"Over time, there can be supply constraints, they can run out of labor, their costs can become too high, or there can be other, competitive problems, like congestion or pollution," said Harrison. "But getting data on that across different cities or a uniform global data set on that—it doesn't exist."
Still, local governments, businesses, urban planners and policymakers are scrambling to figure out how to manage the looming surge in growth. While the evolution of emerging megacities is unpredictable and complex, there are some common attributes that economically successful cities share, such as economic strength, human capital, financial maturity and global appeal.
Urban "agglomerations"—the sprawling population concentrations that spill over beyond political boundaries—are more difficult to manage when multiple, regional governments are involved with no clear overall authority.
That's one reason the most competitive cities will be those with strong institutions—from government to businesses to cultural groups—that can help foster growth and address problems that arise, according to the authors of the Citibank report. "A city's ability to tax, plan, legislate and enforce laws and its willingness to be held accountable by its citizens requires strong institutions," they said.
Nowhere is that more evident in the developing world than in China, where a historic migration set in motion three decades ago is expected to add dozens of new megacities over the next 30 years.
"China is in a unique position in that it has a strong enough government that it can just do what needs to be done," said Harrison. "It's good at planning. If it sees a problem, it will solve it."
But even in China, new arrivals from rural areas don't pay attention to political lines drawn on a map or follow the wishes of city planners. And efforts to keep them back on the farm—from China's hukou household registration system to investments in upgrading rural services—hasn't stopped the ongoing wave of urban immigration.
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