As you look down from the new bypass road winding its way through green hills on the south-western edge of Nairobi, the roofs of Kibera slum resemble thousands of bargesjostlingbow-to-anchor in a busy colonial-era harbour, with traders nipping in and out of sight. The shacks are packed so tight that tin coverings almost touch each other above the alleys, obscuring the dense flow of people below. Hundreds of them are squeezed under each long sheet of tin. Schools are filled to the brim. The surrounding hills may not be green for much longer. Nairobi is eating up land. A new study by Unicef, the un children's agency, points out that, by 2100, on current rates, almost half the children un- der 18 in the world will be African. At the moment, the share is only a quarter. This would be one of the most dramatic demographic shifts in history. By the end of the century, if current demographic patterns continue for another 85 years (which they may not), Africa would have 4.2 billion people, against 1.1 billion today. Nigeria, whose land mass is similar to Pakistan's or Venezuela's, would rise from 180m today to 910m, registering one in 12 of the world's births.
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