How are non-CO2 emissions accounted for?
The combustion of kerosene causes the emission of other greenhouse gases alongside CO2. The most important are nitrogen oxide and water vapour. Nitrogen oxide has an indirect climate effect in that its emission at higher altitudes (over 9,000 m) reduces the atmospheric concentration of methane and produces ozone. Many different attempts to quantify the climate effect of these greenhouse gases caused by aviation exist, for example: Radiative Forcing Index – RFI or Absolute Global Warming Potential - AGWP (Fischer et. al. 2009), Global Temperature Change Potential - GTCP or Integrated Change in Temperature over Time - ICTT (Kollmuss & Crimmins 2009). The first of these measures, RFI, which was introduced into the scientific literature in the 1999 IPCC special report Aviation and the Global Atmosphere, has the problem that only past emissions are accounted for. The other three measures mentioned, take account of the future climate effect of current emissions. However, the effects of the various greenhouse gases emitted are weighted very differently in the three models, depending on the time frame considered (i.e. 20, 50 or 100 years). As CO2 has a comparably long atmospheric life-span, the longer the time frame under consideration, the smaller the factor with which CO2 emissions are to be multiplied to convert the climate effect of other greenhouse gases (which have shorter atmospheric life-spans) into that of CO2.
Due to these and other uncertainties, there is no generally accepted measure to precisely express the climate effect of aviation in a uniform manner, i.e. CO2-eq. Kollmuss und Crimmins, have summarized the state of knowledge and recommend multiplying the CO2 emissions caused by the combustion of kerosene by a factor of (at least) 2, to account for the climate effect of other greenhouse gases.
Fischer, A. et. al. (2009): Flugverkehr und Klimaschutz. Ein Überblick über die Erfassung und Regulierung der Klimawirkungen des Flugverkehrs. Gaia 18:1. 32 – 40.
IPCC (1999): Aviation and the Global Atmosphere. A Special Report of IPCC Working Groups I and III in collaboration with the Scientific Assessment Panel to the Montreal Protocol on Substances that Deplete the Ozone Layer. Cambridge University Press, UK.
Kollmuss, A. & Crimmins, A. (2009): Carbon Offsetting & Air Travel – Part 2: Non-CO2 Emissions Calculations. Stockholm Environment Institute. SEI Discussion Paper.