The pace of major city urbanisation is set to slow, but it will  still see one billion people move into cities as they expand over the next 25 years.

A new study by the Complexity Science Hub and ETH Zurich suggests that the world will have 450 million fewer people living in large cities by the end of the century than current trends indicate. The reason: the growth of large cities slows as countries urbanise.

The world is urbanising rapidly. In 1975, about 11% of the global population lived in cities with more than one million inhabitants.

“Today, we estimate that share to be about 24%,” said Andrea Musso, junior fellow at the Complexity Science Hub (CSH) and PhD student at ETH Zurich.

Projections suggest that over the next 25 years, one billion people will move into cities – roughly the equivalent of adding one New York City every two months.

“That pace matters,” added Frank Neffke, who leads the Transforming Economies research group at the Complexity Science Hub. “Decisions about infrastructure, housing, transport, energy, and climate adaptation all depend on where people will live.”

The study, estimates that by 2100, 38% of the world’s population will live in cities of more than one million people – substantially less than current trend projections suggest.

“Compared with simply extrapolating current trends, our model projects about 450 million fewer people living in million-plus cities by 2100 – a difference much larger than the current population of the United States,” explained Musso.

This gap in projections is driven by the discovery in the paper that urban systems follow a specific life cycle that determines how fast large cities grow relative to smaller cities in a country. Their study shows that in countries that are only beginning to urbanise, large cities grow much faster than smaller ones. People move to places where jobs, hospitals, universities, and other opportunities and amenities concentrate – typically the largest urban centres.

“We found that between 1975 and 2025, cities with more than one million inhabitants in less urbanized countries, including many countries in Asia and Africa, grew about 7.3% faster than the average city in their respective country,” said Musso.

But as a country urbanises further, this magnetic pull weakens.

In highly urbanised countries – including much of Europe and the Americas – cities with more than one million inhabitants grew at roughly the same rate as the national average over the past 50 years. Meaning that smaller and larger cities then grew at similar speeds.

Understanding how quickly cities grow – and particularly how large cities grow – is also important because urban expansion is associated with so-called super-linear effects.

“Studies of US cities show that people in a city of one million spend more than twice as much time in traffic and are nearly three times more likely to contract certain diseases compared to those in a town of 10,000,” Musso continued. “At the same time, residents of the million-person city are more than three times as innovative and almost twice as productive.”

“If moving to a large city leads to a doubling in productivity, then the rise of large cities can be a powerful driver of economic growth,” added Neffke, who is also a professor at IT:U Interdisciplinary Transformation University Austria. “Our findings show that this driver gradually loses momentum as countries become more urbanised.”

“Most earlier studies had to rely on administrative city boundaries,” explained Musso. “But these boundaries are often misleading. Paris, for example, is much larger than the administrative City of Paris. New York is not just Manhattan – or even the five boroughs.”

“We divided the world into small grid cells, each roughly one square kilometre in size, and then determined, based on population and built-up area, whether a cell was urban or rural,” says Musso. “We then linked clusters of urban cells over time and tracked how their populations changed.” This “harmonized way of defining city size is crucial to compare not just individual cities but entire urban systems both across space and in time,” Neffke added.

What surprised the team most was how orderly the pattern proved to be. Cities across different world regions may appear to follow very different trajectories. Yet once differences in urbanisation levels are taken into account, many of those contrasts largely disappear.

Viewed through this lens, countries are not so much following fundamentally different paths as moving through similar stages of development at different points in time.

“Knowing that urban growth follows a predictable life cycle is enormously important for decision-makers,” Neffke concluded. “It can help guide infrastructure planning, climate adaptation strategies, and forecasts of future economic growth.”