[astrodust]

This back-of-the-envelope calculation appears to be off by a factor of two, which isn't bad.

http://en.wikipedia.org/wiki/World_population#Number_of_huma...

[robbles]

this error is because of assumptions about the life expectancy, right?

[jonathanbgood]

Yes, their birth rate assumptions would only yield the population milestones they suggest if the life expectancy was significantly below the reported level.