I have doubts about world population figures. Regions within Nigeria have incentives to inflate their numbers so that they get more seats in government. It's also seen as bad to have population declines, so governments over-report their population. Particularly with respect to central banks, they want to over-report the population because they want their per-capita printing to not look as bad.

Maybe we're already down to 6.5B or so. I'm not sure how you can verify such a big number like 8B.