"...per-capita emissions of carbon dioxide will likely rise in poor countries for some time yet, even in optimistic scenarios. But that is an issue of consumption, not population."
Human consumption of natural resources and emission of CO2 requires humans. More humans equals more of what humans do.
Per-capita CO2 emissions will rise greatly in poor countries as they begin to afford the technologies we take for granted. No matter how efficient cars, appliances, machinery and so on become, increased production and use of them will increase CO2 emissions. It's cornucopian fantasy to think a population increasing by 50% can have a "First World" standard of living without causing serious ecological damage. It's fantasy to think it can be done in the first place.
Even if humans discover means to produce enough energy to support ourselves, even with ZERO damage to the environment, our use of energy will increase and we will cause damages, due simply to our increased numbers if nothing else.