The Honest Broker - Stuck Half Way Through

In their recent Nature article Pielke et al. (2008) convey as a their key message that the IPCC dramatically underestimates the socio-economic effort it would take to transform the 21 century non-policy high emission path into a policy-induced low-emission path. Thus, the core statement of working group 3 in the Fourth Assessment Report of the IPCC seems to begin to totter – that an ambitious climate policy can be pursued with the already existing technologies. What the article of Pielke et al. does not explicitly say but suggests implicitly is that the cost estimates of the IPCC would then also be just as unrealistic as the estimation of the technical and economic challenge. Is the optimistic message that the IPCC announced in 2007 to the whole world finally unfounded? Is the challenge greater than assumed, is climate policy now more expensive than alleged? Did the IPCC finally mislead policy makers? Is the IPCC eventually not the "honest broker" between science and politics and pursues own politics with the authority of science?