Anthropogenic Global Warming disproven by Blackbody physics?

Let's see if someone can find the error in my reasoning here:

1. Blackbodies absorb all incident radiation.
2. Blackbodies absorb and re-emit incident radiation below their cut-off frequency.
3. Blackbodies absorb and turn into heat energy radiation above their cut-off frequency.

Imagine a 300K blackbody surrounded by a perfect mirror, itself at 300K.



The radiation emitted from the blackbody is reflected by the mirror back onto the blackbody. The radiation emitted from the mirror (due to itself being at 300K) is also incident on the blackbody.

The blackbody despite having more than twice the incident radiation cannot heat up past 300K because this would require a 300+K body being heated by a 300K body -- a violation of the second law of thermodynamics.

The best that could be said for a warming effect here is that if the temperature of the blackbody were to fall, then it would be 're-heated' by its own rays trapped in the cavity. So if anything this is a radiative insulator but definitely not a heater.

OK so what does this have to do with AGW?

Well if the surface of the earth is a blackbody and it radiates as such into space, and the atmosphere contains CO2 which absorbs and re-emits 15 micron (and other wavelengths), then we have a similar situation to the above thought experiment, with a couple of important differences:

1. CO2 absorbs and re-emits in both directions. For each incident photon there is a 50% chance it will be ejected to space or emitted back down to the ground.

2. CO2 is not a blackbody itself. (Only dense media behave like blackbodies).

The idea that the earth emits infrared radiation toward the sky which 'back-radiates' it down to earth producing a heating effect is purely fictional. As we have just seen this can't happen due to the second law. At best it could behave as a radiative insulator. However the fact that for each 'reflection' 50% is lost to space would suggest that after just three or four 'bounces' there would be nothing left of the back-radiation, and since this only takes a fraction of a second to occur one cannot say that there is any effect on surface temperature by the presence of a CO2 layer at all.

So to recap:

1. A body cannot be heated by its own reflected rays.
2. CO2 absorbs and re-emits in both directions, sending back as much as 50% of the bands it absorbs.
3. The earth cannot be heated by 'back-radiation' from CO2.
4. Any energy trapped in this system would take a fraction of a second to escape so the CO2 cannot act as a radiative insulator.

Replies from people who understand the physics only please!

 

Explain what you mean by this.

No, read the argument.

I don't think you understand the argument at all. Go back and re-read the initial post.

What I am saying is that thermal radiation from a blackbody at temperature A cannot heat -- at all -- a blackbody at temperature B if temperature A is less than or equal to temperature B. Thus the earth 'heating itself' via 'backradiation' is purely fictional.

Re-read the argument.

 

Agreed, the earth cannot heat itself via 'backradiation'.

AGW theory centers on the 'greenhouse effect' which specifically refers to the 'trapping of thermal radiation' by 'greenhouse gases'. The mechanism proposed by AGW advocates is as follows: The earth radiates its blackbody spectrum. Gas molecules in the atmosphere absorb and isotropically re-radiate certain frequencies in this spectrum. Some of this re-radiated thermal spectrum is directed down toward the earth. The earth is heated by this downward 'back-radiation'. This is their proposed mechanism for warming.

I don't think you understand the mainstream AGW theory.

This is from the IPCC's own webpage:

Read it here: http://www.ipcc.ch/ipccreports/tar/wg1/518.htm and if you're not convinced check any other mainstream source.

The problem with your more general point about CO2 changing the emissivity of the earth as a whole is that it necessarily cannot if my reasoning above is correct. The thermal radiation is not trapped in any way that will lead to less radiation being emitted. If thermal radiation has to 'bounce' (be absorbed and re-emitted) three or four times between the atmosphere and the ground in order to get into space then this does not change the amount that is radiated into space, nor does it change the earth's capacity to radiate it in the first place. The only effect CO2 has in this model is to increase the path length for the radiation... Think about it.

 

No, the heat capacity of the earth is greater than zero. The water bodies on the surface absorb shortwave radiation and retain it as heat energy which raises the temperature of the earth. The water in the atmosphere also does this. Water has a high heat capacity.

Most of it being direct back to the earth? Consider a single cubic cm of CO2 gas in the atmosphere absorbing and isotropically (in all directions) re-emitting 15 micron wavelength. There are two essential directions it can radiate, up and down. This means that approximately 50% (actually slightly less) can be radiated back to the earth, and 50% out to space.

In the IPCC model the earth is correctly modelled as a blackbody or greybody. The albedo of the earth is 0.3. This doesn't change the physics. As the reflective part of the earth only leads to less shorwave energy being converted to heat on the surface it doesn't change anything in favour of AGW. Claiming that the earth isn't a perfect blackbody is a red-herring. Nothing is a perfect blackbody. That fact doesn't invalidate the approximation as a blackbody.

Where to even begin. Try this experiment, you will see it does not work. Further this *is* inapplicable physics because no part of the CO2-surface system is reflecting the CO2 absorption bands. There is, in the 'greenhouse effect' only absorption and emission of spectra, no reflection. Further, there's no radiative heat transfer between the chicken and foil in direct thermal contact with it. All that will happen is the chicken will conduct its heat to the foil and the foil will conduct its heat to the air. Almost nothing will be lost to thermal radiation.

Again this counter-argument is incorrect, but not only because the physical analogy is wrong. Let me propose a counter-analogy which will work better for your understanding of my argument.

