Ice-supersaturation in these regions is often too small to allow cirrus to form naturally, so aircraft act as a trigger to form cirrus clouds. Aviation-induced aerosol present in exhaust plumes and accumulated in the background atmosphere may indirectly affect cirrus cloud cover or other cloud properties throughout the atmosphere. Observations and models are not yet sufficient to quantify the aerosol impact on cirrus cloud properties. Satellite and surface-based observations of seasonal and decadal changes in cirrus cover and frequency in main air traffic regions suggest a possible relationship between air traffic and cirrus formation. Observed cirrus cover changes have not been conclusively attributed to aircraft emissions or to other causes.
Contrails cause a positive mean radiative forcing at the top of the atmosphere.
They reduce both the solar radiation reaching the surface and the amount of longwave radiation leaving the Earth to space. Contrails reduce the daily temperature range at the surface and cause a heating of the troposphere, especially over warm and bright surfaces. The radiative effects of contrails depend mainly on their coverage and optical depth. In the current atmosphere, the direct radiative forcing of accumulated aircraft-induced aerosol is smaller than that of contrails. This forcing is likely positive and may be larger than that from line-shaped contrails.