Note: This post originally appeared at Ensia.

The 1960 street map of Lagos, Nigeria, shows a small coastal city surrounded by a few semi-rural African villages. Paved roads quickly turn to dirt, and fields to forest. There are few buildings over six floors high and not many cars.

It’s likely no one foresaw what would happen between 1960 and today. In just two generations Lagos grew 100-fold, from fewer than 200,000 people to nearly 20 million. Today it is one of the world’s largest cities, sprawling over nearly 1,000 square kilometers (400 square miles). Vastly wealthy in parts, it is largely chaotic and impoverished. Most residents live in informal settlements or slums. The great majority of them are not connected to piped water or sanitation. The city’s streets are choked with traffic, its air is full of fumes, and it produces more than 10,000 metric tons (11,000 tons) of waste a day.

But new research from Canadian academics, though replete with caveats, suggests that the changes Lagos has seen in the last 60 years may be nothing to what may take place in the next 60 years. If Nigeria’s population continues to grow and people move to cities at the same rate as they are moving today, Lagos could be the largest metropolis the world has ever known, home to up to 100 million people. By 2100 it is projected to have more people than California or Britain today, and to stretch hundreds of miles with enormous environmental effects.

Hundreds of far smaller cities across Asia and Africa also could 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.

A projected decline for Europe and the Americas

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 in 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% 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.

Rapid growth can bring problems

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.

“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.

While many economists argue that population growth is needed to create wealth and that urbanization reduces environmental impact, others fear cities are becoming ungovernable and too unwieldy to adapt fast enough to rising temperatures and sea levels, pollution, water shortages, and ill health of inhabitants.

Will mega-cities be part of the problem — or the solution? What can they do to maximize the benefits of urbanization while minimizing the downsides? This look at seven cities on five continents, each at a different stage of development, can shed valuable light on what it might take to do it right.

John Vidal was environment editor of The Guardian for 27 years. He is based mainly in London and has reported on climate change and international environmental issues from more than 100 countries. He is the author of McDonald’s, Burger Culture on Trial.