Hi adb. Your post is very timely, as you bring up precisely the next point I was thinking of considering in the Gerlich and Tscheuschner paper. This is another case in which Gerlich and Tscheuschner get elementary thermodynamics wrong.
I see some other posters have given good concise comments that may help show where your analysis fails. In the meantime, I've been working on this longer post.
As an aside, I'll note that this paper seems to have a lot of popular interest mainly because it feeds into widespread public skepticism about so-called "global warming". That particular topic is one that seems to bring up a lot of passions and vehemence. There's a place for that, of course, but I'm hoping to avoid the whole question of
changing climate in this thread. Gerlich and Tscheuschner are attempting to refute the conventional physics of a simple fixed atmospheric composition without worrying at all about changes in the atmosphere from one year to the next.
Let's keep that in mind. The question at issue is: the surface of the Earth is warmer – on average – that the surface of the Moon. Why? It's a basic physics problem, and we should be able to give a reasonable answer without any worry at all about how climate is changing. It's only after we have a credible understanding of the physics of the present that there is any basis for looking at the much more subtle problem of changes to present conditions.
Let's look at what you have introduced from G&T.
adb said:
It is easy to find fault in areas of minor detail in a paper as long as that of Gerlich. The point of his paper is that he has shown the "classic" atmospheric greenhouse model as depicted by the IPCC, to be utter nonsense.
Here's the IPCC atmospheric greenhouse model:
https://www.msu.edu/course/isb/202/ebertmay/drivers/ipcc_greenhouse.jpg
(IPCC 2001)
We may describe this as:
1. A warm body (the earth) radiates heat to a cool body (the atmosphere)
2. The cool body "back-radiates" (IPCC term) heat to the warm body.
3. This process continues perpetually, with heat flowing round and round in a continuous cycle.
4. The result of this perpetual process is that the warm body becomes warmer.
What is most amazing is that both alarmists and skeptic scientists have taken the above blatant 2nd Law of Thermodynamics violation at face value for so long.
The first thing to note here is that the first three points above are simple facts of life. The fourth, however, is incoherent, and stands as a basic confusion made by Gerlich and Tscheuschner. More detail can be added to the picture, but there's no violation of the second law here.
- The surface of the Earth really is warmer than that atmosphere above it. And the surface of Earth does radiate heat up into the atmosphere. If we consider a more complete picture, the Earth also transfers energy from surface into the atmosphere by convection, and by latent heat of evaporation; but radiant transfer is the largest part of the energy flux from surface to atmosphere. Basically, convection transfers a certain amount, latent heat transfers about three times as much, and longwave radiation transfers four or five times more than latent heat. The net flow of heat and energy is from surface to atmosphere. At night, especially over the land, there can be a low level "inversion" involving a transfer of energy back to the surface from the bottom 500m or so of the atmosphere. This helps damp out the oscillations of temperature between night and day. Over all, however, it is a perfectly sensible observation that the atmosphere is cooler than the surface, and that there is (by the second law) a net flow of energy from the surface up into the atmosphere.
- A cool body really does "back-radiate" to a warm body. Anything with a temperature will radiate, and this will involve a flux of energy from a cold body to a warmer one. What the second law requires is that the flow back in the other direction from the warm body to the cold one must be larger. The two way transfer of radiant energy exchange between a hot body and a cold one is a standard thermodynamic problem, and it always involves a small flow from cold to warm combined with a larger flow from warm to cold.
- The exchange of energy between the surface and the atmosphere really does continue perpetually, or at least until the Sun runs out. It has to, by basic thermodynamics, because of the net flow of energy coming in from the Sun, which must be dissipated. But the only energy radiated from the atmosphere is energy it receives from the surface and from the Sun; and ultimately all the energy involved comes from the Sun, with an atmosphere or without.
- The fourth point "a warm body becomes warmer" makes no sense. G&T are confusing a shift from one stable condition to another with an ongoing increase in temperature. Greenhouse DOES NOT involve making things warmer and warmer in some kind of perpetual motion. It is rather a part of all the more or less stable cycles of temperature. A planet with an atmosphere will be warmer on average than one without, but in both cases the surface temperature cycles from day to night and season to season, without a long term trend. Greenhouse effects result in a higher mean temperature for this dynamic equilibrium.
This point about the dynamic equilibrium is fundamental. Try this highly simplified example, which would be a reasonable exercise for an introductory course in thermodynamics.
