Montana State study reveals detailed demographic information for world’s cities

BOZEMAN - The world's urban population increased by 785 million people between 2000 and 2020, but that tells only part of the story. In a first-of-its-kind study, a team led by geographers at Montana State University examined the demographics of individual cities to obtain insights into their populations that can't be discerned by numbers alone.

"Two cities can have the same number of people in them but different types of population structures - one could be very old, and one could be very young," said Andrew Zimmer, the primary author of "Global Divergence in Urban Demographic Change and Migration Patterns," which was published by the journal Nature Cities this week. "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 said 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. 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," Zimmer said.

Demographic information is also useful in assessing economic potential and political stability, Zimmer said. For example, large, working-age populations can drive productivity in cities, while aging populations can strain social welfare systems in other urban areas. Cities with large populations of younger people may experience social and economic pressures or even conflict related to employment, housing or governance.

Zimmer, now a geospatial scientist at Oak Ridge National Laboratory in Tennessee, led the study from 2023 to 2025 when he was a postdoctoral researcher in MSU's Department of Earth Sciences in the College of Letters and Science. Together with his adviser, MSU earth sciences assistant professor Cascade Tuholske, and researchers from the University of Michigan and University of Louisville, he analyzed available population data to identify migration trends and demographic changes in more than 10,000 cities worldwide from 2000 to 2020.

One of the data sources was WorldPop, 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 in MSU's Research Cyberinfrastructure core facility to process "tons of data across 20 years in about a week," Zimmer said. That computing effort was funded in part by MSU's Office of Research and Economic Development and the Geospatial Core Research Facility, a university-wide resource that provides advanced geospatial technologies, data analysis and expertise to support interdisciplinary research.

"This was our first geospatial project after Tempest was launched," said Tuholske. "The project couldn't have been completed without it."

Among their many findings, Zimmer and his co-authors determined that, from 2000 to 2020:

• Globally, 45% of urban population growth was attributable to migration.
• Smaller cities remained younger than larger cities, especially in Africa.
• 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.

Though all the world's cities were part of the study, the paper focuses mostly on the developing world.

"Developed countries changed more slowly during the study period. The populations in their cities don't change much, while cities in Africa and Asia, and to some extent Latin America, are rapidly expanding. Their structures are changing quite a lot," Zimmer said.

He noted that analyzing populations by city provides more useful information than looking at national averages.

"There are large differences between regions within countries, such as Nigeria, because fertility, migration, economic opportunity and social conditions vary across space," Zimmer said. "A national average can hide those differences."

Tuholske said the study is important because it provides a high-resolution understanding of urbanization.

"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."

Zimmer's and Tuholske's work was funded by an early career NASA Land-Cover and Land-Use Change grant, for which Tuholske is a co-investigator. Nina Brooks, an assistant professor at the University of Michigan, is the grant's principal investigator.