As the UN report points out, the drop in fertility in the Third World has stopped dropping. Every Western country and every western Social Justice Warrior is doing everything possible to prevent mortal disease in the Third World. Fair enough, but they are not having the same luck with birth control, which seems a lesser priority.
A parallel exits with other species: in North America the two mammal species which are most overpopulated are deer and raccoons, each for the same reason: we have killed or greatly reduced the numbers of their predators, and we feed them (incidentally, through agriculture: zillions of acres of corn, soybeans, etc..) Likewise, we try to reduce mortality from disease in the Third World and we give much food aid. The population, not surprisingly, increases. Arithmetic works.
The question, I think, is not "Well, what should we do about those things," but rather, "What will we do when the 2 Billion or 5 Billion people arrive? The problem is that we address these matters with one eye closed and looking in the wrong direction. That won't change.