Carbon dioxide and nitrous oxide from aircraft have long been recognised as contributors to both global warming and the destruction of the ozone layer. Now scientists are concerned that condensation vapour trails can freeze in the cold upper air and become thin lines of cloud similar to cirrus cloud. Cirrus cloud traps heat and may make a significant contribution to global warming. In areas with heavy air traffic the effect may be magnified.

Commercial and military aircraft emit an estimated 230m tonnes of water vapour annually. 40% of this is discharged in the mid latitudes of the northern hemisphere at cruising altitudes of about 10km. If the scientists are right, the problem will escalate rapidly. Air traffic is forecast to double within 15 years.