That is by volumn, yes.. but we know that how much CO2 will effect items, and how much CO2 is being pumped into the atmosphere. The fact that water vapor is a green house gas means that the excess CO2 causes a positive feedback loop, amplifying the effect of the CO2.
Yes, under laboratory conditions it does. It does so by increasing the water content in the air.
The radiative spectra from CO2 is absorbed in the first few microns of water, where the water skin is more apt to evaporate. Increased air temperatures also hold more moisture.
This is no doubt, a positive feedback is with H2O forcing. There are two major problems with the ignorance of stopping there with it though. The first is that we would have a runaway feedback if this exceeded 50%, and the alarmists like to claim more that 50%. Since the spectra from H2O and CO2 operate the same with the water skin area, if this were to happen, the runaway scenario would happen without changes in CO2 forcing.
In the laboratory, the 3.71 W/m^2 of forcing for a doubling of CO2 might be correct. However, H2O from the oceans also have a negative feedback.
There is a reason why CO2 is well mixed in the atmosphere up past 80 km, and H2O drops close to zero before 10 km. It's because CO2 is a gas at all the atmospheric temperature ranges, and H2O is either a liquid of solid through the atmospheric temperature ranges. Temperature does not affect how much CO2 is in the air, but does affect how much H2O can be absorbed before precipitating out.
That's why we have clouds!
The extra water absorbed at sea level rises, and because there is more, when it reaches higher elevations, there are more clouds too. More clouds reduce the solar heating of the earths surface, and since this is source of energy for the greenhouse effect, it is a negative feedback to the greenhouse effect.
Both positive and negative forcing. Overall, the positive forcing is probably marginally greater than the negative forcing.
This is why all the alarmists models fail. They do not consider the indirect results of the sun, assuming it's power driving the greenhouse effect remain constant, and they also refuse to accept how the extra humidity reduces the clear sly areas.