Anyone help with this one?

John. Can you help with this? In the accounts of the formula for change in the  radiative transfer of co2 (delta F = alpha logn(C2/C1), the alpha term, ie the coefficient for co2, is given as 5.31, sometimes 5.35. How is this derived?  

I'm guessing it's calculated from the line-by-line calculations in those papers that theoretically calculate the radiative forcing from CO2 but am hoping someone just lob in an answer before I have to go digging through papers.

Discussed here.  More details are available in the link to Myhre 1998.

Myre et al 1998 use several different models, a Line-by-Line model, a Narrow Band Model and a Broad Band Model. The result, 5.35 is actually a best fit to the calculated curves. As Riccardo pointed out in another context, 5.35 only applies between 300-1000 ppm. He used Modtran to look at a much larger range up to 1000's of ppm and it gave 4.8

So although the reationship is logarithmic, it isn't derived from first principles. Rather you do the number crunching in the RT models and curve fit.

This whole suggestion box forum is proving quite useful!

Can either of you give a link to where Riccardo discussed this?  I'm pretty interested in that result, and I'd like something I could reference.