Why Africa’s population explosion cuts both ways
By 2050, the number of people living in Africa will have doubled from the 1.25 billion today to more than 2.5 billion. And that can go one of two ways. If countries can put all these young people to work, economies can be transformed and millions lifted out of poverty. But if that opportunity isn't taken, migration could dramatically increase and radical extremist groups will have a pool of poor, angry, disaffected young people to swell their ranks.
So how big a deal is it? Which way is it going to go? And how can African countries cash in on the so-called "demographic dividend"?
As BBC Africa Correspondent, my team and I have spent months researching and travelling the continent to try and tell this story which will be broadcast over a week of special reports on BBC World News, BBC World Service and BBC.com.
We began exploring the challenges and opportunities facing the continent in Niger, where women have an average of 7.6 children each - the highest number in the world. In Nigeria, we saw the rise of the megacities and how planners can cope with massive urbanisation.
In Kenya, we witnessed the huge efforts being made to grow more - and better - crops, despite the extra challenge of climate change. Finally, in Ethiopia, we looked at the grand plan for industrialisation - to put hundreds of thousands of people to work and continue the upward trajectory of Africa's fastest growing economy.
Niger's changing traditions
One thing you notice in every village in the Zinder region of southern Niger is just how many children there are. The fertility rate here is even higher than the national average, and the government and aid agencies are together trying to change traditions and encourage people to have fewer kids.
On either side of Angoual Gao village, two meetings were taking place: girls from as young as ten sitting in a quiet corner of a covered compound, and men at "husband school" sitting under a tree. They were all talking about the same things - contraception, marrying young in a place where half the girls wed before they're 15, spacing the birth of children and thinking about why having so many children is so much part of life.
At a mobile clinic in a remote rural area women passed around condoms and IUDs and one even volunteered to have a three-year contraceptive implant injected into her arm in front of everybody. Habits and attitudes are changing, but only slowly -Niger's population will more than triple in size over the next 30 years.
And this will have global consequences, as the main migrant route to Europe is through Niger and Libya, and militant Islamist group Boko Haram recruits throughout this stretch of the Sahel.
Nigeria's rapid urbanisation
We travelled just over the border to northern Nigeria where it's a similar picture - people leaving the villages and heading to the town for the hope of work and a better life. Before 2050 Nigeria will overtake America to become the world's third largest country - as now, more than half the population will be under 25.
Nobody knows exactly how big Lagos is, but it's growing at an astonishing rate. There's an amazing energy surging through the city, but also a lot of poverty. We met a Los Angeles city planner who's come home to try and help transform the slums and saw the contrast between the poor water-front neighbourhoods and the fancy new developments, which now compete for land.
Transport networks are being retrofitted to cope with the influx of people from across West Africa, and some great lessons are being learned about how to keep mega-cities moving.
Agricultural innovation in Kenya
Like many parts of Africa, maize meal is the staple crop in Kenya - a colonial era introduction that doesn't cope well with the more frequent droughts that climate change is bringing. Pigeon peas, sorghum and millet are traditional crops, but they fell from favour, and now efforts are being made to bring them back through rural cookery classes.
Cross-breeding the best strains of Africa's top 101 crops is an ambitious project, but a new DNA sequencing machine in Nairobi is speeding things along. The yield of the average hectare of land could easily be increased by a factor of four, the experts tell us - and a reality TV show is trying to persuade the young to stay on the farm.
Ethiopia's industrial revolution
Creating jobs is the key, but the architect of Ethiopia's new industrial revolution, Arkebe Oqubay says 20 million jobs need to be created every year on the continent for economies to keep up with population growth. In Africa's biggest industrial park we met the international investors betting on Ethiopia as the next place after south-east Asia to build big textile factories to clothe the world.
Africa's population explosion is a huge opportunity - with strong leadership and investment, it could turn the continent around.