Spencer seems to view this trivial observation as a sign of "backradiation" from the sky. The argument is that without this "backradiation" the cavity temperature would drop below 290 K towards 0 K.
But this is an illusion or play with confusing words: What happens is that the cavity reads
the temperature of the sky through its blackbody spectrum, then compares with its own
temperature and decides to cool off towards the sky temperature, if it happens to be lower,
and warm up if it happens to be higher. This is illustrated in Mathematics of Blackbody Radiation, Without Backradiation in the equation for internal energy (related to temperature T by E ~ T^4) :
- E_t = int (F^2 - RU_tt^2) dx
with the integral of local difference (F^2 - RU_tt^2) between incoming and outgoing radiation intensity driving the temperature (and not int F^2 dx - int RU_tt^2 dx).
The same happens when you open the door to your garden: Your are not struck by any backconvection/conduction from the garden, are you?
You may compare your own salary with that of a more successful competitor and ask for a compensating increase, but you don't compare to somebody with zero salary, because it serves no purpose.
It is remarkable that climate science even among skeptics can be in such a complete state of confusion. Of course, Spencer does not respond to my comments on his experiment. Evidently there is not any "backradiation" here either.
The confusion is a result of the abdication by physicists from the throne of rational mechanics,
because the 2nd Law could not be given a correct understandable formulation, only as a deep mystery beyond human comprehension. The abdicated physicists instead flocked into string theory. And string theory physicists have no reason to say that "backradiation" is non-physical and accordingly don't do it.
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