The world's population should be at most 50 million.

[Alan V]

I = P A T

The environmental impact (I) equals the population (P) times the affluence (A) times the technology (T) used to acquire it.

Population is a multiplier.  One-tenth of the population means you have reduced the impact, whatever it is, by nine-tenths.

The poorest 1 billion are 3% of our carbon footprint (and I'm rounding up bils here to say 10bil, since we'll be there sooner or later), there's one tenth of our -future- population that would not reduce impact of climate change by nine tenths if it were completely removed.  

Similarly we can reduce our affluence or change our technologies to have lower impacts and thus reduce climate change.

https://en.wikipedia.org/wiki/I_%3D_PAT

I'd go with changing our tech and fixing our distribution before I told the poors to stop fucking like that would actually do something.

The Chinese are the biggest CO2 polluters today because of their huge population, even with a fraction of the per capita footprint of Americans.  Both China and India want to increase their affluence significantly this century, which is a real problem for controlling climate change.  They are still increasing their burning of fossil fuels.

Studies have shown that increasing education for girls is the most effective way to reduce populations.