I must admit my simple calculation was based on compound interest, where the principal doesn't "die off", so the figure of 140 years might not be accurate
I'm assuming the percentage shown in the graph is based on living population, that is, it takes into account deaths as well.
To reduce the population, the birth rate would obviously have to be lower than the death rate.
It's an interesting one - but if all births stopped for two generations, the human race would probably be doomed to a slow extinction unless we figured out how to clone ourselves...
