Hundreds of far smaller cities across Asia and Africa could also grow exponentially, say the Canadian demographers Daniel Hoornweg and Kevin Pope at the Ontario Institute of Technology. They suggest that Niamey, the barely known capital of Niger — a west African country with the highest birth rate in the world — could explode from a city of around 1 million people today to be the world’s seventh largest city with 56 million people in 2100. Sleepy Blantyre in southern Malawi could mushroom to the size of New York City today.

Under the researchers’ extreme scenario, where fertility rates remain high and urbanization continues apace, within 35 years over 100 world cities will have populations larger than 5.5 million people. By 2100, say the authors, the world’s population centers will have shifted to Asia and Africa with only 14 of the 101 largest cities then in Europe or the Americas.

Cities in the Global South are growing much faster than they did in industrialized countries 100 or more years ago for several reasons. Fewer children now die young. Migration from rural areas is speeding up because there’s less new farmland to be opened up. Many countries encourage urbanization to grow their economies. As a result, cities today are being challenged to keep pace with the largest wave of urban growth in history and the need to provide water, sanitation and power to all those people.

It’s impossible to know how cities will grow, but the stark fact, according to the United Nations, is that humanity is young, fertile and increasingly urban. The median age of Nigeria is just 18, and of all Africa’s 54 countries is under 20. The fertility rate of the continent’s 500 million women is 4.4 births; meanwhile, 50 percent of India’s population is under age 25, and Latin America’s average age is just 29.

Recent U.N. projections expect the world’s population to grow by 2.9 billion — nearly that of China and India today — in the next 33 years and possibly by a further 3 billion by the end of the century. By then, says the U.N., humanity is expected to have developed into an almost exclusively urban species with 80 to 90 percent of people living in urban areas.

The Canadian researchers observe that urbanization can have environmental benefits in the form of economies of scale; that it is associated with better access to education and health care; and that it can boost the economy. But they also note that cities can become more vulnerable as they grow. And, says Robert Muggah, research director of the Igarapé Institute in Rio de Janeiro, a global think tank, urbanization can rapidly outpace society’s capacity to accommodate it.

“Cities are generating a challenge on a level never seen before.” — Robert Muggah

“Many fast-growing cities urbanize before they industrialize,” Muggah says. “It took centuries for cities in the Global North to do this. In the south we are seeing a doubling of population in 25 years. Most cities [today] don’t have the infrastructure, employment base or productivity to manage this growth. You see a maze of informal settlements, completely overloaded infrastructure. Cities are generating a challenge on a resource level never seen before. The fastest growing cities may be hit the hardest.”

Whether the world’s major cities develop into endless, chaotic slums, with unbreathable air, uncontrolled emissions, and impoverished populations starved of food and water, or become truly sustainable depends on economies, technology and how they respond to population growth and environmental risk.