However, many prior posts have shown this to be false for a number of reasons, including two posts quoting the Feynman lectures on statistical mechanics of a Boltzmann Distribution pure N2 atmosphere, and the HS post, "Why Greenhouse Gases Don't Affect the Greenhouse Equation or Lapse Rate," which also calculates a pure N2 Boltzmann Distribution for Earth.
We will now use a couple of illustrations for smarties or dummies to understand why the so-called 'greenhouse gas' water vapor cools, not warms, the Earth surface by up to ~25C via changes in heat capacity (Cp) alone (not even including additional cooling from latent heat transfer or clouds). We will also show why a pure N2 atmosphere without any greenhouse gases would have a surface temperature ~25C warmer than the present, due to a much steeper lapse rate.
Recall that the dry adiabatic lapse rate formula is a very simple, linear relationship whereby the change in temperature (dT) with change in height from the surface (dh) is solely dependent upon the gravitational acceleration constant (g) divided by the heat capacity of the atmosphere at constant pressure (Cp):
dT/dh = -g/Cp
And note that change in temperature dT is inversely related to change in heat capacity (Cp). Since water vapor has a much higher heat capacity Cp than air or pure N2, addition of water vapor greatly decreases the lapse rate (dT/dh) by almost one-half (from ~9.8K/km to ~5K/km), thereby cooling, not warming, the surface by up to 25C.
In our hypothetical 1st atmosphere consisting only of N2 plus addition of < 1% water vapor, we assume the addition of water vapor creates a wet adiabatic lapse rate of 5K/km, the same as the wet adiabatic lapse rate on Earth. By calculating the center of mass as the HS Greenhouse Eqn does, and calculating the fixed 255K equilibrium temperature between the Earth and Sun, we can then calculate the entire tropospheric temperature profile from the surface to tropopause, and replicate the 1976 US Standard Atmosphere model:
|Thought experiment 1 of a N2 atmosphere with < 1% GHG water vapor. Note for easy illustrative purposes only, the actual numbers are rounded slighly, e.g. the actual height of the center of mass is ~5.1km rather than 5.0 km, and the actual dry adiabatic lapse rate is ~9.8K/km, not 10K/km.
Note in the above "greenhouse atmosphere," there is a ~33C "greenhouse effect" from the 255K center of mass to the ~288K surface, as well as an even larger "anti-greenhouse effect" of negative 35K from the 255K center of mass to the ~220K top of troposphere. Thus, gravity has not added any energy to the atmospheric system; gravity has simply redistributed the available energy from the only source the Sun, more towards the surface and less towards the top of the troposphere. That is the gravito-thermal greenhouse effect of Poisson, Maxwell, Clausius, Carnot, Boltzmann, Feynman, US Std Atmosphere, HS greenhouse eqn et al, and has no dependence whatsoever upon IR emission/absorption from greenhouse gases.
Now lets consider a hypothetical Earth atmosphere without any greenhouse gases, consisting solely of pure N2. We again use the dry lapse rate equation above, since obviously N2 is affected by gravity (g) and has a heat capacity (Cp). In this pure N2 Boltzmann distribution, the lapse rate can thus be calculated as ~10K/km, essentially the same as our present atmosphere dry lapse rate (9.8K/km).
For illustrative purposes only, the atmospheric mass of a pure N2 atmosphere is close to that of our present atmosphere, and thus the center of mass is also located near ~5km in the troposphere. However, since the lapse rate is much steeper in a pure N2 atmosphere, the "greenhouse effect" is about 50K from the 255K center of mass to 305K surface, and the "anti-greenhouse effect" is also ~50K from the 255K center of mass to the ~205K top of the troposphere, producing a ~100K temperature gradient from the surface to top of the troposphere:
Thus, we find the net effect of the addition of < 1% 'greenhouse gas' water vapor was to cool, not warm the surface of an N2 atmosphere by up to ~25C.
Thus, the Arrhenius radiative greenhouse theory (which confuses the cause with the effect) is once again demonstrated to be unphysical and falsified, and the Maxwell et al gravito-thermal greenhouse effect once again vindicated. One and only one of these two competing greenhouse theories can be correct, otherwise the observed effect would be double (66C) that observed (33C). The Maxwell et al theory is the only option which does not violate any laws of thermodynamics.