Fig. 1
We notice that CO2 absorbs at certain wavelengths: most prominently around 4.26 μm and 15 μm. The lesson that we and most students get taught is that when this infrared radiation is absorbed, the CO2 molecule vibrates. Through collisions with oxygen and/or nitrogen molecules, the vibrations are transported to these molecules. Within these molecules, the vibrations are converted into heat, and this is the heat that is supposedly released by the influence of CO2. The common argument is that the continued exposure of light with, say, wavelength 15 μm from both earth and sun, is absorbed, and this absorbed radiation is continuously converted to heat during the interaction and numerous collisions with other gas molecules….
Some observations, related to water, water vapor and water droplets, appear to support this theory. Fine! We know that water has absorption in the UV- and the IR regions and that this is converted into heat. Hence the burning of your skin after exposure to UV and evaporation of water from the top layers of water (oceans). This absorbed heat, spread evenly over the planet by the water of the oceans and water vapor and clouds, sustains our lives on Earth. The point is that with water, there is mass, even if it is just a tiny little drop. The question is whether we can simply assume that the observed effects of absorption of radiation by liquids, namely the appearance of heat, must be the same as with ideal gases.
Since regular measurements of CO2 began, ca. 60 years ago, about 100 ppm of CO2 were added to the atmosphere, from whatever: source
Every extra molecule of CO2 is surrounded by 10000 other gas molecules. CO2 behaves approximately as an ideal gas, meaning there is no sticking together of molecules like we have with water vapor. It diffuses equally into all directions of a vessel. There is also no release of heat due to condensation by CO2. At the pressures and temperatures normally found in the lower atmosphere, CO2 does not condense into a liquid. According to the relevant gas law, there can be no accumulation of that gas in the lower atmosphere, and subsequent condensation, to contribute to the greenhouse effect caused by the water on the planet. In my opinion, one molecule in 10000 has no mass to absorb any heat and pass it on. Any such effect is probably minimal, totally unmeasurable and negligibly small.
So, what does happen to an ideal gas that is irradiated and has absorption in its IR spectrum? Carefully look at Fig. 1. There is no transmission of light with wavelength 4.26 and 15 μm through the molecule. Exposure to specific radiation of that wavelength will not pass through. The proposed theory is that the photons here are re-emitted or re-radiated. Let us call it back-radiation.
This back-radiation by certain substances at certain wavelengths is also the principle of infrared and UV & visible spectrophotometry. I remember that as a student I sometimes secretly lifted the cuvette holder a bit during a determination to see what happens when you turn the wavelength knob on the absorbing wavelength when measuring the standard solutions. I could see that the path of light is obstructed. The light goes back, mostly in the direction of the source, but also a bit round. That is why in my days we used to talk about ‘extinction’. The light does not go through the cuvette, but it goes extinct at the absorbing wavelength. The transmission of the radiation at that wavelength is obstructed. In spectrophotometry, the degree of extinction at a certain wavelength is therefore an indication of the concentration of a certain component (Lambert-Beer).
At one stage during my employment – I remember this was in fact at the very initial stage of micro chip manufacturing – I was tasked with controlling the quality of the incoming nitrogen. One of the tests involved the measurement of CO2 where I used an FTIR spectrophotometer, able to bombard my gas sample with radiation of 4.26 μm. As it happened, the amount of CO2 in the nitrogen was in the range of about 100 ppm. Now, like most of you, I sometimes made the mistake of forgetting….to switch off….. Never mind that, coming back the next morning I always found there was no change. I could repeat the test and still get the same result. This observation demonstrates that continuous irradiation at 4.26 μm did not produce any heating of the gas sample. Thinking back now, you can imagine that if the theory of those CO2 molecules exposed to 4.26 μm radiation continuously vibrating and turning that absorbed radiation into heat were true, it should affect all nitrogen molecules and eventually the contents of my cuvette should cook or explode… None of this has ever happened to me during any spectrophotometric measurements that I conducted…..
I also consulted ChatGPT on this and eventually he or she admitted:
‘So your experiment gives you a solid observation—CO₂ strongly attenuates 4.26 µm radiation—but the conclusions about where the absorbed energy goes, need to be tested independently’.
Let us have a look at that! Take note that CO2 also has absorptions in the near infrared region. Scientists can pick up the back-radiation of this via the dark side of the moon. Here is the report:
Turnbull et al
I show Fig. 6 (bottom) of the above-mentioned report here:
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