water vapour and GHGs

anna v (20:43:39) :
CO2 is a trace greenhouse element . period. the tail does not wag the dog.

BUT consider ozone. Even less than a trace gas. But if it were reduced by a few percent globally UVb would be playing hell with our genes, plant growth, plankton survivability. 1% change in O3==2% increase in UV.
From Wiki O3 gives 3-7% of the GHG effect from only 0.00006 percent of the atmosphere (0.6ppm)
Is this not a case of the hair on the flea in the tail wagging the dog? CO2 could be similar?

Stephen Wilde (23:50:41) :
The composition of the air is merely an enabler. Once it has served it’s function in permitting the creation of liquid oceans and a hydrological cycle it’s significance becomes marginal.

You are making some really wild leaps here.
As you said energy balance is everything. The earth is a "grey" body radiator and if the atmosphere were suddenly lost radiation away would be determined by the various emissivities of the different terrain. And temperature will stabilise when grey body radiation out = radiation in. This would bottom out at about -18C.

Adding atmosphere makes it more complex so lets assume all GHGs are removed but the same atmospheric pressure were present (O2, N2, H2 are non GHGs). Without GHGs thermal radiation from the grey body will not be absorbed so only conduction and thence convection will heat the air. With no absorption by GHGs the grey body + atmosphere will still loose the same amount of radiation as if the atmosphere were not present (possibly more as the conducted heat to air will also radiate from the air).
So without any GHGs but with an amosphere the earth will radiate as before - as a grey body.

Major GHG effect (wiki)
water vapor, which contributes 36–72%
carbon dioxide, which contributes 9–26%
methane, which contributes 4–9%
ozone, which contributes 3–7%
Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for water vapor alone, and between 66% and 85% when factoring in clouds.[8] ...
The Clausius-Clapeyron relation establishes that air can hold more water vapor per unit volume when it warms.

If we now add only normal concentration of water vapour to an earth without other GHGs assuming it starts at the average of 13-15degC However this is with a full compliment of GHGs. Water vapour only provides 72-85% of the GHG effect so the temperature will fall. Falling temperature will reduce the water vapour content of air which will lower the GHG effect and the temperature will fall. Falling temperature will reduce the water vapour content of air which will lower the GHG effect an the temperature will fall etc. - a positive feedback that will eventually lead to very low levels of water vapour and hence we will be back at near grey body temperature.

If we now add the other GHGs into the atmosphere then the grey body radiation is reduced and temperature rises. Rising temperature leads to more water vapour which leads to higher temperature etc. We then end up with a warming grey body. At some point the grey body radiation out will equal the incoming radiation and we have temperature "stasis".

water in the atmosphere
http://weather.cod.edu/sirvatka/1110/Unit2_1110.pdf