A seismic shift in demographic trends is transforming the world into an increasingly African place, creating huge economic opportunities, as well as new risks for political instability and extreme poverty, a United Nations agency says.

"The future of humanity is increasingly African," says a report by the UN children's agency to be issued on Tuesday, based on revised population forecasts that reveal an unprecedented demographic shift this century.

Africa accounted for only 9 per cent of the world's population in 1950, but by the end of this century about 40 per cent of all humans (and nearly half of all children) will be African, heralding one of the fastest and most radical demographic changes in history, the report says.

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While every other continent is seeing a slower rise in births, or even a decline, UNICEF projects that 1.8 billion babies will be born in Africa over the next 35 years, and the total African population will nearly quadruple to about 4.2 billion by the end of the century.

Africa could reap a massive demographic dividend from its bigger labour force and relatively fewer dependents, the report says. The population boom could "transform the continent, breaking centuries-old cycles of poverty and inequality."

But the opposite is also possible, and requires urgent discussion soon, UNICEF warns. "Unless investment in the continent's children is prioritized, the sheer burden of population expansion has the potential to undermine attempts to eradicate poverty through economic growth, and worse, could result in rising poverty and marginalization of many if growth were to falter."

The new projections are based on the latest revised numbers from the UN's population division, showing an even stronger shift in child demographics in Africa's direction.

Until recently, the UN had predicted that one-third of the world's children would be living in Africa by mid-century, yet that prediction is now believed to be an underestimate. Instead, 37 per cent of the world's children will be African by 2050, and more than 40 per cent of births will take place in Africa.

The population explosion will be biggest in West Africa, especially in Nigeria. By 2050, Nigeria alone will account for an astounding one-tenth of all births in the world, the report says, and its total population will reach nearly a billion by the end of the century.

Fertility rates are declining in Africa, but they remain far higher than anywhere else in the world, the report says. At the same time, life expectancy and child survival rates have drastically improved in Africa, helping explain the population boom.

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"Within 20 years, Africa will have its first generation of children who can expect to reach pensionable age," UNICEF says, predicting that African life expectancy will reach 65 years in the next two decades. (By contrast, in the 1950s, African life expectancy was less than 40.)

As part of these trends, Africa will become increasingly urbanized and crowded. In 1950, its population density was just eight persons per square kilometre. By mid-century, Africa will hold 80 people per square kilometre. Its megacities will soar in size, with the population of Lagos nearly doubling to 24 million by 2030 and Kinshasa growing from 12 million to 20 million in the same period.

Much of the population boom is occurring in the poorest and most fragile countries. The world's highest fertility rate is believed to be in the impoverished West African nation of Niger, where the average woman has 7.5 children, the UNICEF report says. The next-highest fertility rate is in a neighbouring country, Mali, where the average woman has 6.8 children.

The report calls for "courageous and determined action" to face the challenges of the African population boom. It cites, for example, the continued lack of contraception for many African women. About a quarter of all women in marriages or unions in sub-Saharan Africa lack the reproductive heath services they need, the report says. It also calls for stronger programs to improve the education of girls and to end child marriage.