Existing cirrus clouds - the thin, wispy clouds that usually form above about 5,500 metres - are likely to reflect more of the Sun’s heat following the passage of aircraft, according to a study published in Nature Communications this week. The findings indicate that the condensation trails (contrails) that form in an aircraft’s wake can increase the optical thickness (the degree to which a cloud modifies radiation passing through it), and thus cooling effect, of existing cirrus clouds.

The effect clouds have on climate is determined by their optical thickness: thicker clouds reflect more radiation and have a net cooling effect. Although the contrail plumes emitted by aircraft have been shown to be optically thin, the degree to which aircraft influence existing cloud remains uncertain.

Kevin Noone and colleagues combine real aircraft flight track data representing the major connections between the west coast of the United States and Hawaii for the years 2010 and 2011 and CALIPSO satellite observations that can detect optical changes in cloud. After accounting for the effects of wind transport, the authors show that there is a statistically significant 22% increase in the optical thickness of cirrus cloud within the analysed air traffic flight tracks compared with areas adjacent to these paths.

Although the thickening mechanism remains unknown, the authors speculate that it might occur as a result of soot, emitted within the contrail plume, which can act as nuclei around which clouds grow. Further work is needed to determine the causal mechanism and to quantify the climatic effect of these findings.