RE: Why we're running out of water
A huge problem with all population reduction schemes is that everyone wants OTHER people to reduce THEIR population. You can't keep cheaters from benefiting at everyone else's expense unless you've got a despotic government willing to engage in forced abortions and forced sterilization. You have to see famines, epidemics, wars, and disasters as wonderful opportunities to cull the herd. And, let's face it, the human race would be better off in the long term if some people weren't allowed to breed at all. Genetically, we're not all created equal.
These are issues that ZPG proponents need to address, and not just at the surface level.