Air traffic condensation trails, or contrails, are believed to have a net atmospheric warming effect1, although one that is currently small compared to that induced by other sources of human emissions. However, the comparably large growth rate of air traffic requires an improved understanding of the resulting impact of aircraft radiative forcing on climate2. Contrails have an effect on the Earth's energy balance similar to that of high thin ice clouds3. Their trapping of outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) is partly compensated by their reflection of incoming solar radiation (negative radiative forcing). On average, the longwave effect dominates and the net contrail radiative forcing is believed to be positive1,2,4. Over daily and annual timescales, varying levels of air traffic, meteorological conditions, and solar insolation influence the net forcing effect of contrails. Here we determine the factors most important for contrail climate forcing using a sophisticated radiative transfer model5,6 for a site in southeast England, located in the entrance to the North Atlantic flight corridor. We find that night-time flights during winter (December to February) are responsible for most of the contrail radiative forcing. Night flights account for only 25 per cent of daily air traffic, but contribute 60 to 80 per cent of the contrail forcing. Further, winter flights account for only 22 per cent of annual air traffic, but contribute half of the annual mean forcing. These results suggest that flight rescheduling could help to minimize the climate impact of aviation.
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We thank Q. Fu for providing the basic radiative transfer code and C. Eyers for the AERO2k data. Radiosonde data was provided by the British Atmospheric Data Centre (BADC). A. Tompkins (ECMWF) provided us with analysis data. N.S. was supported by the Department for Transport, and G.R. by the Department of Trade and Industry, and Airbus. Author Contributions N.S. was the principal researcher. G.R. performed analysis of contrail observations. P.F. led the research with significant contributions from K.S.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Stuber, N., Forster, P., Rädel, G. et al. The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing. Nature 441, 864–867 (2006). https://doi.org/10.1038/nature04877
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