The world, we’ve been told, is urbanizing rapidly. But the picture painted by researchers—dense, blocks of high-rises, new towers rising like staircases as cities evolve—obscures the true growth patterns reshaping the globe. Metropolises aren’t simply building up, they’re expanding out, and doing both at unprecedented rates that have serious consequences for the economy and the environment.

“Upward and Outward Growth: Managing Urban Expansion for More Equitable Cities in the Global South,” released yesterday by the World Resource Institute, paints a picture of cities building and ballooning. Researchers examined growth patterns for nearly 500 urban areas, using satellite imagery and radar data to measure growth vertically and horizontally. The results suggest land-use and density may be one of the defining issues of the next few decades.

“Some expansion of cities is inevitable to accommodate urban population growth,” said Anjali Mahendra, co-author of the new paper. “But the type of unmanaged, incessant growth in land area we are seeing not only leads to inequities in access to core urban services, like water, power and sewage, but also to greater economic and environmental risk for the entire city.”

Cities will have tripled in size between 2000 and 2030, and are on track to reach a projection of 80 percent growth in the next 18 years. The need for smart, dense, and sustainable growth is paramount, the report argues. Otherwise, the current state of unmanaged growth in land area will not only creates more inequality, but add to economic and environmental risk.

Many researchers and think tanks have argued that cities can save the environment; groups such as C40, a network of cities collaborating on environmental policies, have released studies showing that cities can combine growth with ambitious efforts to decrease carbon emissions.

“Upward and Outward Growth” suggests that efforts by cities like Paris or London to cut emissions can make a difference. But it also notes that significant action is required to steer development in the global south—which is threatening agricultural lands, biodiversity hotspots, or important wetlands and urban flood plains—in a more sustainable direction.

Recent United Nations forecasts suggest the urban population in developing areas will double between 2010 and 2050, from 3.6 billion to 6.7 billion, with a third of this growth taking place in China, India, and Nigeria.

“We talk about flooding in Jakarta and Indian cities, but people don’t tie any of this back to land use,” Mahendra told Reuters. “But now there’s enough evidence that all of this is occurring because of overdevelopment where services aren’t available.”

The challenge of expanding cities

According to the report, many cities in the global south have seen their populations surge, leading to growth that’s rapid and unplanned. Mahendra says this is development at the behest of developers, rather than by smart policy.

The effect of this unregulated growth is distorted land markets, people getting priced out, and significant spikes in informal settlements and areas without access to core transit and infrastructure services.

The paper’s authors found that for most city dwellers, access to services like water, sewage, and paved roads drop sharply just three miles outside of city centers, leading to increased costs and less sustainable practices. Roughly 25 to 70 percent of urban dwellers in the global south use informal arrangements to procure these core services. Researchers found that while housing on the edges of the city can often be 40 percent cheaper than living in a city center, households ended up paying double for transportation.

This unregulated construction, especially in urban flood plains or near bodies of water, can multiply the damage of flooding, monsoon, hurricanes, and other weather patterns exacerbated by climate change.

Cities with higher incomes, especially in China, have seen significant upward growth, with new high-rises and towers. But, just like any other city, this expansion has driven up land values, pushing lower-income residents out toward the periphery.

Reshaping urban expansion

When even many U.S. cities, with their considerable resources, have struggled with the side-effects of sprawl, can developing metropolises cope with significant spikes in population and size?

Mahendra says cities like Mexico City and Johannesburg have already shown leadership on this front. And some cities in the global south are helping subsidize affordable housing, as long as it’s being constructed in areas with access to infrastructure and employment. But, for the most part, private, developer-led growth is still the rule rather than the exception.

The answer, says the report, is smart growth and densification, which includes more robust property tax systems to channel investment, and the introduction of development-impact fees and air rights for multistory construction to help regulate sprawl.