7 Billion Humans | National Geographic Magazine: Dharavi Slums | Largest Slums in the World | Mumbai, India

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Dharavi Slums | Largest Slums in the World | Mumbai, India

The world’s population will hit seven billion on Oct. 31. Or maybe not until next year. Or maybe it has already happened… That lack of calendar clarity highlights how much remains unknown about the world’s population today—which makes projecting future population even tougher. Both the U.N. and the Census Bureau build their figures using a mix of national census data, population surveys in developing countries and demographic expertise… But not every nation carries out a census, and the U.N. says it hasn’t been able to use census data more recent than 2005 for countries that contain 62% of the world’s population. It has no data, census or otherwise, for 25% of the world’s people since 2005.

 —Carl Bialik, The Wall Street Journal, October 22, 2011