Seun Dupe sits with her newborn twins in a maternity hospital in Lagos, Nigeria. Nigeria is Africa's most populous country with more than 160 million people. UNICEF estimates that number will swell to nearly 1 billion by 2100. Sunday Alamba/AP hide caption

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London's financial district, known as the Square Mile. Will it be one of the first dominoes to fall when society can no longer sustain itself? Peter Macdiarmid/Getty Images hide caption

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The crowded scene on "Black Friday" at Macy's in Manhattan. The U.S. population is projected to hit 400 million in 2051, Census says, up from 321 million in 2015. Stan Honda /AFP/Getty Images hide caption

itoggle caption Stan Honda /AFP/Getty Images