Comment Re:Wow... (Score 1) 473
The medium-variant projection for 2050 is more certain than for 2100 because people who will be 40 years and older in 2050 are already born. According to the medium variant, it will take 13 years to add the eighth billion, 18 years to add the ninth billion and 40 years to reach the tenth billion. According to the high variant, an additional billion would be added every 10 or 11 years for the rest of this century.
Original source here: http://esa.un.org/unpd/wpp/Other-Information/Press_Release_WPP2010.pdf More data here: http://esa.un.org/unpd/wpp/P-WPP/htm/PWPP_Total-Population.htm