[DuskStar]

What would be "overpopulation" by your definition? No way to grow enough food, resulting in mass famines? No physical space for people, even after tiling the planet with skyscrapers?

Or would "unsustainable carbon emissions and ecological damage" be enough? And if that's enough, aren't we already there today?

[Isinlor]

I would accept decline in more than 1% of world population over 10 year time span caused by ecological collapse as overpopulation.

But the important question is what would you do about possibility of overpopulation however it is defined?

If you want to invest in health care and education in Africa - awesome! You have my full support. For example educating woman has excellent side effect of reducing fertility rate.

If you think about enforcing population restrictions, then you should think harder. How do you imagine that would look like on a global scale if you can't get USA to stick to Paris Climate accord? Some country with nuclear weapons threatening mutual assured destruction if some other country doesn't stick to 1 kid policy? Ridiculous.