I was shocked that few (if any) seem to get this. About what is only one of the most written-about topics (global temperatures).
Clive Best is an expert. And one of the saner ones. Yet in 2019 he wrote this:
“The physical reason why increasing CO2 apparently produces a logarithmic forcing is that the central lines rapidly get saturated way up into the stratosphere, the strongest of which can then even cause cooling of the surface.”
I bet he started to type “Apparently the physical reason why …” then put the ‘apparently’ in the wrong spot. I say this because Best is certain CO2 produces logarithmic forcing. But not certain of why its logarithmic. If you read the quote above making this switch it makes sense. Even though it’s wrong (IMO).
My understanding has 2 parts. First: yes, of course, the IR photons zip around redirected in pseudo-random directions ‘all the way up to the stratosphere’. Why wouldn’t they?
Second: asfaik the cooler air does not then impact the whole thing for degrees of freedom of heat are not shared much in the sky between IR and kinetics.
Rather the logarithmic quality, observed since the 1920’s in graphs (whether via compression or increase in a GHGs) comes from the redirection itself. It is not linear because half the time the re-direction is up! Nor is it some relative value based on stratospheric temperatures.
It is measured to match E, the natural log. The logarithm arises as the relation between 3 dimension and the vectors. It’s mathematical. They could do lab tests with an old style pinball machine, changing the number of posts and using tiny balls and the results would also graph showing E.