I’ve always accepted the ~2.7x multiplier applied to personal aviation emissions, in order to account for the non-CO2 effects of high-altitude jet emissions (including methane, ozone and contrails). Having looked at some of the literature, however, for CRAGs footprinting, it seems it’s not all that simple.

The original source for aviation forcings was the IPCC’s Aviation and the Global Atmosphere (1999). The relevant section is 6.2.3. Alternative Indexing of Aviation’s Climate Impact-RF Index which says:

RFI is a measure of the importance of aircraft-induced climate change other than that from the release of fossil carbon alone. RFI ranges between 2.2 and 3.4 for the various E- and F-type scenarios for subsonic aviation and technical options considered here

Jardine (2005) (the ECI report commissioned by Climate Care) suggests 1.9 as the better figure, based on the TRADEOFF project . On page 7:

The IPCC calculate the change in radiative forcing of aviation emissions since pre-aviation times to be 0.049 W/m2 (See Figure 5).. This corresponds to a radiative forcing index of 2.7 as the total radiative forcing of 2.7 times that of CO2 alone (0.018 W/m2). However, a recent study (TRADEOFF) has updated this figure and a value of 1.9 is now the best-quantified estimate of radiative forcing index of aviation emissions.

Sausen et al. (2005) compare the IPCC (1999) and TRADEOFF results in more detail, and their Fig. 1 gives a good breakdown of the components of the aviation forcing and the level of uncertainty and understanding. The estimates for O3, CH4 and (especially) contrail forcings have fallen, meaning the total radiative forcing (0.048 W/m2) is 1.9 times that of the CO2-only forcing (0.025 W/m2), rather than 2.9 according to the IPCC (1999). Cirrus cloud forcing is highly uncertain and not included in these totals. Its likely range is 0.01 to 0.08 W/m2.

Although these effects do undoubtedly scale up the climatic effects of aviation emissions, Forster et al. (2006) 4 note that using the RFI method to quantify it for emissions trading is rather suspect. Normally, you’d use Global Warming Potentials which define the effect of a unit greenhouse gas over a defined timescale. Instead, the RFI measures the cumulative globally-averaged forcing by aviation emissions up to a certain year. It doesn’t define the timescale of the climatic effect, which is important where non-CO2 forcings operate over variable timescales. And to be fair, we should also include non-CO2 forcings of other sectors, like heating or electricity.

Unfortunately GWPs are unsuitable for most aviation forcings too (Forster, 2006; IPCC, 1999; Jardine, 2005). So it’s very much a case of “watch this space” for a reliable metric to scale up aviation emissions. Until then, we have to fudge it.

There’s a good overview of this area in the Workshop on the Impacts of Aviation on Climate Change: A Report of Findings and Recommendations (Section 2.3.2, p.25)
Forster et al. (2006) It is premature to include non-CO2 effects of aviation in emission trading schemes. Atmospheric Environment 40:1117-1121

IPCC (1999) Aviation and the Global Atmosphere: 6.2.3. Alternative Indexing of Aviation’s Climate Impact-RF Index [online]. Available from: http://www.grida.no/climate/ipcc/aviation/071.htm#623 (accessed 12/2/2006)
Jardine C. N. (2005) Calculating the Environmental Impact of Aviation Emissions [online]. Available from: http://www.climatecare.org/_media/documents/pdf/Aviation_Emissions_&_Offsets.pdf (accessed 12/2/2006)

Sausen, R. et al. (2005) Aviation radiative forcing in 2000: An update on IPCC (1999) Meteorologische Zeitschrift 14: 555-561 - available from: http://folk.uio.no/gunnarmy/paper/sausen_mz05.pdf