In a quarter-century, at the rate Nigeria is growing, 300 million people — a population about as big as that of the present-day United States — will live in a country roughly the size of Arizona, New Mexico and Nevada

Nigeria’s unemployment rate is nearly 50% for people in urban areas ages 15 to 24 — driving crime and discontent. In October 2011, the United Nations announced that the global population had breached seven billion and would expand rapidly for decades, taxing natural resources if countries cannot better manage the growth. Nearly all of the increase is in sub-Saharan Africa, where the population rise far outstrips economic expansion. Of the roughly 20 countries where women average more than five children, almost all are in the region. In Nigeria, experts say, the swelling ranks of unemployed youths with little hope have fed the growth of the radical Islamist group Boko Haram, which has bombed or burned more than a dozen churches and schools in 2012. Internationally, the African population boom means more illegal immigration, already at a high, according to Frontex, the European border agency. There are up to 400,000 illegal alien Africans in the United States. Sub-Saharan Africa, which now accounts for 12% of the world’s population, will account for more than a third by 2100, by many projections.