Urban areas are expected to grow by 80 percent by the end of the next decade, and unless they grow up rather than out, they could be in trouble, according to a new report from the World Resources Institute and Yale University.
But poor land records, rampant speculation and weak or corrupt implementation of regulations means that cities are using land inefficiently, increasing inequity and environmental risk as new residents take matters into their own hands, co-author Anjali Mahendra told the Thomson Reuters Foundation.
"We talk about flooding in Jakarta and Indian cities, but people don't tie any of this back to land use. But now there's enough evidence that all of this is occurring because of overdevelopment where services aren't available," said Mahendra, director of research at the institute's Ross Center for Sustainable Cities.
When cities grow at the behest of developers rather than guided by policy, she said, the result is development at the periphery.
That eats into agricultural or forest lands, and expands into areas that are far from jobs and largely unconnected to municipal services like water and public transit.
Access to services drops "sharply" just 5 kilometers (3 miles) from many city centers, according to the report.
For the study, the researchers used a first-ever technique to combine satellite images tracking how 499 of the largest cities in the developing world have expanded outward with radar data of how those cities have also grown upward.
The idea is that horizontal growth, while necessary, is far less efficient than vertical growth, which also ensures that new development takes place where city services are available.
Past research has typically looked only at outward growth, "ignoring inner-city redevelopment and changes in density," said co-author Karen Seto of the Yale School of Forestry and Environmental Studies.
The results show that among developing countries, vertical growth is almost completely confined to Chinese cities, while urban areas in Africa and South Asia are seeing very little.
That's a concern because some 90 percent of urbanization through 2030 is expected to take place in Africa and Asia, according to the report.
The international community has started to discuss these issues at the highest levels, and some cities do offer strong models for responding to these trends, particularly in Latin America, said Mahendra.
She pointed to moves by Mexico City and Johannesburg to offer development incentives only within certain predefined areas within the city, and Thailand's law allowing new development to occur only where city services exist.
"But that's very, very rare in the Global South, where it's still very much private developer-led, outside of the purview of land regulations -- if they even exist," Mahendra said, noting that any sense of urgency "really hasn't sunk in yet with lawmakers."