First, technically what matters is not PPM but parts per cubic area. PPM is a proxy. Except with discussion of saturation this does not matter. The density of the air in the future — whether you are taking about the surface or above — will effectively be what it is now. That is, average barometric pressure is pretty constant. It is easier to use PPM instead of referencing how close C02 molecules are to each other.
Experiments back in the 1920s showed this. Scientists measured the (heat) emissivity of enclosed tubes of air, then increased PPM then measured a second time – the emissivity went down. Then, they did a third test, they went back to the lower PPM but increased air pressure instead. The result was the same decrease in emissivity. The same Greenhouse Effect. QED it’s how close these molecules are to each other that matters.
Which makes perfect sense since the absorbed photon is re-emitted in a pseudo-random direction. And the closer the next C02 molecule is, the sooner there will another re-direction.
How fast, you ask? … I read somewhere an estimate that one of these infrared photons traveled about 10 meters before being absorbed again (not 10 meters between molecules but between actually getting reabsorbed, which requires specific conditions. Taking roughly 1 hundred-millionth of a second.
This was one some physics board, and was clearly nonsense. The tubes in labs are shorter than that. Let’s do a thought experiment of 10 cm — which would take 1 millionth of a millionth of a second. Elsewhere I have read this is the speed of the absorption-reemission. So that would be the distance of interest. For instance, if true now, in 2021, a lot of time the photons are arriving at the appropriate angle, even as the last one is leaving!
Before absorption the C02 molecule are triangular, after redirection they are momentarily linear – then they snap back (so to speak) . This temporary change of shape is the ‘vibration’. C02 has three shapes aka 3 ‘vibration modes’.
Why doesn’t top alarmist site SkepticalScience at least say this is wrong? .. probably because it isn’t. It is in textbooks. Maybe they don’t agree that the GHG absorption barely works with a flat triatomic molecule?
From where I sit it seems clear they pretty much need to be triangular in order to act as a GHG and thus to me it seems it is essential for climate models to know how long does it take for the molecule to snap back in shape?
My impression this varies based on the precise frequency of the photon and this is fractionally longer for the top end of the IR band so we want the correct average time. There is math and it called is called the Schwarzchild equation.
The point is, there will be a density such that when the photons arrives the C02 molecule wont be ready to receive it! This will happen everywhere effectively all at once because IR moves at the speed of light and this point is called ‘optical saturation’. As meaningless ghosts IR photons will fly by any CO2 molecule which has not yet snapped back.
After this point that moving the C02 molecules closer together (via increasing the famous PPM) cannot increase redirection. Because all the molecules will constantly be in the wrong vibrational mode. note: this saturation ceiling will arrive asymptotically.
According to Happer’s math this is what we get. Not the green but the difference between black and the red. [and notice that all the molecules involved are triatomic!]
If this is true it means there is absolutely no climate emergency.
Happer, by the way, is possibly the top atmospheric laser expert in the world… And one needs to understand everything about optical re-direction in order to design a laser. Which he did. His invention was declassified some years ago and is now used for new kind of earthbound astronomy.