When I was involved with the Millennium Assessment scenarios group (where I was co-ordinating lead author for the chapter on future human well being) a co-chair completely shut down discussion on population and tried to do the same (but with less success) on the fortress world and the possibilities of civilization collapse. Paul Raskin (lead author of the Great Transition) was unimpressed; he left. This was in 2002. I was too early in my career to follow suit and had to endure this co-chair's control for many meetings.
Note: I resigned from my unpaid role with the UNEP in October 2017, in part because the new chief scientist for the UNEP is Chinese.