IPCC is a policy making body

The IPCC is a policy making body, since its Terms of Reference limit it to investigation of only human-induced climate change and its Summary for Policymakers defines what can be included in the 'science' parts of its reports. The NIPCC report cites many instances of scientists objecting to these limitations. 

http://www.sepp.org/publications/NIPCC_final.pdf

The role of the IPCC “is to assess on a comprehensive, objective, open and transparent basis the latest scientific, technical and socio-economic literature produced worldwide relevant to the understanding of the risk of human-induced climate change, its observed and projected impacts and options for adaptation and mitigation” (emphasis added) [IPCC 2008].

http://www.ipcc.ch/pdf/ipcc-principles/ipcc-principles-appendix-a.pdf

Changes (other than grammatical or minor editorial changes) made after acceptance by the Working Group or the Panel shall be those necessary to ensure consistency with the Summary for Policymakers or the Overview Chapter.

The Synthesis Report is composed of two sections as follows: (a) a Summary for Policymakers and (b) a longer report. The IPCC Chair will lead a writing team whose composition is agreed by the Bureau, noting the need to aim for a range of views, expertise and geographical representation. An approval and adoption procedure will allow Sessions of the Panel to approve the SPM line by line and to ensure that the SPM and the longer report of the Synthesis Report are consistent, and the Synthesis Report is consistent with the underlying Assessment Reports and Special Reports from which the information has been synthesised and integrated. This approach will take 5-7 working days of a Session of the Panel.

The Session of the Panel will review and adopt the longer report of the Synthesis Report, section by section, i.e. roughly one page or less at a time. The review and adoption process for the longer report of the Synthesis Report should be accomplished in the following manner:

- When changes in the longer report of the Synthesis Report are required either to conform it to the SPM or to ensure consistency with the underlying Assessment Reports, the Panel and authors will note where changes are required in the longer report of the Synthesis Report to ensure consistency in tone and content. The authors of the longer report of the Synthesis Report will then make changes in the longer report of the Synthesis Report. Those Bureau members who are not authors will act as Review Editors to ensure that these documents are consistent and follow the directions of the Session of the Panel.

- The longer report of the Synthesis Report is then brought back to the Session of the Panel for the review and adoption of the revised sections, section by section. If inconsistencies are still identified by the Panel, the longer report of the Synthesis Report is further refined by the Authors with the Assistance of the Review Editors for review and adoption by the Panel. This process is conducted section by section, not line by line.