- Take a flat surface which radiates as a black body, and which is heated by a bath of incoming radiation at 240 W/m2. Calculate the temperature of the surface when it comes into equilibrium with the radiation.
- Add a barrier between the surface and the incoming radiation. Let the barrier absorb 25% of the incoming radiation, and transmit 75%. Let the barrier absorb 80% of the radiation coming up from the surface, and transmit 20% of this. (This is physically sensible. Many materials are transparent to one wavelength and opaque to another.) Assume that the barrier is "thin", with the same temperature on either side, so that it radiates equal amounts of energy in either direction, and that all energy transfer is radiant. Calculate the resulting temperature of the surface.
Note that adding the barrier makes the surface warmer by comparison with conditions without the barrier. This is analogous to a greenhouse effect, whereby an atmosphere makes a planet warmer than it would be without an atmosphere. It does not, however, result in a continuous and perpetual increase in warming at the surface. It just shifts things to a new dynamic equilibrium.
To calculate this correctly, you will have to consider a flow from the surface to the barrier, and also a smaller back-flow from the barrier to the surface.
G&T have confused the "warming" effect of an atmosphere, which is simply a shift of the prevailing conditions to a new and higher mean temperature, with a perpetual motion machine, which actually generates an additional source of energy. The greenhouse involves no extra energy; merely a redistribution of the same energy which comes always from the Sun.
Many will shout that all bodies radiate ... yes they do but NETT heat flow is always from hot bodies to cool bodies (without the input of work), not the reverse. Note also that the 2nd Law does not care about the wavelength of radiant heat.
Quite right; and all of that remains true for an atmosphere that absorbs infrared and transmits visible light. The total energy flow is obtained by taking all the sources of energy, without distinguishing wavelengths. They are all equally important in the energy balance. Different wavelengths can be absorbed or transmitted in different ways, but the second law applies in the same way regardless. You just need to know the energy transferred between the parts of the system, without regard to wavelength.
When an atmosphere absorbs infrared and transmits visible light, it ends up being heated from a planet's surface… because that is where the infrared is coming from. Then, by the second law, it must be cooler than the surface. And it is. Equivalently, the planet's surface must be warmer than the atmosphere. And it is. This is the greenhouse effect.
Atmospheric gases do absorb radiation from the sun and the earth. NETT radiation from the cool daytime atmosphere is to space. The Sahara desert in daytime has a very low "greenhouse gas" concentration above it, yet contrary to greenhouse theory, it is a hot place rather than a cool place.
That's not a contradiction with greenhouse theory at all. The Sahara is extremely cold at night. That is, there are large changes from night to day. The main reason for this is the lack of an ocean to damp out the changes. The drier atmosphere also reduces this damping effect. So in the day time, temperatures rise very quickly, and then drop again at night.
Don't mix up the damping effects of day/night oscillations with the greenhouse effect. They are different things. Also, don't think that that greenhouse concentrations are "very low" in absolute terms. The local greenhouse effect may be reduced, but it's still very much there. Also, because temperatures flow from one part of the planet to another, you simply cannot treat regions in isolation, with greenhouse affecting one place and not another.
Night time, rotation of the earth, convection, conduction, latent heat all add greatly to the complexity of climate model. However the basic daytime atmospheric greenhouse model as presented by the IPCC and most textbooks, is nonsense.
No, it isn't. And do recall, this is NOT about climate change. It is simply about what you should expect the average temperature of a planet to be.
You do agree that the Earth has a higher average temperature than the Moon, I guess. And yet we get the same solar energy. Indeed, the moon absorbs more solar energy than the Earth, because the Earth has ice and clouds and so on that reflects more light without absorption.
If the whole planet was a blackbody at a uniform -18C, then it would be radiating away the same amount of energy as we receive from the Sun. By Holder's inequality, with any redistribution of temperatures that maintains the same net outward radiation, the average temperature will be LESS than -18C. G&T actually get this bit right, without understanding the implication. The surface of the Earth is has an average temperature of more than -18C, and it is, by basic thermodynamics, therefore radiating away more energy per unit time than is received from the Sun. That's a fact. It's data.
Now by the laws of thermodynamics, that energy MUST be coming from somewhere. You cannot propose some kind of perpetual energy machine to supply more energy.
The solution is analogous to the example I have given you above. There's a barrier, above the surface, which in accord with the laws of thermodynamics generates no new energy. Its effect is to raise the net energy flux at the surface above what you would have without the barrier.
Cheers -- Sylas