An analysis of more than 10,000 cities worldwide reveals facts, figures and trends that governments can use to better to support their populations that would otherwise be obscured in national averages
Study: Global divergence in urban demographic change and migration patterns (DOI: 10.1038/s44284-026-00447-7)
The world's urban population increased by 785 million people between 2000 and 2020, but that tells only part of the story. Now, a research team including an expert from the University of Michigan has dug into the demographics of more than 10,000 individual cities to obtain insights that can be lost in the aggregate.
"Our results show that national averages obscure substantial differences between cities and that globally consistent city-level demographic data can provide important insight for urban planning, climate adaptation and development," said Nina Brooks, an author of the new study and assistant professor at the U-M School for Environment and Sustainability, or SEAS.
The team's analysis, supported in part by NASA, revealed that, from 2000 to 2020:
- Globally, 45% of urban population growth was attributable to migration.
- Migration drove growth primarily in megacities, whereas natural population dynamics-births and deaths-were the dominant factors in smaller cities.
- Smaller cities remained younger than larger cities, especially in Africa.
- Developing cities, especially in Asia, Africa and Latin America, are changing more quickly than developed cities.
- Urban dependency ratios-the number of children and older adults relative to the working-age population in cities-generally declined over the study period because working-age adults moved from rural to urban areas.
For example, Dubai in the United Arab Emirates has a population of more than 2 million and a huge working age demographic. Its dependency ratio dropped from 0.33 in 2000 to 0.18, the lowest in the world, in 2020. In Mardadi, Niger, however-a city where a high percentage of its 330,615 inhabitants are younger than working age-the dependency ratio grew from 1.15 in 2000 to 1.24, the world's highest in 2020.
Such demographic insights can help governments and agencies evaluate a city's strengths and vulnerabilities, as well as determine how to allocate resources to best serve citizens, the team said. A heat wave, for example, could have a very different impact on a city with lots of very young and elderly people.
"My work is focused on the human lens-these things matter because of people," said Andrew Zimmer, lead author of the new analysis in the journal Nature Cities. "This is one of the first efforts to map demographic details to learn more information about who lives in these cities and how that's changed over time."
Zimmer led the work as a postdoctoral researcher at Montana State University and is now a geospatial scientist at Oak Ridge National Laboratory in Tennessee.
Another interesting finding of this more granular analysis, Brooks said, was how individual cities could often counter the popular narratives around population at the national level. For example, in higher-income countries in parts of Europe and East Asia, national trends show an aging population and there are prevalent concerns about countries falling off a "population cliff."
Elsewhere, worries and fears are being stoked around "youth bulges," especially in African countries, Brooks said. There, people are saying there will be insufficient jobs to support large young populations, which could lead to political unrest and economic instability.
"But when you look across the cities where people are living, you see that's just not the case uniformly," Brooks said. "So I think this study has highlighted how important it is to drill down a little more and get a more nuanced understanding of those dynamics."
To enable the analysis, the team had to meld multiple datasets to get comprehensive data not just on demographics, but also the spatial extent of cities. One of the data sources was WorldPop, produced by a research group at the University of Southampton in England.
WorldPop provided what Zimmer calls "pixelated" demographic data that looks at 1-square-kilometer areas-or "pixels"-of populated areas around the globe. Data from these pixels, such as year, age and sex, were combined with urban boundary data to yield information about population change and structure for every city on Earth.
To blend the datasets, the research team used the Tempest supercomputer at Montana State's Research Cyberinfrastructure core facility.
"This was our first geospatial project after Tempest was launched," said study co-author Cascade Tuholske, assistant professor at Montana State. "The project couldn't have been completed without it."
And while the study is a technical achievement, Tuholske, too, emphasized that the massive datasets and computational power are ultimately used in service of people around the planet.
"Urbanization is connected to economic growth, to conflict, to environmental sustainability and to changes in agricultural systems," he said. "It aligns many aspects of human dynamics across the planet."
Written by Diana Setterberg, Montana State University News Service
