In order to properly understand some of the aspects of the greenhouse debate, it is important to make clear the concept of equilibrium contra non-equilibrium. No real world system is ever in equilibrium, there are always disturbances. In the case of the earth these disturbances are, among many others, the rotation of the earth, the non-uniformly distributed sunlight, the moon, etc etc. So why do we use the concept of equilibrium at all. The way I see it is that the equilibrium is the imaginary state that the system strives towards but never reaches. And in order no know how the non-equilibrium state evolves we need to know its ultimate goal. If there is a temperature gradient, the system will strive to erradicate it as fast as it can. Or will it?...
Let's make things simple for a while. Let's take away all non-uniformities that create disturbances in the earth's thermodynamic system. No rotation, uniform sunlight, uniform oceans, no poles, no equator, no moon. What would it be like? The greenhouse hypothesis says that this system would be in equilibrium, but the equilibrium would be profoundly affected by the amount of greenhouse gases present in the system. The fact that things will be slighty different with a change of composition of the atmosphere is of no surprise, but the strange thing is that GHEH implies that this equilibrium would be characterized by a temperature gradient (or a "radiative-convective equilibrium") and that this temperature gradient can be considered caused by the greenhouse gases. Strange isn't it, an equilibrium with a temperature gradient, like a refrigerator working without electricity. But hey, so what, the real world atmospheric temperature gradient is indisputable. Or maybe it isn't?..
GHE proponents have of course developed a cunning way to get out of this dilemma. They argue that an equilibrium with a temperature gradient is not a violation of the 2nd law, since the earth is not in equilibrium anyway. Wicked isn't it :)
But let's not argue about that now. What do the skeptics say? Some skeptics say that this equilibrium which I sketched on before will indeed be characterized by a temperature gradient but it will be caused by gravity. Since the total energy of each molecule follows a Boltzmann distribution the molecules at higher altitudes will be slower than those at lower altitudes giving rise to a temperature gradient. Convincing isn't it? The only problem is that it is wrong, as was shown long ago. Given the assumptions, gravity doesn't cause a non-uniform temperature, instead it creates a non-uniform chemical potential. To make things even more complicated (and interesting) the chemical potential is temperature dependent. (Check out the previous post "On the temprature distribution of an ideal gas under the force of gravity"). This cannot be dismissed easily since even under the Navier-Stokes approach to fluid mechanics, the isothermal air parcel is a very stable and physical concept that can be extended to an infinite space domain.
There is however another aproach. One could claim that there is an atmospheric lapse rate because the earth is not in equilibrium and thats that. At the moment, Claes Johnsson and I discuss the consequences of this assumption. In this case the incoming sunlight is treated as an energy source heating the surface and the important question becomes how the thermal transport properties are affected by an increase in the optical activity of the atmosphere, characterized by a absorption/emission parameter A. Does an increase in A lead to larger radiative heat transport and flatter lapse rate or does it lead to increased isolation and a steeper lapse rate. The first corresponds to cooling, the latter to warming. A simple observation could give a hint to the answer to that question, consider two bodies with temperatures T1 and T2 that radiate against each other with an intensity AT^4. The net heat transfer is thus
dQ = A(T1^4 - T2^4)
The heat transfer increases as we increase A, or what is your opinion? The thriller continues...