INSOFAR as scientists have been concerned over the years about the effects of aircraft on global warming, they have tended to concentrate mainly on emissions of carbon dioxide and other greenhouse gases from the engines.
In recent times, however, they have begun to look at another way in which these flying machines may contribute to the trend for rising global temperatures. Their attention is focused on the condensation trails or "contrails", those elongated streaks of cloud frequently seen to mark the path of high flying aircraft against a clear blue sky.
Although we nowadays take them very much for granted, contrails were a phenomenon completely unknown to our ancestors. Perhaps the nearest thing in concept was the heavenly escalator dreamed of by the Patriarch Jacob in the Old Testament, along which choirs of angels made their way to and fro between this world and the next. Jacob's ladder with its "shining traffic" was, as the poet Francis Thompson imagined it, "pitched betwixt Heaven and Charing Cross."
The modern contrail is made mundane in origin. Water, a byproduct of the burning of hydrocarbon fuels, is ejected in vapourised form in great quantities behind a jet aircraft. As it mixes with the surrounding air, the water vapour increases the relative humidity, and if the temperature is low enough, condensation results in the formation of the familiar trailing cloud.
Moreover, since the process of condensation cannot take place until the exhaust gases have been cooled sufficiently by their surroundings, which takes a little time, if you look closely you will notice that the trail only begins to appear some 50 to 100 metres behind the aircraft.
Now these trails of water droplets are very similar to the wispy cirrusclouds, those that we sometimes call mares tails. And cirrus clouds tend to trap the Earth's heat, increasing slightly the temperature of the atmosphere beneath them.
Moreover, contrails can encourage more clouds to appear, since they contain particles of ice and soot and act as condensation nuclei on which more droplets can form. This multiplies the effect, to the extent that one study of the skies over Europe has estimated that the origins of up to one tenth of the cirrus in evidence can be traced to aircraft.
All in all, there is a strong suspicion that contrails may be making a significant contribution to global warming, and that their effect may be highly localised, concentrated in areas where air traffic is heaviest.