Water Vapour vs(?) CO2

It is true that water vapour causes more warming than CO2. But what is also true is that the concentration of water vapour in the atmosphere is determined by the sea surface temperature and so is not something we can directly affect. So what water vapour does is provide a feedback mechanism that amplifies the effect of any other influence (such as that of CO2). Fortunately the feedback effect diminishes with increasing temperature and so just leads to a new higher equilibrium temperature rather than causing a run-away. But the fact that water vapour acts like an amplifier does explain why tweaking the small CO2 direct effect causes a much bigger end result than that which is frequently suggested by uninformed and over-confident denialists.

Source: (178) Alan Cooper’s answer to Is it true that water vapor causes more warming than CO2? If so, why is CO2 still considered a main cause of global warming? – Quora