Having established the real reason for the Greenhouse Effect (yes, there is one), we now have to address why the Greenhouse Effect is less marked than we'd predict using basic physics alone.
We'd predict a temperature at sea-level of about 300K and a temperature of about 200K at the top of the atmosphere, the precise numbers depending on what altitude you choose as starting point (where actual temperature is equal to effective temperature = 250 - 255K) and whether you assume dry lapse rate of 8K/km or 10K/km.
1. As it happens, the Greenhouse Effect is less than this - average temperature at sea-level 288K and average temperature at the top of the stratosphere 213K - the 'moist lapse rate' is 6.5K/km, not 8K/km or 10K/km.
So water vapour is reducing the Greenhouse Effect by about 12K at sea-level, and so it can't be a 'greenhouse gas' (to the extent there even is such a thing).
2. As a matter of fact, humid places not only have smaller day/night temperature ranges than drier places at the same altitude and latitude, they are also on average slightly cooler overall.
3. The main reasons for this are as follows:
a) When water evaporates from the surface, extra kinetic energy (heat) is converted to latent heat of evaporation/condensation. When water vapour condenses back to water droplets higher up, it converts latent heat of evaporation/condensation back to actual kinetic energy (heat). That's a one-way mechanism.
b) Clouds have a cooling effect in the day time (reflecting solar radiation so it never reaches the hard surface) and a warming effect at night (when clouds reflect some light and some infra red radiation back to the hard surface). That's a two-way thing, but the balance is a cooling effect (see point 2). To complicate matters, the cooling effect is stronger in summer than it is in winter (when it possibly has a slight warming effect).
4. The Consensus argues that water vapour in general absorbs infra red radiation which would otherwise leave the surface and go out to space. Some of this infra red is reflected back to the hard surface and warms the hard surface up a bit more. This is quite possibly true, but it is nigh impossible to split the overall 12K cooling effect into the main cooling effects (latent heat and day time clouds) and the main and minor warming effects (night time clouds and reflected radiation) and any attempt to do so would be subject to so many estimates and intelligent guesses as to be meaningless.
5. The Consensus also argues that more water vapour will lead to a self-reinforcing cycle - higher hard surface temperature leads to more water vapour; more water vapour reflects more radiation to hard surface; this warms hard surface even more; etc etc. Logic tells us that this can't happen, or else it would have done long ago.
There is no need to speculate about what would happen if sea-level temperatures rise by one or two degrees - the tropics are tens of degrees warmer than the Poles, and most of the tropics are oceans with plenty of water available, and there hasn't been a self-reinforcing cycle there; the overall picture is that water vapour and actual water are a self-regulating, moderating influence.
During the Little Ice Age, temperatures were barely affected in the tropics but went down a lot at higher and lower latitudes. Since then, temperatures have bounced back more at very high and very low latitudes (i.e. North and South Pole, where there is very little water vapour) and haven't risen much in the tropics (very humid). Which again points to water vapour being a moderating influence.
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