Imagine two parallel perfect mirrors extending to infinity in all directions. If you were to introduce a ray of light between the mirrors, it would bounce forever between them and never escape. However now imagine one mirror has evenly spaced holes in it. In fact 50% of the mirror is missing. So that everytime the ray hits that mirror it has a 50% chance of going through the mirror and a 50% chance of being reflected.

Now lets suppose you introduce a continuous beam of light into the system. Does the amount of light in the system increase without limit? Consider it in terms of simple energy: Does the ingoing flux match the outgoing flux? The answer is that it does because the 50% chance of being passed by the second mirror applies to ALL of the radiation incident on that mirror. If there is twice as much radiation trapped between the mirrors as is entering, then there will be the same amount of radiation leaving as entering the system.

To draw this back to the 'greenhouse effect', consider the CO2 layer like that same mirror with the holes in it, and the ground like a hot mirror. The ground gives off thermal radiation, which has 50% chance of getting out to space, the other 50% bounces back to the ground where it is unable to heat the ground due to having insufficiently short wavelength and thus is bounced back to the CO2 layer where once again it has 50% chance of getting out to space. You can see how the analogy fits the two mirrors. Once 200% of normal infrared emissions are trapped between the two mirrors, the outgoing flux will be the same as if no second mirror existed at all.

As a caveat: there is no reflection in the AGW model or greenhouse effect. Reflection is an entirely different physical process to absorption and re-emission. By reflection or 'bounce' I mean absorption and re-emission, and by mirror I mean something that will perfectly absorb and re-emit radiation -- i.e. a blackbody with a peak radiation shorter than the incident wavelength or a CO2 molecule where the incident wavelength is in its absorption bands. It is just a useful analogy to describe what is going on, but definitely not to be confused with actual reflection which is a different physical process.

 

The reason the dark side of the earth doesn't freeze to death is more likely to be due to the heat capacity of water in the atmosphere and on the ground.

But thanks for chipping in. I would still like to hear a proper physical refutation of the original argument if you think you have one.

 

I don't think you understand my argument or the physics. I encourage you to go back and re-read the thread more carefully.

Firstly no radiation is reflected by anything in the greenhouse model. There is a certain portion of incident solar radiation which is actually reflected by the earth to space due to earth's albedo -- but this is not what is considered when discussing the 'greenhouse effect.'

Secondly, for all bodies in thermodynamic equilibrium energy in must equal energy out. If what I think you are saying were true then the earth would have a net inflow of energy at all times which would cause it to heat quickly to the temperature of the sun.

If you read the IPCC model (or study astrophysics) you will learn that their argument is not that energy-in does not equal energy-out, but rather that in order to maintain that energy balance when certain frequencies of thermal radiation are prevented from escaping the earth, the temperature of the earth must increase. This would be fine except that the physical mechanism proposed does not work by careful analysis -- CO2 is not preventing the escape of radiation to space, rather it is increasing the path length thermal radiation must travel in order to escape.

Please do not assume that because I disagree with the mainstream 'consensus' that I don't know what I am arguing. On the contrary I have taken two years of university level physics and I am keen to debate this properly with anyone who thinks they know their stuff on AGW.

 

The 50% refers to the absorption bands which are over 95% saturated.

Let me clarify for you: Of the thermal radiation emitted by the earth which is absorbed by the CO2 in the atmosphere, 50% is re-emitted out to space and 50% is re-emitted down to the ground.

Thermal radiation leaving the earth which does not interact with CO2 is irrelevant for the purposes of debating the validity of the greenhouse effect mechanism.

"Provides energy that is not in thermal equilibrium"? This sentence doesn't make sense unless you mean that the energy is hotter (shorter wavelength) than that sent back by the earth.

Let me clarify the system for you:
In the IPCC model the sun heats the earth via shortwave radiation -- which the atmosphere is transparent to. The earth heats up due to this shortwave radiation and then emits a characteristic blackbody spectrum back to the sky. The CO2 layer(s) then absorb and re-emit parts of this blackbody spectrum in both directions (up and down).

To simplify the system we can replace the sun with an internal heater within the surface of the earth. The earth still behaves as a blackbody and still emits cavity radiation. This does not effect the applicability of the greenhouse effect.

With your constant you begin by talking about albedo (percentage of light reflected by the earth) then you try to make it the absorption co-efficient of CO2.

Let's get this straight. The greenhouse effect as defined by the IPCC involves no optical reflection -- none. Only absorption and re-emission. These are completely different phenomena. Your constant is physical nonsense. Go back and read the physics.

Nope, still wrong. Read the argument you are a proponent of / learn the physics. Nobody claims that there is a difference between the rate of energy gained and the rate of energy lost. Their argument is that the rate of energy emitted by the earth must equal the rate of energy absorbed by the earth in order that the earth maintain thermodynamic equilibrium. And this is quite correct. Their contention then follows that if you prevent the escape of some thermal radiation from the earth then the temperature of the earth must rise in order to compensate SO THAT energy out can continue to equal energy in.

I cannot stress enough that no one is claiming that energy out doesn't equal energy in. If you took even a moment to think about it, the earth would quickly reach the temperature of the sun if it emitted less energy than it takes in.

I think I'm done debating this unless someone with an actual physics background can come up with a concise and accurate counter-argument. If this happens please PM me as I probably won't check this thread.

Incidentally in case anyone is intrigued by the argument, there is direct evidence for its validity. Look up an experiment performed by Wood in 1909, where two identical greenhouses were constructed, one using glass (which is opaque to infrared) and one using rocksalt (which is transparent to infrared). The rocksalt greenhouse was hotter. This is direct reproducible counter-evidence for the greenhouse effect and is consistent with the analysis put forward in this thread.