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 barges jostling bow-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 under 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.

“The future of humanity is increasingly African,” says Unicef’s report, which shows a “massive shift in the world’s child population towards Africa”. The number of Africans under 18 may swell by two-thirds, to reach almost a billion by 2050, even if child-mortality rates remain relatively high. The new figures assume a reduction in fertility rates over time, as prosperity increases.

Yet Africa seems unusual in that economic growth during the past decade has not cut fertility as much as it has done elsewhere. Fertility rates in some African countries have stalled, instead of falling continuously, as happened throughout East Asia and Latin America.

The Unicef report is based on recent research that suggests that Africa is facing a population boom, nearly quadrupling it by the end of the century. The Population Research Bureau, a Washington-based outfit, said last year that Africa’s population would more than double to 2.4 billion by 2050, with the sub-Saharan part of it, currently 900m-plus, swelling to 2.2 billion. This is because Africa’s total fertility rate—the number is children a woman can expect to have in her lifetime—is 4.7. The figure in America is 2.0; in East Asia 1.7.

The prospect of so rapid a rise has triggered a host of Malthusian fears, on the continent and elsewhere. If Africa is struggling to feed a billion people, it is hard to see how it could feed 4 billion in future. Though the past decade has been relatively peaceful, perhaps a population boom will fuel civil strife. And surely, some fear, the expanding masses will destroy what is left of Africa’s flora and fauna, wiping out endangered species and pouring concrete on primal forest and grassland.