While a city is not a living organism, it behaves very much like one. Its metabolic processes may be manifested in growth spurts, metamorphosis over time and even decay. Researchers using satellite imagery have tracked the vital signs of six major global cities, detecting a distinctive "urban pulse" in each.
The researchers looked at Dubai, Lagos, Mexico City, Mumbai, Seattle and Shenzhen using a new way to document dynamic changes unfolding in each of these cities in near real-time.
Historically, experts have relied upon aggregated and infrequent data to document urbanization, such as a yearly census, annual economic figures or a map showing how a city's footprint has changed over a decade - essentially using specific outcomes as metrics. But the scientists behind the new study said such an approach provides an incomplete understanding of a city and can miss the nuances as a metropolis evolves, Reuters reported.
"We got the inspiration from the human pulse, which tells us different information about our health than weight or height," said study lead author Zhe Zhu, a professor of remote sensing and director of the Global Environmental Remote Sensing Laboratory at the University of Connecticut's Department of Natural Resources and the Environment.
"The urban pulse measures the high-frequency process of development, and therefore we can spot early warning signs of economic stress or stagnation before they become full-blown crises," Zhu said. "We compare traditional metrics to looking at a heart attack - the outcome - whereas the 'urban pulse' is like monitoring the daily lifestyle and vital signs leading up to that heart attack - the process."
The biggest takeaway from the study, published in the journal Proceedings of the National Academy of Sciences, is that urbanization is not smooth and steady, the researchers said.
"Urbanization is actually 'spiky,' meaning that it happens in abrupt, intense bursts, or 'cyclical,' moving through boom-and-rest phases that don't match annual seasons, or 'asynchronous,' as different neighborhoods in the exact same city develop at completely different, uncoordinated times," said study senior author Karen Seto, a Yale University professor of geography and urbanization science.
"This is important because, for decades, researchers have characterized cities through static maps," Seto said.
CITIES WITH DIFFERENT CONDITIONS
The researchers used dense and high-frequency satellite imagery from the US space agency NASA's Landsat and the European Space Agency's Sentinel-2 satellites. They tracked physical changes in the cities such as new building construction, demolition, major infrastructure improvements and expansion into green spaces.
"We selected cities with a wide range of political-economic conditions including the state-led development of Shenzhen, the market-driven growth of Seattle, the informal expansion of Lagos and the megaprojects of Dubai," Zhu said.
Shenzhen, formerly a small fishing village near Hong Kong that has become a megacity, exhibited the highest magnitude and intensity of growth, characterized by massive and clustered spikes reflecting rapid, state-led mobilization of capital.
Dubai, the most populous city in the United Arab Emirates, also showed huge growth.
In Nigeria's largest city Lagos, the "pulse" was highly fragmented, with long periods of inactivity punctuated by brief and intense surges.
Seattle, the largest metropolis in the US Pacific Northwest, reflected a market-driven pulse of redevelopment and densification.
Mumbai, India's financial and commercial powerhouse, and Mexico City, North America's most populous city, proved to be highly resilient and showed less disruption during global shocks such as the COVID-19 pandemic than the others.
"Just as a human pulse reacts to illness, our data captured the exact moment COVID-19 triggered a synchronized 'cardiac arrest' in development worldwide. But the recovery was entirely unequal," Zhu said.
"Shenzhen saw a sharp, coordinated dip followed by a rapid rebound. Lagos experienced a muted pulse that transitioned into smaller, incremental changes. Meanwhile, cities like Mumbai and Mexico City showed much less of an impact. It showed us that global shocks don't manifest the exact same way in every city's 'body,'" Zhu said.
The researchers see practical applications for their method.
"For urban planners and policymakers, it functions as a diagnostic tool. Instead of reacting to a crisis after the fact, they can see exactly when and where a neighborhood's 'pulse' is slowing down and intervene early to prevent infrastructure collapse or economic decay. It also prevents cities from overheating their labor and material markets," Seto said.
