Changes in cirrus cloudiness and their relationship to contrails

Condensation trails, or contrails, formed in the wake of high-altitude aircraft have long been suspected of causing the formation of additional cirrus cloud cover in otherwise clear skies. Contrail-generated cirrus clouds alter the radiation budget in the same manner as natural cirrus clouds of equivalent optical depth and microphysical properties. Because commercial air traffic is increasing globally, any radiative effects from contrails may also be on the rise. To estimate the maximum possible impact of contrail-generated cirrus on the Earth’s radiation budget, trends in cirrus cloud amounts are correlated with the density of high altitude air traffic using both surface and satellite-generated cirrus cloud amounts. A minimum estimate is derived using the coverage based on linear contrail observations. Cloud radiative forcing is estimated by assuming that the contrail-generated cirrus clouds have the average microphysical properties derived from contrails with satellite data. The results provide a range of estimates that can be used to evaluate the current state of contrail effects and to determine the potential for future radiative forcing. Increases in cirrus cloudiness are well correlated with air traffic patterns with the radiative forcing being greatest over the USA. These estimates suggest that contrails may be an important component of climate change, at least at the regional scale.