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konfab
Honorary Master
- Joined
- Jun 23, 2008
- Messages
- 36,120
You are using infrared radiation and heat interchangeably. This is incorrect.
You need to think about energy, specifically electromagnetic energy in the form of electromagnetic waves. Energy will be at different frequency bands of the waves, some of it will be UV, some of it will be visible, and some of it will be infrared. How this is distributed is what we will discussing next.
To start at the source of the problem. Lets talk about the sun:
The sun pretty much is a perfect blackbody emitter of electromagnetic radiation. That is, it emits radiation across the spectrum almost perfectly as matched by the theoretical distribution of wavelength given by Planck's law.
https://en.wikipedia.org/wiki/Planck's_law
This is given by the black line in the graph. Sunlight at the top of the atmosphere is given by the yellow graph.
Next part you need to understand how molecules absorb and emit electromagnetic energy. Electromagnetic energy consists of waves (for this purpose at least, I am not going into quantum mechanics here). To explain it briefly, molecules act as resonators for electromagnetic waves. Not to differently than a string of a musical instrument is to a sound wave. If you pluck a string of a musical instrument, it is effectively converting the energy of you plucking it, to an acoustic wave at the resonance frequency (or note) of that string. In a similar way, if you play back a note's resonance frequency it will absorb it and turn it into mechanical energy which you can visualise for as a vibration.
Molecules act in the same way for electromagnetic waves. If you agitate molecules with energy (like electricity), they will resonate at a bunch of different frequencies and produce an electromagnetic wave. If you agitate molecules with an electromagnetic wave that it will resonate at, you get energy out (which is usually heat).
Last thing you need to know before we put everything together is that the earth is also a blackbody emitter of radiation. Except it is at a much cooler temperature. Cooler temperatures means more energy is at longer wavelengths(more red). It is at these longer wavelengths where CO2 has its resonance frequencies.
So putting all of this together, you can see what is going on in the following figure.
Solar radiation comes in and as you can see, the CO2 bands don't absorb it much of it. When that energy is absorbed by the earth and then re-emitted, it falls into CO2's frequency spectrum and thus the energy is blocked by CO2 molecules instead of radiating back out into space.
To use an analogy, if we look at the visible spectrum, certain materials absorb difference frequencies of light at different rates. Materials that appear "white" to us do not absorb any energy from the visible spectrum, whilst materials that appear "black" absorb all the energy in the visible spectrum. This is why if you only shine visible light on a black or a white surface, the white surface will be cool whilst the black surface will be hot. This is because the black surface has absorbed the energy that was given to it. CO2 would be a greyish colour in the infrared spectrum, which means it absorbs infrared radiation.
This is about as pure physics as you get. And is why it is fundamentally incorrect to state that CO2 has nothing to do with global temperature. The physics says it must have something to do with it. The degree to which CO2 is responsible is a different story, and there are a million other variables at play.
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