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 urbanize.
IN SHORT:
- An estimated 38% of the world's population will are projected to live in cities of over one million by 2100 – 450 million fewer than current trends suggest
- In the early stages of a country's urbanization, large cities grow much faster than small ones – but in highly urbanized countries cities grow on average at almost the same rate, regardless of their size
- The researchers built two major new datasets based on satellite and census data, using geographic rather than administrative city boundaries
The world is urbanizing fast. 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%," says 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," says 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 new study, published in PNAS, now 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," says Musso.
KEY DRIVER: URBAN GROWTH SLOWS DOWN
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 urbanize, 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 centers.
"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," says Musso.
But as a country urbanizes further, this magnetic pull weakens.
In highly urbanized 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.
THE COSTS AND BENEFITS OF BIG CITIES
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," says Musso. "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," says 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 urbanized."
GEOGRAPHIC, NOT ADMINISTRATIVE, CITY BOUNDARIES
To study urban growth consistently across countries and over time, the researchers used satellite and micro-level census data.
"Most earlier studies had to rely on administrative city boundaries," explains 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." An interactive data story by Musso illustrates how a single city can take on very different shapes depending on which boundary is used: https://megacities.ch/
Because administrative definitions vary widely across regions and countries, the team decided to build a new city dataset from the ground up, resulting in two major new datasets.
The first covers 99 countries worldwide – representing about 94% of the global population in 2025 – and spans the period from 1975 to 2025. Using satellite imagery, it tracks cities as they physically expand across the landscape.
The second reconstructs the history of American cities from 1850 to 2020. It draws on more than 500 million individual census records and matches them to roughly 40,000 historical places.
"We divided the world into small grid cells, each roughly one square kilometer 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 states.
A PREDICTABLE URBAN LIFE CYCLE
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 urbanization 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 states. "It can help guide infrastructure planning, climate adaptation strategies, and forecasts of future economic growth."
ABOUT THE STUDY
The study " Large cities lose their growth advantage as countries urbanize " by A. Musso, D. Rybski, D. Helbing, and F. Neffke was published in PNAS (doi: 10.1073/pnas.2529430123).
"Are we all going to live in megacities?" – an interactive data story by Andrea Musso visualizing the study's data and findings: https://megacities.ch/
ABOUT THE COMPLEXITY SCIENCE HUB
The Complexity Science Hub (CSH) is Europe's research center for the study of complex systems. Drawing on large-scale data across economics, medicine, ecology, and the social sciences, CSH develops quantitative methods to understand the interconnected networks that underlie society – from financial markets and supply chains to public health and urban development. The goal is to provide a rigorous basis for navigating the challenges of an increasingly complex world.
Members of the Complexity Science Hub are: AIT Austrian Institute of Technology, BOKU University, Central European University (CEU), IT:U Interdisciplinary Transformation University Austria, Medical University Vienna, TU Wien, TU Graz, University for Continuing Education Krems, Vetmeduni, WU Vienna and WKO.
