billy, quantity is the most important factor and it's the effect of each molecule of water vapour versus the effect of each molecule of CO2 times the number of molecules that is the relevant equation. The fact that the resident time of each water vapour molecule is only relevant if the departing molecule is not replaced. However that is not the case as we know that there is a certain large percentage of water vapour in the atmosphere all the time. What you are in effect saying, is that 1 person who stays in a smoking room chain smoking for 8 hours has more effect on the atmosphere of the room than 2000 who come and go over the same period, only staying for one cigarette.
