In a blogpost from Roy Spencer published some time ago, he defends the existence of a natural greenhouse effect. The entire post can be read here. I will pick out a few passages that concerns the second law of thermodynamics. Spencer writes the following:
"A second objection has to do with the Second Law of Thermodynamics. It is claimed that since the greenhouse effect depends partly upon cooler upper layers of the atmosphere emitting infrared radiation toward the warmer, lower layers of the atmosphere, that this violates the 2nd Law, which (roughly speaking) says that energy must flow from warmer objects to cooler objects, not the other way around."
There is indeed a formulation of the 2nd law that states the following:
Heat flows spontaneously from higher to lower temperature
He goes on:
"There are different ways to illustrate why this is not a valid objection. First of all, the 2nd Law applies to the behavior of whole systems, not to every part within a system, and to all forms of energy involved in the system…not just its temperature. And in the atmosphere, temperature is only one component to the energy content of an air parcel."
What Spencer wants to say with this is somewhat obscure. The formulation I stated above can be found in standard textbooks on thermodynamics and is pretty straightforward. Furthermore he states that the 2nd law applies to all forms of energy, not just the temperature. First of all, temperature is not a form of energy but relates to energy and entropy by the formula
1/T = dS/dE.
Secondly, the energy he refers to that is not included in the "temperature", does he mean the potential energy? Probably. Well, the potential energy could be included in the heat capacity of the gas, indeed, in statistical mechanics one observes that the heat capacity of an ideal gas in a gravitational field increases to 5/2kT per constituent particle to be compared with the value 3/2kT holding without the field. This accounts for the potential energy of the gas.
Furthermore he writes:
"Secondly, the idea that a cooler atmospheric layer can emit infrared energy toward a warmer atmospheric layer below it seems unphysical to many people. I suppose this is because we would not expect a cold piece of metal to transfer heat into a warm piece of metal. But the processes involved in conductive heat transfer are not the same as in radiative heat transfer. A hot star out in space will still receive, and absorb, radiant energy from a cooler nearby star…even though the NET flow of energy will be in the opposite direction.
In other words, a photon being emitted by the cooler star doesn’t stick its finger out to see how warm the surroundings are before it decides to leave."
This is even more obscure. What precisely is the difference between conductive heat transfer and radiative heat transfer? Is it that when a hot metal plate looses heat to a colder plate it does so because it has first measured the temperature of the colder plate and concluded that it was lower that its own?
We will return to discuss this issue at length later. Stay tuned..