In terms of AGW if you sift through the hyperbole to the actual basics of the AGW proposition it works like this.
Lets define some variables.
We will lump all the things that affect climate into one generic lump, and call it K, for known variables and driver affecting climate.
So we observe that in the late 20th century, ΔT != f(ΔK).
That is temperature changes can not be accounted for by currently known variables.
So we pick on one known unknown, CO2 gas concentration (lets call that C) and try
ΔT = f(ΔK) + f(ΔC).
Sadly this does not work either. The effect of CO2 is too small to account for observed ΔT!
Now at this point you have a known unknown . Your model does not work as it is. So you have two choices.
(I) introduce an unknown feedback term to multiply the effect of CO2. (lambda factor) giving you an equation of the form:- ΔT = f(ΔK) + λ f(ΔC). This is the essence of the IPCC model.
(ii) simply accept that there is in addition to carbon dioxide, something else (U for unknown) going on as well giving rise to climate changes, so we have : ΔT = f(ΔK) + f(ΔC) + f(ΔU).. This is the essence of say - Svensmark et al's hypotheses where cloud cover is modulated by cosmic rays etc etc.
There is an important point here. Both models assume an known unknown element. In the IPCC model its a feedback system - positive feedback as well. It is not clear whether in fact they don't make a rather logically absurd assumption here as well, because the lambda factor should multiply ANY climate change due to other effects. I.e. the proper IPCC equations should be of the form:
ΔT =λ( f(ΔK) + f(ΔC)).
The problem is that this leads to a very unstable climate - which is simply not borne out by the historical record.
In short the 'CO2 is not the whole story' models of Svensmark and co actually fit the data better.
So if you compare the two possible models you have the following broad conclusions from the IPCC
- most warming in the late 20th century was man made
- feedback only applies to carbon dioxide created warming. Or the historical records don't fit.
- the lambda factor necessary to make the data fit this model produces really scary climate change predictions, justifying massive expenditures on products Al Gore and his chums sell.
Whereas lumping the known unknowns into a separate function with NO overall positive feedback gives rather different conclusions
- most warming in the late 20th century was not man made
- climate change will happen independently of any human attempts to stop it
- there is no point spending money on products and technologies to make the attempt: better is to spend the money on dealing with it
- we can't say whether or not scary climate change is happening/will happen, or not.
Naturally enough the political movements and industries geared to taking your money and spending it on stuff that either doesn't work at all, or doesn't work well enough to make any real difference (but we have to make the attempt, right?) have a lot riding on the IPCC general equation form.
Feedback is essential to keep the green machine on track printing money.
THAT'S why everyone loves it. Nothing else produces scary warming. In fact the non lambda models tend to suggest that we are more likley to be about to enter a global cooling period with far more drastic impacts on human life.