Notice that these are estimates. The Journal reports:
The biggest human increase in modern history is under way in Africa. On every other continent, growth rates are slowing toward a standstill for the first time in centuries, and the day is in sight when the world’s human population levels out.
But not here — not yet.
Some 2.5 billion people will be African by 2050, the U.N. projects. That would be double the current number and 25% of the world’s total. There will be 399 million Nigerians then, more than Americans. When the century closes, if projections hold, four out of 10 people will be African.
Billions of them will be living in cities that are today small towns. The land of open spaces that was Africa will have blended into one big megalopolitan web.