I have been majoring here on Climate Sensitivity, which I make simple with the Three Babies Test.
In engaging with climate sceptics and deniers, I challenge them on sensitivity.
Over on Steve Goddard's the ill-named Real Science blog, a sceptic puts forward Roy Spencer's lecture on clouds as evidence for low CS.
I have written about Roy Spencer quite a bit on my blog, but I summarised it on Goddard's blog, and paste my reply here, not least in case it gets lost, since Steve seems to be a bit rude.
It is good to notice that Roy Spencer (RS) accepts that doubling of CO2 concentrations means a 1*C rise in earth temperature. Basic textbook physics.
First, a CS of 0.5*C cannot explain the atmospheric response to Pinatubo: http://greenerblog.blogspot.co.uk/2011/10/can-roy-spencer-account-for-pinatubo.html
Second, whatever the net feedback effect of cloud, whether strong negative as Roy claims, or weak positive as Dessler claims, the effect is going to diminish in a warming planet:
3rd, RS is alone in believing that clouds vary randomly, rather than being influenced by ocean temperatures.
4th RS' model (yes, he used a model) has no realistic ocean, no El Niño, (ocean current cycle) and no hydrological cycle, and it was tuned to give the result it gave." In short, it uses a partial, not a complete view of the data.
5th , he does not supply error margins - and when his figures are reworked with error margins, his discrepancy vanishes.
6th, even if he is granted a negative feedback from clouds (and see 2nd above) there are also positive feedbacks from water vapour, albedo, soil CO2, and methane. The latter two are not yet integrated into models, since they are not yet quantifiable, as I understand.
7th there is the matter of the several other independent lines of inquiry which all point to a CS in the region of 3*C. That really is the clinching argument. When many diverse and independent lines of inquiry point to a figure, it is pretty clear that a single line of inquiry pointing to a different figure, especially if it does not even supply error margins, is not going to overthrow the main figure.
8th Ling proposes that tropical clouds have a weak positive feedback rather than the strong negative feedback proposed by Lindzen.