Be sure to read Part 1 first, now …
DEFINING THE rGHE THROUGH THE ERL.
How is the rGHE defined in the most basic way? If you have a planet with a massive atmosphere, the strength of its “greenhouse effect” is defined as the difference between its apparent planetary temperature in space and the physical mean global temperature of its actual, solid surface. The planet’s apparent temperature in space is derived simply from its average radiant flux to space, not from any real measured temperature. It is assumed that the planet is in relative radiative equilibrium with its sun, so is – over a certain cycle – radiating out the same total amount of energy as it absorbs.
If we apply this definition to Venus, we find that the strength of its rGHE is [737-232=] 505 K. Earth’s is [288-255=] 33 K.
The averaged planetary flux to space is conceptually seen as originating from a hypothetical blackbody “surface” or ‘radiating level’ somewhere inside the planetary system, tied specifically to a calculated emission temperature. This level can be viewed as the ‘average depth of upward radiation’ or the ‘apparent emitting surface’ of the planet as seen from space. Normally it is termed the ERL (‘effective radiating level’) or EEH (‘effective emission height’).
The idea behind the ERL is very simple, but does it accord with reality? The apparent planetary temperature of Venus in space is 231-232K, based on its average radiant flux, 163 W/m2. Likewise, Earth’s apparent planetary temperature in space is 255K, from its mean flux of 239 W/m2. In both of these cases, the planetary output is assumed to match its input (from the Sun), so one ‘easy’ method one could use to derive the apparent temperature of a planet is by simply taking the TSI (“solar constant”) at the planet’s (or moon’s) particular distance from the Sun, and multiply it with 1 – α, its estimated global (Bond) albedo, a number that’s always <1, finally dividing by 4 to cover the whole spherical surface. Determining the average global albedo is clearly the main challenge when going by this method. The most common value provided for Venus is 0.75, for Earth 0.296.
But does the resulting value really say anything about the actual planetary temperature? If the planet absorbs a mean radiant flux (net SW) below its ToA, then how this flux affects the overall system temperature very much depends on the system’s total bulk heat capacity. If it is large, the flux will have little effect, if it’s small, the flux will have a bigger effect.