A comment on Claes' answer to Roy

In a recent post Claes Johnson attempts to answer the following question from Roy Spencer:

How does the surface 'know' how opaque the atmosphere is before it 'decides' at what rate it should emit IR?

There is one point which I would like to make here which seem to have escaped many GHE-skeptics. In a previous post I constructed a simple radiation model, which does not necessarily come close to the real situation, but which nevertheless highlights something important. In the model in question no part of the system 'knows' what goes on anywhere else, the only things each part knows is its own temperature and absorptivity. The model also contains 'backradiation'. Moreover, the 'backradiation' taken alone does in fact slow down the rate of cooling in the system. The question is now: Does the model reproduce anything like the greenhouse effect? The answer is: It doesn't. In the model the temperature lapse rate flattens as the absorptivity/emmisivity increases with the consequence that the system cools, which is a clear deviation from the so called GHE.

The reason for this is probably the following: The amount of backradition can never exceed the radiation that at the same time is lost to outer space. Thus the backradiation cannot trap energy in the system since it is always associated with an 'out-radiation' that is equally big. 

The real difference with the GHE and reality is thus probably much more subtle than both Roy and Claes wants it to appear. To be honest, I have not quite understood the supposed mechanism of GHE although I have tried, but maybe I will succeed in the future to completely disentagle the mathematical structure of it. I very much encourage the mathematically inclined audience to also make such an attempt, since the present 'wordy' discussion on 'backradition' has not managed to clear the confusion. 

My suspicion though is that the greenhouse effect is based on a mechanism of reflection rather than absorption-thermalization-thermal reemission. Hence it is formally more akin to radiation pressure, but that remains to be clarified. Good luck.

Cloud or No Cloud

Much confusion surrounding the debate concerning the fundamentals of the greehouse theory, in my opinion can often be traced back to a careless hopping between concepts from equilibrium- and non equilibrium thermodynamics respectively. Here I will present a simple thought experiment that might highlight this issue. Consider a body which by regulated inner chemical reactions maintains a temperature of 37 degrees Celsius. Now consider two situations:

1. The body is placed in vacuum (outer space)

2. The body is surrounded by a nitrogen cloud which has a temperature of 20 degrees Celsius.

In case 1 it is obvious that the body will radiate energy to space at a rate which depends on its temperature. The exact law governing the magnitude of this radiation is not really important here, but let's suppose that it follows a T^4 law. But what about case 2? It seems obvious that the body will transport heat to the gas due to the temperature difference but how much will it radiate? Greenhouse theory given an unequivocal answer to that: It will still radiate as much as it did in vacuum, and since there are no greenhouse gases around there will be no back-radiation. In case 2 we are thus left with the conclusion that the body loses more heat to the surrounding than it did in case 1. But does this really make sense? I can think of several ways to argue that the body will in fact not radiate nearly as much as it did in vacuum, and I will for the moment leave it to the reader to think about this. If, however, you believe that the surrounding gas will reduce the amount of radiation it does have implications for the greenhouse effect, it means that the application of an equilibrium radiation law, like the Stefan-Boltzmann law, to a surface which is not equilibrated to the surrounding gas (with or without greenhouse gases) is invalid.

If you are still hesitant I might just through out the question quite bluntly:

Which would you like to try, cloud or no cloud?