I frequently get people becoming wildly irrational and upset at me when I suggest that the current population is not supported by the earth sustainably, let alone a much larger one. They'll angrily insist that the earth's "carrying capacity" is 20, 50, 100 billion people! Or that countries should be looking to increase their populations, or that natural population stability and decline is a terrible thing.
As though we have not _already_ caused (and are continuing to cause) a great extinction event almost entirely before any significant effects from greenhouse gas emissions have made an impact.
Habitat destruction, chemical pollution, mining, mineral and fossil fuel depletion, water depletion and contamination, farming practices and monocultures, human interference, etc., are all just horrific, are responsible for massive destruction and extinction of the environment, and aren't all suddenly going to get magic'ed away the instant we somehow get the GHG pollution thing under control (if we ever do).
The funny thing about that is, if our population was a very healthy, say, 5% of its current level, greenhouse gas emissions would be practically a non-issue too. The problem would literally be solved. Gradually moving toward cleaner and more renewable sources of course should and would happen, but while that was responsibly and efficiently changing, there would be no imminent climate catastrophe occurring.
EDIT: It's happening again.