There are three gases emitted by aircraft which contribute to global warming: H2O, CO2 and NOx The most obvious is the water vapour (created by burning jet fuels -see below) which forms condensation trails -clouds of frozen ice crystals. Since the air in the upper troposphere (the level at which most commerical planes fly) is naturally very dry, water vapour emitted by aircraft can make a big difference. Sometimes the contrails cover the whole sky as shown by this satellite photo taken over Germany, and the average coverage in this region is about 6%. Have you ever wondered, why the sky is so much clearer in remoter locations?
Although these contrails reflect a little sunlight away from earth, they reflect back to earth much more invisible infra-red (heat) radiation which would otherwise escape to space -and therefore they have an overall warming effect. This is hard to measure accurately, because the contrails eventually spread out and become indistinguishable from natural cirrus clouds.
Not all of the water vapour forms contrails, but water is itself a "greenhouse gas" which also traps this outgoing infra-red radiation. Each water molecule traps much more heat and also survives much longer at this height than it would do at sea-level.
Jet-fuel - kerosene - is a mixture of substances produced by distilling crude oil, which can be represented by C13H28 (this is assumed in the "how much" calculations"). The chemical equation for burning it is as follows:
2C13H28 + 40O2 =>26CO2 + 28H2O
So you can see, that for every 14 water molecules produced, the aircraft must also emit 13 of CO2. This is also a greenhouse gas and will stay in the atmosphere warming the earth for an average of 100 years, some of it for 1000s of years. There's no way that you can get the energy from such fossil fuel without producing that much CO2. It's not a by-product that can be "scrubbed" from the exhaust.
A Boeing-747 can burn over 200 tons of fuel in one flight.
A simple "rule of thumb" to remember, is that a plane uses about as much fuel, and therefore produces about as much CO2, as would every passenger driving one car the same distance. So next time you're thinking of flying thousands of miles, think how much petrol you would need to put in a car to go so far. Since take-off uses a disproportionate amount of fuel, short-haul flights emit a bit more per passenger-kilometer, and long-haul flights a bit less.
Note: the car comparison is for typical european cars, not for the less efficient american gas-guzzlers. Trains produce, on the other hand, about 1/3rd as much CO2 per passenger-kilometer
But fuel use is not the end of the story. Aircraft also emit NOx gases, a product of combustion of atmospheric nitrogen and oxygen in the very hot jet engines. NOx is not in itself a greenhouse gas but it catalyses the production of ozone (O3), which is a powerful greenhouse gas. Nearer the ground, ozone also leads to the formation of photochemical smog which is harmful to human health.
In the stratosphere, on the other hand, ozone is valuable as it blocks harmful ultra-violet radiation. Unfortunately NOx emissions from aircraft which venture into the stratosphere (such as concorde) catalyse the destruction of ozone there. Confused? -Yes, ozone chemistry is complicated, but in a nutshell, NOx emissions are bad news -producing ozone where we don't want it, and destroying it where we do..
To make matters even more complicated, NOx emissions also catalyse the destruction of methane (CH4), another greenhouse gas, although this cooling effect is much smaller than the ozone warming effect (see IPCC graphic below). Aircraft emissions of sulphate aerosols also have a slight cooling effect, but also contribute to acid rain.
It's not easy to quantify the exact greenhouse warming due to water vapour and NOx emissions from aircraft, but a scientific consensus is now emerging, that the total warming effect of all emissions (CO2, H2O and NOx) put together, is in the range 2-5 times greater than that of CO2 alone. This is confirmed by analysis in the "Special Report on Aviation" recently published by the "Intergovernmental Panel on Climate Change" (IPCC) in April 1999.
Dr Ben Matthews