Urbanization
The rate of urbanization is currently about 1.3 million new city dwellers a week, 70 million a year, still apparently accelerating. The world was 3% urban in 1800, 14% urban in 1900, 50% urban in 2007, and probably headed in the next few decades to around 80% urban, which has been the stabilization point for developed countries since the mid-20th-century.
Almost all the rush to the cities is occurring in the developing world (though the countryside continues to empty out in developed nations). The developing world is where the greatest poverty is, and where the highest birthrates have driven world population past 6.5 billion.
Women liberated by the move to a city drop their birthrate right on through the replacement rate of 2.1 children/woman. As a result, there will be another billion or two people in the world total by midcentury, but then the total will head down.
“Cities — Global Population Shrinkage And Economic Growth,” by Stewart Brand