How overpopulated is the planet, really?

[GliderGuider]

12. Nope.

Everyone gets to decide for themselves what their final disposition may be. Nature will decide for those who don't.

Regarding higher populations and the assumptions:

Ever since agriculture began, we've been strip-mining the soil. That's where the extra energy to boost the population - as well as the initial planetary damage - came from. If we'd stuck to doing it with oxen and sticks, it might have been tens of thousands of years before the negative effects became visible. But we didn't.

I use a very strict definition of sustainability - something like, "The ability of a species to survive in perpetuity without damaging the planetary ecosystem in the process." This principle applies to a species' own actions, but not to external forces like Milankovich cycles, asteroid impacts, plate tectonics, etc. In fact, in order to completely fulfill this definition, even my numbers could be too high by up to an order of magnitude.