I'm have a really drawn out discussion (read: argument) on a different forum with a poster who says that because CO2 causes about 1/4 of greenhouse effect (in terms of radiative forcing), that must mean it has a 4x feedback factor.
Can anyone else see the flaw in this logic, or is it just me?
I assume he is drawing a direct link between the 1/4 and the 4x? If so, I agree with you that the logic is flawed. Not sure what point he is trying to make? Surely by that logic the other 75% would also have 4x feedbacks for each quarter...?
Yes, as far as I can tell he's working on the implicit assumption that CO2 is the only driver and all else is feedback, and although he denies this I can see no other explanation.
This logic makes sense only if you consider all non-condensible GHGs (not just CO2), and assume that if you took out all of those, the remaining water vapour would result in a greenhouse effect so small as to be negligible.
This paper suggests that 20% or so of the greenhouse effect would remain if you took out all the CO2 and let the system equilibriate. As a first approximation maybe the 4x is okay, but it sure helps (particularly if arguing with ardent, die hard sceptics) to be as clear and accurate as possible on all points.