouds for the North Atlantic region and over subtropical Southeast Asia, with the focus on the occurrence of ice-supersaturated air masses and the potential of contrail formation.The underlying data base builds on more than 7 years of continuous in-situ observations by the European research infrastructure IAGOS (www.iagos.org) which measures, among others, temperature, RHice and ice cloud particles, on instrumented passenger aircraft, and covers the period from June 2014 to December 2021. Information on cloud coverage and cloud thickness were taken from ERA5 global reanalysis by means of the cloud ice water content (CIWC). The separation of clear-sky and in-cloud flight sequences was achieved by applying a novel ERA5 CIWC based cloud index validated by IAGOS and research aircraft in-situ RHice observations as well as by process simulations.The analysis shows that conditions promoting long-lived contrails are fulfilled most often in regions already covered by subvisible or visible cirrus: ~90% over the Northern midlatitudes and almost 100% in the Southeast Asian subtropics, approximately equally distributed among visible and subvisible cirrus clouds. A conceptual analysis shows that subvisible cirrus and clear-sky cover ~10% of the cruise altitude over Northern midlatitudes (< 2% in the subtropics) and contrails within these regions are expected to cause additional warming. However, most contrails in the thicker, visible cirrus, only slightly enhance the cirrus warming effect or possibly reverse it to cooling. Our results suggest that potential flight rerouting concepts for contrail avoidance need to consider cirrus cloud coverage in addition to ice- supersaturation, which is currently the primary criterion for rerouting.References:Petzold, A., Khan, N. F., Li, Y., Spichtinger, P., Rohs, S., Crewell, S., Wahner, A., and Krämer, M.: Most long-lived contrails form within cirrus clouds with uncertain climate impact, Nat Commun, 16, 9695, doi: 10.1038/s41467-025-65532-2, 2025.Seelig, T., Wolf, K., Bellouin, N., and Tesche, M.: Quantification of the radiative forcing of contrails embedded in cirrus clouds, Nat Commun, 16, 10703, doi: 10.1038/s41467-025-66231-8, 2025.