Night flights by aircraft are much more damaging to the environment than air travel during the day, a study shows.
Aircraft vapour trail in the sky above London
The reason, says a UK team, is that vapour trails from aircraft have a greater warming effect during darkness.
Writing in the journal Nature, they say cutting night flights could help minimise the climate impact of the aviation industry.
Winter flights have a bigger effect on global warming than flights during the rest of the year, they add.
The warming effect is due to condensation trails (contrails) created by aircraft. They have two opposing influences on the climate: warming from trapping heat leaving the Earth like a blanket, and cooling from reflecting sunlight back into space.
Although these two factors are balanced out during daylight to a certain extent, on average the warming (greenhouse) effect slightly exceeds the cooling effect, contributing a small amount to global warming as a whole.
However, at night the cooling effect does not apply, producing a bigger contribution to global warming.
"During the night-time, we don't have the Sun out and so the greenhouse warming effect is no longer balanced," said principal researcher Nicola Stuber, of the University of Reading.
"That's the reason why night time flights have such a large contribution to the daily warming effect."
Dr Stuber said the impact of contrails on the climate had been known for some time, but the importance of flights during different seasons and at different times of the day had not been investigated until now.
In collaboration with researchers at the University of Leeds, she looked at aircraft flying over a site in southeast England bound for the North Atlantic flight corridor.
The team predicted whether or not the aircraft would create contrails by studying atmospheric conditions such as air humidity.
They then created a climate model to indicate the overall effect on climate change at different times of the day and night, and in different seasons.
They found that night flights during the hours of 6pm and 6am, which make up a quarter of total air traffic, contributed between 60 and 80% of the annual warming.
Winter flights, which represent 22% of the yearly total, contributed 50% of the annual warming - largely because contrails are more likely to form between December and February when air humidity is higher.
"Aircraft currently only have a small effect on climate," said Dr Piers Forster of the University of Leeds.
"However, the fact that the volume of air traffic is set to rapidly grow in coming years makes it important to investigate the effects of contrails on our climate."