Global Radiative Forcing from Contrail Cirrus

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Notes:
“Aviation makes a significant contribution to anthropogenic climate forcing.” p.54

“We show that the radiative forcing associated with contrail cirrus as a whole is about nine times larger than that from line-shaped contrails alone. We also find that contrail cirrus cause a significant decrease in natural cloudiness, which partly offsets their warming effect. Nevertheless, net radiative forcing due to contrail cirrus remains the largest single radiative-forcing component associated with aviation.” p.54

“Contrail cirrus initially form behind cruising aircraft as line-shaped contrails and transform into cirrus-like clouds or cloud clusters in favourable meteorological conditions, occasionally covering large horizontal areas. They have been tracked for up to 17 h in satellite observations. They remain line-shaped, and therefore easily distinguishable from natural cirrus, for only a fraction of their lifetime. The impact of aircraft soot emissions on cirrus in the absence of contrails depends on the ice-nucleating properties and the ice-active number concentration of soot-particle emissions.” p.54

“Contrail cirrus form and persist in air that is ice-saturated, whereas natural cirrus often require high ice supersaturation to form. This implies that in a substantial fraction of the upper troposphere, contrail cirrus can persist in supersaturated air that is cloud-free, thus increasing high cloud coverage.” p.54

“Over central Europe, contrail-cirrus coverage is largest, reaching up to 10%. Although the level of air traffic over the east coast of northern America is as large as over central Europe, contrail-cirrus coverage in the former region is lower, reaching 6%. It is mainly the coverage due to contrails older than 5 h that is smaller over the USA than over Europe…” p.54

“A large fraction of contrail cirrus is optically very thin (solar optical depth <0.02) and can therefore neither be detected by a satellite nor seen with the human eye from the ground.” p. 55

“The global net radiative forcing of contrail cirrus is roughly nine times that of young contrails, making it the single largest radiative-forcing component connected with aviation.” p.56

“Contrail cirrus change the water budget of the surrounding atmosphere and therefore can have an impact on natural clouds.” p. 56

“Locally, the decrease in natural-cirrus coverage (over Europe and the US) amounts to up to 10% of the natural-cirrus coverage or up to 20% of the contrail-cirrus coverage. Furthermore, in the main contrail-cirrus areas of North America and Europe, the optical depth of natural clouds is significantly (at the 95% significance level) reduced by up to 10% owing to the presence of contrail cirrus.” p.57

“Clouds are influenced by small-scale processes that cannot be resolved by a large-scale climate model and which therefore need to be parametrized.” p. 57