According to conservatives, the greatest disaster is falling birth rates. This is, apparently, going on internationally Japan, France, Germany, America, etc. I think what they really mean is that white birth rates are declining.

The earth functions best at about 2 billion people, but can sustain itself reasonably well at about 4 billion people.

The things that could wipe out that number of people in tthe near future would include, nuclear warfare and a near fatal pandrmic.

I don't think huumanity is capable of organising itself when it comes to large numbers, and I doubt planetary cooperation is possible.

I would love to see a western politician try to convince an African shaman that burning the leaves of some tree won"t savee the world.