Direct GHG Emissions from Aviation:
CO2 and Water Vapor

Carbon Dioxide

CO2Carbon dioxide (CO2) is emitted during the combustion of aviation fuel (kerosene) in direct proportion to the kerosene consumed: 3.16 kilograms CO2 are produced per kilogram of kerosene burned. Given the current condition of the carbon cycle, natural CO2 sinks (e.g. oceans and vegetation) absorb CO2 from the atmosphere at approximately half the rate that anthropogenic atmospheric CO2 emissions are produced, leading to a net accumulation. The resultant long-lived CO2 spreads globally and affects climate independent of where the emissions originated. CO2 is the leading anthropogenic (human-induced) GHG and its warming effects are well understood; it is therefore often used as the basis for comparison of all other emission effects (see Radiative Forcing Index and Global Warming Potential). The climate response to CO2 emissions is independent of where emissions occur; CO2 from aircraft has the same effect as CO2 from other ground level sources.

Water Vapor

Water vapor (H2O) is another GHG that is emitted during air travel. Most subsonic aircraft water vapor emissions are removed from the atmosphere through precipitation within one to two weeks and therefore cause short-lived, regional effects. These effects are greater at high altitudes (i.e. a stronger climate response occurs when water vapor is emitted in the upper stratosphere than in the lower stratosphere ) where water vapor stays longer and can accumulate. This would have climate implications if air travel was expanded into these higher altitudes, but this is currently not common with commercial aircraft. Current science indicates that the warming effect of water vapor from air travel emissions is small. (For information on the effects of condensed and frozen water, see contrails and cirrus clouds.)

leading to a net accumulation: Climate change can also affect the capacity for land and oceans to act as carbon sinks, another example of a positive feedback mechanism.

long-lived CO2: CO2 does not have a mean atmospheric lifetime. CO2 emissions are absorbed by oceans and by the terrestrial biosphere. About half of CO2 emitted into the atmosphere is removed in the first 30 years, a further 30% is removed within a few centuries and the remaining 20% will typically stay in the atmosphere for many thousands of years.

lower stratosphere: The stratosphere lies between about 10 km (6 miles) and 50 km (31 miles) altitude above the Earth’s surface at moderate latitudes. At the poles, the stratosphere starts at about 8 km (5 miles) altitude.