The EPA’s Endangerment Finding relied entirely upon IPCC Assessment Reports (and, to a lesser extent, USGCRP Assessments) as supplying more than enough evidence that rising atmospheric CO2 concentrations have caused changes in various measures of climate and that without steps to reduce anthropogenic CO2, climate changes will get event worse in the future. Having already explained how both the IPCC and USGCRP have developed into science advocacy organizations, rather than assessment institutions, in this chapter I begin my brief critical analysis of the output produced by the IPCC. I focus on the IPCC because its reports are the primary basis for not only the Endangerment Finding but for precautionary US climate policy more generally.