If you could do population management at a worldwide level, you would look at the current density and available resources, yes? That would be the most viable and survivable solution. Perhaps tweaked a little towards actual habitable land, because steep rocky mountains can be pretty hard to try and survive on. But that's besides the point; We're already at 508(!) humans per km2 in The Netherlands, in Canada by comparison that's 4 per km2, in the US it's 33 per km2, in France 106/km2, on Ibiza 227/km2 and in Denmark there are around 132 humans/km2. Just some examples. I find it absurd that not one world leader with half of a brain comes up with decent dividing of habitable land by now. Not even a proposition to that…