Fig 5. Future stationary population size depends on when fertility increases to replacement.

(a) displays global population size (left) and annual births (right) after the population has stabilized. TFR first falls toward the given level after 2100 and then rises to replacement at either 2125, 2150, or 2175. (b) displays population size over time if the long-run TFR before increasing to replacement level is 1.66, demonstrating how even a few decades change in timing of this hypothetical transition could alter long-run population sizes by billions.