Another IPCC contributor with think tank connections is Ross McKitrick who says "far from being an open network of top experts it has turned itself into a narrow clique of like-minded activists."
I'd like to venture why Professor McKitrick feels that way. McKitrick served on Working Group I which deals with the physical science basis for anthropogenic global warming. McKitrick's field is the dismal science of economics.
McKitrick is far more at home at the Fraser Institute . In 2001 this libertarian think tank published "Global Warming A Guide to the Science" an opus which staes "A review of the scientific literature concerning the environmental consequences of increased levels of atmospheric carbon dioxide ... have produced no deleterious effects upon global climate or temperature." It also tells us "There is no clear evidence, nor unique attribution, of the global effects of anthropogenic CO2 on climate." Although this guff was published in 2001 it didn't stop Professor McKitrick from contributing to the IPCC's 2007 report.