In 2020, Dubai’s working-age adults outnumbered its dependents so dramatically that just 18 people under 15 or over 64 relied on every 100 working-age residents—the lowest dependency ratio on Earth. Meanwhile, in Mardadi, Niger, that number soared to 124 dependents for every 100 working adults, the highest in the world. These extremes, uncovered in a groundbreaking analysis of over 10,000 cities, reveal how national statistics can mask life-altering realities for urban populations. Led by researchers from the University of Michigan, Montana State University, and Oak Ridge National Laboratory, the study published in Nature Cities offers the most detailed portrait yet of how cities are growing, shifting, and straining under demographic change.
Between 2000 and 2020, the world’s urban population swelled by 785 million, but that growth was far from uniform. Forty-five percent of it came from migration, not births—a force that reshaped megacities like Dubai, where an influx of working-age laborers transformed the city’s social and economic fabric. Smaller cities, especially across Africa, grew more through natural increase and remained significantly younger. In Asia, Africa, and Latin America, urban demographics are evolving faster than in wealthier nations, challenging long-held assumptions. While countries like Japan and Germany are often said to be hurtling toward a "population cliff" due to aging, the city-level data shows many of their urban centers are still drawing younger workers. Conversely, fears of destabilizing "youth bulges" in Africa don’t hold up across all cities—some are aging, others stabilizing.
The study’s power lies in its precision. By combining satellite-derived urban boundaries with high-resolution demographic data from WorldPop—a project based at the University of Southampton—the team analyzed population trends in 1-square-kilometer pixels across the globe. They processed this massive dataset using the Tempest supercomputer at Montana State, enabling them to map not just how many people live in a city, but who they are: their age, sex, and how those demographics have shifted over two decades. One of the most striking patterns was the transformation of the Arabian Peninsula, where working-age sex ratios skewed heavily male due to labor migration, a trend invisible in national averages.
These insights are more than academic—they are vital for planning resilient cities. A high dependency ratio, like Mardadi’s, signals a need for schools, pediatric care, and youth employment programs. A low one, like Dubai’s, may mean booming economies but also strained housing and social services for transient workers. As climate change intensifies, knowing a city’s demographic vulnerabilities could determine how well it survives a heatwave or flood.
"Our results show that national averages obscure substantial differences between cities," said Nina Brooks of the University of Michigan, "and that globally consistent city-level demographic data can provide important insight for urban planning, climate adaptation and development." As urbanization accelerates, this granular view may be the best tool we have to build cities that truly serve the people within them.