Following on from blackbody discussions, here is Christopher Monckton’s simple account of how we know the IPCC is exaggerating climate sensitivity. This comes from page 12 of Moncktons: Regulation without reason (on the Canadian coal regulations.)
Are the IPCC’s global-warming projections proving accurate?
IPCC (2007, scenario A2) expects 3.4 C° manmade global warming to 2100. The calculations so far in this paper have assumed that the IPCC is right. Environment Canada does not ask any questions about the IPCC’s global-warming projections. Officials should have made some allowance for IPCC overshoot.
Since 1750, whence IPCC dates our influence on climate, a recent study (Blasing, 2011) shows 3 W m–2 of forcing from our greenhouse-gas emissions, less –1 W m–2 from non-GHG influences (IPCC, 2007). Global temperature had risen by 0.5 C° from 1750-1983 (Hansen, 1984), with a further 0.3 C° since (HadCRUt3, 2011). Of this 0.8 C° warming, 50 to 100% may be manmade. Thus, the 261-year transient climate sensitivity parameter is (0.4 to 0.8)/(3 – 1) = 0.2 to 0.4 C° W–1 m2. Multiplying by the forcing at CO2 doubling, i.e. 5.35 ln 2 (Myhre et al., 2001, cited by IPCC, 2001, 2007), gives transient sensitivity of 0.75 to 1.5 C° by 2100, when CO2 concentration will have doubled. Dividing this value by 0.7, the fraction of all forcings attributable to CO2, allows for non-CO2 forcings. Expected warming would thus be 1.1 to 2.1 C° to 2100, 32 to 62% of IPCC’s central estimate.