## Discussion > Where's my best evidence?

Richard, if there is no response in one of the bands, even if it is the minor band, it makes a difference to the 3.7 watts I had always believed until I went looking for its derivation. Your name is on the chapter, perhaps you can tell me whence it comes and how much is apportioned to each band. I'll be looking at the downwelling IR data later. But if the smoking gun is there why are we all arguing about solitary larches and cherry-picked model output for our evidence?

(Yes, I said cherry-picked. If you want to dispute that, just pick one, any one, beforehand and we'll look at that. It is not helpful to say 'one of our models predicted that' when you can choose post facto.)

Jul 18, 2012 at 3:01 PM Dung

Everyone agrees that the warming effect of CO2 is logarithmic but nobody apparently knows at what level CO2 ceases to have any real effect. So everyone ignores this?

If it is a simple logarithm formula there must be a simple derivation - right? But apparently not.

Dunno if the following is of any interest. (I imagine probably not, as I most posters on this thread are probably familiar with it all, but I'll post it all the same.)

A while ago, I posted a query on Science of Doom asking:

(...)Wikipaedia gives a formula that I have often seen referred to for the forcing due to CO2 (said to be a 1st order approximation):Delta F = 5.53 ln ( C / C0 ) W m^-2

It gives a reference [Myhre et al., New estimates of radiative forcing due to well mixed greenhouse gases, Geophysical Research Letters, Vol 25, No. 14, pp 2715–2718, 1998] which is inaccessible to me – I have no library access and no budget to access paywalled papers.

I remember that this formula is quoted by IPCC, who give the same reference.

My question: Is the derivation of this formula available on Science of Doom, or elsewhere online? I am interested to know the method used to derive it and what assumptions and approximations were used in its derivation.

scienceofdoom replied:

Martin A,

The most important point about this formula is it is calculated as a curve fit to a number of points on a graph. So don't think of it as a derivation, more a "handy ready reckoner".

(...)

Each point on the graph was calculated from solving the radiative transfer equations through the atmosphere. These equations can only be solved via numerical methods – no analytical solution can be derived.

Nick Stokes added:

Link for Myhre's paper.

Martin, that explanation may be summarised as 'because we say so.'

Simon. being a good representation of any fule, I know that. I just can't do it. It ought to be a b. Where the hell does the p come from?