The preceding posts lead to the conclusion that the Earth (and Venus) including atmosphere up to a pressure of 0.2 - 0.3 bar have a TOA temperature at the tropopause equal to the bolometric temperature determined by the distance to the Sun, as a minimal temperature.
The surface temperature would then be determined by a lapse rate observed to be 6.5 C/km resulting from atmospheric thermodynamics driven by radiative forcing of the Earth surface, to be compared with the dry adiabatic lapse rate of g/c_p = 9.8 C/km with g gravitational acceleration and c_p the heat capacity of air at constant pressure.
The thermodynamics in the atmosphere would thus have the effect of reducing the dry adiabatic lapse representing a possible state without radiative forcing and thermodynamics, and thus an effect of reducing the surface temperature.
Doubling the atmospheric CO2 is by IPCC estimated to correspond to a radiative forcing of 2- 4 W/m2, to be added to the 180 - 40 = 140 W/m2 effectively absorbed by the Earth surface with 180 incoming and 40 directly outgoing through the atmospheric window. The effect on the surface temperature would then be determined by the lapse rate with the bolometric temperature of TOA at the tropopause unchanged because the distance to the Sun is unchanged.
The effect of additional effective radiative forcing of the Earth surface would be more active thermodynamics which would tend to further reduce the lapse rate and thus the Earth surface temperature.
Climate sensitivity as the increase of the Earth surface temperature upon doubling of CO2, would thus be negative: More CO2 would tend to be cooling rather than warming, but the effect would probably be so small that it could not be observed. Climate sensitivity would thus seem to be non-positive and the risk of global warming would (very likely) be small (with a most likely value of 0).
(This insight is now quickly eating its way into the minds of both people and politicians and global warming hysteria is already history).
Compare with the climate sensitivity of + 3 C by IPCC, which is obtained by a combination (i) radiative forcing increasing the bolometric temperature, as if the Earth was moved closer to the Sun and (ii) positive thermodynamics feedback, as if thermodynamics could slow down by additional forcing.
The IPCC view is presented by its Swedish representative Lennart Bengtsson with the following key argument:
- .... the Earth energy balance can temporarily be changed by reduced radiation to outer space by increased concentration of greenhouse gases.
PS Notice that in the IPCC and LB greenhouse gas argument, the TOA would be put at 5 km at a bolometric temperature of - 18 C corresponding to 240 W/m2 outgoing radiation, and would then be shifted upwards to cooler levels under increased concentration of greenhouse gases and then eventually cause surface warming. But there is no TOA other than the tropopause (as concerns thermodynamics), and shifting an artificial TOA up or down would lack physical meaning.