David Adam, environment correspondent for The Guardian writes: A nationwide ban on night flights would significantly reduce the aviation industry's impact on the climate, a new study shows. Scientists have found that the warming effect of aircraft is much greater when they fly in the dark, because of the effects of the condensation trails (contrails) they leave.

Piers Forster, an environmental scientist at the University of Leeds who led the project, said: "Night flights are twice as bad for the environment. If the government wanted to reduce the likely impact of aviation on climate then it could ensure that more flew during the day."

Writing in the journal Nature today, Dr Forster and his colleagues say aircraft contrails enhance the greenhouse effect because they trap heat in the same way as clouds. During the day, their warming effect is not as pronounced because contrails reflect sunlight back into space, which helps to keep the planet cool. This means contrails are responsible for about half of the aviation industry's impact on climate. Dr Forster added: "Aircraft currently only have a small effect on climate.

However, the fact that the volume of air traffic is set to grow rapidly in coming years makes it important to investigate the effects of contrails on our climate." Shifting all UK night flights to the daytime would save the equivalent of 2.5% of the UK's annual carbon dioxide emissions, he said. The team studied flights crossing the UK at night, not takeoffs and landings from its airports, but campaigners say both will increase as air traffic increases. The number of overnight takeoffs and landings at so-called designated airports - Heathrow, Gatwick and Stansted - are currently restricted, but flights into other airports face few controls.

The scientists monitored air traffic over the UK and worked out that, although one in four flights occurred between 6pm and 6am, they contributed 60-80% of the warming that could be attributed to contrails. Winter flights had more effect than those in the summer, contributing 50% of the warming despite providing only 22% of traffic. Nicola Stuber, a meteorologist at Reading University, said the warming effect of contrails was roughly the same as that caused by the carbon dioxide emitted from an aircraft's engines.

The team looked at contrails that lasted for an hour or more over south-east England, passed by aircraft heading for the north Atlantic. They combined flight data with measurements from weather balloons to predict whether flights would form contrails or not. They found that contrails formed more easily when conditions high in the atmosphere are very humid, as they are during the winter.


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