The IPCC doesn't write a single report. The have 3 different working groups, each writing their own report. The first group deals with the science, the second deals with the impacts, and the third deals with mitigation. Obviously, the 3rd one is the most politically influenced.
You forgot the fourth output from the groups: the "Synthesis Report", subtitled "Summary for Policymakers". This is where most of the information comes from that the media reports to the public.
How do they exactly phrase their call for centralized authority ? What's the page number ?
Oh, it's subtly sprinkled throughout the reports as well as in companion plans from climate-change related UN organizations. For instance, on Page 17 of the summary is this not-so-subtle admonition to create a global governance structure:
Climate change has the characteristics of a collective action problem at the global scale ... Effective mitigation will not be achieved if individual agents advance their own interests independently. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation.
And there's more on page 32 that hints at centralized planning as a requirement to avert global disaster:
International cooperation is critical for effective mitigation, even though mitigation can also have local co-benefits. Adaptation focuses primarily on local to national scale outcomes, but its effectiveness can be enhanced through coordination across governance scales, including international cooperation.
What do you think all the "cooperation across all governance scales" is all about, anyway? How would you interpret "Effective implementation depends on policies and cooperation at all scales", and calling for new "effective institutions and governance"? Hmmm? All over the place in those reports.