When hydrocarbon fossil fuels are burnt water vapour is always a combustion product, but this is not where we necessarily want it. When fuel is burnt in jet engines it often leaves visible contrails which can be seen from time to time in weather satellite pictures, such as this one, taken at 06:35 on 1st July, 2006. In the vicinity of major air routes these can spread out to form a continuous sheet of altocirrus cloud. This affects the climate: during the three days following the 11th of September 2001, when all civilian air traffic was grounded in the USA, the ground temperature throughout the United States rose 1°C as a direct result of the absence of altocirrus formed from jet contrails. In addition, the water emitted by high flying jets condenses in the troposphere as ice crystals. If these are swept up into the stratosphere they provide additional reaction sites where chlorine radicals from CFCs and HCFCs destroy ozone.
To put some numbers on this, consider just the air traffic across the the north Atlantic. Combining published data for the Boeing 747 400 with values from the table gives an estimate that each trans-Atlantic flight injects 69 tonnes of water and 155 tonnes of carbon dioxide into the troposphere at the typical cruise altitude of 11 km. Statistics from the UK's National Air Transport Services, which manages air traffic in the eastern side of the North Atlantic air route, yield an estimate of 120,848 trans-Atlantic flights in 2009. Assuming these are all 747s or equivalent then commercial flights over the north Atlantic alone are responsible for adding 8 megatonnes of water to the troposphere each year. They also produce 19 megatonnes of CO2. Its worth noting that New Scientist (Damp blanket, In Brief, 13 September, 2003, p26) reported that the water vapour content of the stratosphere between 15 and 50 km in altitude has increased by 50% over the past 50 years and that the cause of 80% of this increase is a mystery. There was no mention of the contribution made by air traffic.
New Scientist (24 Feb, 2007 p32, Green sky thinking) provides some global figures. In late 2006 there were about 85,000 commercial flights a day, or 31 million flights a year. Each year these flights burn 130 megatonnes of jet fuel, adding 400 megatonnes of carbon dioxide and 180 megatonnes of water vapour to the troposphere. This figure is growing 5% annually.
