A little while ago, I ventured into aircraft CO2, and as good as said that the climate impact of aircraft fuel use should be weighted up by a factor of 2-3 because of various side effects: I thought of water vapour being dumped in the stratosphere. It turns out I’m wrong on that: at the altitudes planes fly at that effect is small, and the extra radiative forcing is dominated by ozone (via NOx) and contrails. There is lots of uncertainty, but a figure of about *3 for the radiative forcing is plausible. The govt website http://actonco2.direct.gov.uk/index.html says it uses a factor of two.
I claim credit for realising that you have to worry about how long the effects last, but someone else has to point out to me more explicitly that its not the radiative forcing that matters but, effectively, the global warming potential, which is to say the time-integrated radiative forcing. And since contrails are short lived, and ozone not long-lived, those effects are small-ish. So I now think that the weighting factor to be used should be much smaller – perhaps so small as to be indistinuishable from 1.