Layman struggles with Science

[Richard111]

Learning about the relationship between peak emission wavelength and temperature has scrambled my poor mind. In my post above you see how a simple formula derived from Wien’s Law gives the point of maximum intensity for a given temperature in degrees Kelvin.

I had to re-read Claes Johnson's essay on Computational Blackbody Radiation to try and sort out my mind. I have selected the following text from his essay as I feel these statements encapsulate the area of my confusion.

Quote:A blackbody acts like a transformer of radiation which absorbs high-frequency radiation and emits low-frequency radiation.

The temperature of the blackbody determines a cut-off frequency for the emission, which increases linearly with the temperature:

The warmer the blackbody is, the higher frequencies it can and will emit.

Thus only frequencies below cut-off are emitted, while all frequencies are being absorbed.

These statements seem quite clear and straight forward. I will now attempt to write down my understanding of them using a thought experiment.

Imagine two black bodies, A and B, radiating at each other. Body A is at 288K (+15C) and body B is at 273K (0C). Peak emission from body A will be at 10.0625 microns and body B is at 10.6154 microns. So body B is radiating at at a longer peak wavelength (cooler) than body A. The temperature readings confirm this anyway.

Body A is emitting at the shorter wavelength (higher frequency, higher temperature) but it is intercepting all radiation from body B. Since A is already radiating at all frequencies arriving from B it cannot store that energy as heat.

Body B is receiving frequencies above its peak wavelength so it can store that energy and warm up. But body B can store only that excess energy that is above its own peak emission level. So the warming is real but small.

To summarise: body A is recieving radiation from B but NOT warming. B is seeing radiation from A and being warmed by the slight excess in energy.

Eventually both bodies reach the same temperature and radiation levels. They will have reached equilibrium. They will continue to cool at the same rate, each counteracting any change in the other.

All of the above, as I see it, agrees with Claes Johnson's explanations.

Now for my area of confusion. Both bodies are emitting and absorbing at the longer, cooler, wavelengths without any exchange of energy.

Let us now return to the real world where body A is the earth's surface, well 1 square metre of it will do, and body B is the atmosphere above that 1 square metre. I realise these are not "black" bodies so the emissions will be less. Either way, both bodies will be radiating at each other way above the 15 micron level which equates to a peak temperature of -80C (193K)!

We are reliably informed that CO2 radiates happily at 15 microns but so what? Only in the Antarctic at midwinter will we possibly see temperatures of -80C, the rest of the world is much warmer and will therefore not respond to any 15 micron radiation.

Will anyone please explain to me what AGW is all about? It cannot be about CO2 as that operates in the very cold part of the IR spectrum.