A simple argument is put forth against the idea that the radiative properties of an atmosphere somehow serve as the CAUSE of elevated steady-state planetary surface temps.
The presence of an atmosphere on top of a solar-heated planetary surface forces the steady-state temperature of that surface to be higher than if the atmosphere weren’t there, because it acts like a layer of thermal INSULATION interposed between the surface and its ultimate heat sink – space. The surface heat loss will simply always be lower – at any given surface temperature – with an atmosphere on top than without.
Even a 100% IR-active atmosphere wouldn’t, however, be able to make any difference whatsoever to the surface temperature if the ATMOSPHERIC temperature were equal to that of space itself.
In other words, it is an absolute requirement for the atmosphere in question to be capable of producing an insulating effect on the solar-heated surface at all that it be WARMER than space. What sets an atmosphere apart from the vacuum of space, after all, is its (thermal) mass, making it able to be HEATED by the surface on which it rests, thus naturally allowing it to gain a temperature much higher than that of space and therefore much closer to that of the surface itself.
- So for any physical property of an atmosphere, if it – on balance – helps in making it warmer than space, if it contributes positively to its net heating, then it effectively acts to promote the atmosphere’s insulating effect on the solar-heated surface below.
Which leads us to the crux of this argument: An atmosphere’s RADIATIVE properties, specifically, will not on balance contribute positively to the dynamic heating/cooling budget of that atmosphere; which is to say that an atmosphere containing IR-active constituents will end up COOLER on average than an atmosphere not containing such IR-active constituents. The radiative properties of an atmosphere’s IR-active constituents simply – and quite naturally – help cool the atmosphere to a much greater extent than they help in heating it …
And so, insofar as an atmosphere’s radiative properties are not contributing positively to the net heating of that atmosphere, they also cannot be CAUSING the atmosphere to exert an insulating effect on the planetary surface.
So what is the cause? The atmosphere’s TEMPERATURE is. Those particular physical properties of an atmosphere that actually help making and keeping it warmer than space, that contribute positively to its dynamic net heat budget. The ability to absorb IR photons is not such a property. Because an ability to absorb IR photons is always accompanied by an equal ability to emit IR photons. And so it takes something else, something more, to turn that inherently dual process into a non-zero-sum game and have energy starting to accumulate. Because that’s what we want, that’s what we need. The capacity for holding on to energy, storing it internally over time. Not just the catch-and-release part.
The point here is that an atmosphere is good at both catching and releasing energy in the form of radiative heat, but only good at catching energy in the form of conductive heat and evaporative heat (latent heat of vaporisation). Which means that the two latter modes of heat transfer dynamically promote the accumulation of energy within the atmosphere, while the former mode does the opposite.
In short, an atmosphere’s IR-active constituents are capable of ridding the atmosphere of excess energy in the form of heat, while its non-IR-active constituents aren’t; they are capable of capturing heat coming in, but not capable of releasing it again. For that, they need their IR-active siblings.
Let’s put some numbers to this.
Our atmosphere’s global annual heat budget looks like this (flux values from Stephens et al., 2012):
Qin(total) = QSW(from sun) + QLW(from sfc) + Qcond(from sfc) + Qevap(from sfc) → 75 W/m2 + 32.4 W/m2 + 24 W/m2 + 88 W/m2 = 219.4 W/m2
Qout(total) = QLW(to space) → [239.7 – 20 =] 219.7 W/m2
Qin = 34.2% + 14.8% + 10.9% + 40.1% = 100%
Qout = 100%
What can we read from this?
The atmosphere can only be radiatively heated if it is significantly able to absorb radiation. Likewise, it can only radiatively cool if it is significantly able to emit radiation.
Our atmosphere on average absorbs a SW heat flux (Qin(SW)) from the Sun worth of 75 W/m2 and a LW heat flux (Qin(LW)) from the surface worth of 32.4 W/m2, totalling 107.4 W/m2, which is quite exactly equal to 49% of its entire heat gain.
At the same time, though, our atmosphere on average emits a LW heat flux (Qout(LW)) to space worth of 219.7 W/m2, which makes up 100% of its total heat loss.
In other words, our atmosphere’s radiative Qin is less than half as large as its radiative Qout. On average, it cools more than twice as effectively via radiation as it heats via radiation. The non-radiative heat transfer mechanisms, on the other hand, only heat the atmosphere. They don’t cool it at all. On average.
Which essentially means that the strictly radiative processes operating in our atmosphere aren’t working towards making it warmer at all. They’re actively and continuously working rather towards cooling it down. Specifically compensating for the exclusive heating effect resulting from the non-radiative processes going on …
And that, my friends, is basically the end of the argument 😎