Humans have been flocking to cities for centuries. Two hundred years ago, just 2% of the global population lived in urban areas; today, more than half the population does.
Experts expect this trend to hold throughout the 21st century. Although urban areas cover only a small fraction of habitable land, a novel urbanization model suggests that fraction could increase sixfold by 2100, which could have serious consequences for the health of the planet. Cities produce roughly three quarters of the world’s carbon emissions. But just because urbanization has been linked to climbing emissions in the past doesn’t mean it needs to be going forward, experts say.
“There is a way to continue to have prosperous urban centers while keeping emissions down,” said Nadine Ibrahim, the Turkstra Chair in Urban Engineering at the University of Waterloo in Canada. “The key thing is to eliminate a fossil fuel–based economy.”
Indeed, the new model of urbanization found that a fossil fuel–dependent world will convert 3 times more land for urban use by 2100 than one that chooses a more sustainable development path.
A Long-Lasting Mark on the Planet
“The decisions we make around cities are so important because they will far outlive us,” said Ibrahim, who was not involved in the new study. Urban areas are where humans tend to have the most direct and long-lasting effects on the environment. When cities expand into natural habitats, they can threaten biodiversity, alter water cycles, and influence atmospheric dynamics; once land is covered with human-made structures, it rarely reverts back to a natural landscape.
In turn, urban areas themselves are shaped by environmental stresses. Where a city is located determines which risks the population will face, from sea level rise to extreme precipitation to heat stress. The new study’s findings raise several questions about what future urbanization means for the environment and for human society.
“It’s hard to think about either one of those unless you can organize your knowledge in a fairly detailed spatial framework,” said Mark Montgomery, a professor of economics at Stony Brook University in New York who was not involved in the study. “If the area that we call urban is expanding in the way this paper suggests, what are the effects on emissions, biodiversity, heat itself? Who is going to be exposed to flooding? All of those things can come into the picture once you’ve got the basic spatial framework underneath that these other kinds of information can be overlaid upon.”
Capturing the Evolution of Urbanization
To create that spatial framework, the authors began by creating forecasts of new urban land development with country-level data under five different socioeconomic scenarios (known as the Shared Socioeconomic Pathways): sustainable, middle of the road, regional rivalry, inequality, and fossil-fueled development. The model categorized countries as either rapidly urbanizing, steadily urbanizing, or urbanized.
But a broad view of urbanization obscures the complexity on the ground: The processes driving urban growth vary between cities and over time. “Urban studies have been more local because the process is actually quite different from place to place, even within countries,” says Jing Gao, an assistant professor of geography and spatial science at the University of Delaware and lead author on the new study. “With natural systems, once you discover the physical rules, [the rules] hold over time, but society changes.”
To account for this variation, Gao and her coauthor Brian O’Neill, a professor at the University of Denver, created a two-tiered model. The team divided the globe into 375 regions and modeled urban land development decade by decade on the basis of the local processes involved in urban growth. The result was a model that can link regionally specific urbanization processes with country-level socioeconomic projections that many scientists and policy makers use to better understand what kind of action we need to take to prevent and prepare for climate change.
A Model of Urbanization for a New Century
The results support previous research predicting that urbanization will grow throughout the 21st century. Over time, most countries shifted toward a more urbanized state. But the amount of urban land change—how much of the natural world is supplanted by asphalt, cement, steel, and other human-made materials—depended greatly on whether or not we can wean ourselves off fossil fuels. If the world follows a sustainable pathway, urban land will roughly double by 2100. If it follows the business-as-usual pathway, urban land will grow by 1.6 million square kilometers—a sixfold increase over today.
The new study was published last month in Nature Communications.
Michail Fragkias, an associate professor of economics at Boise State University in Idaho who was not involved in the study, cautions that the research doesn’t yet account for economic and environmental shocks (including a pandemic and climate change) or our response to those shocks.
“We rely a lot on a perspective of urbanization that was formulated in the last century [and] does not take into account a global environmental change as a fundamental driver of urbanization. We have been looking extensively at one of the directions of the connection—how urbanization affects the global environment, either globally through emissions or locally through land use change,” he said. “We haven’t been looking at the reverse, of how global environmental change comes back to affect urbanization.”
Gao hopes that other researchers will use the framework they’ve developed here to begin to investigate those questions. “This isn’t fortune telling. Our objective is not to tell people exactly how it will be,” Gao said, but rather how it might be, depending on decisions we make. “We hope people can use this in impact analyses of alternative policy choices, to orient society toward a favorable outcome.”
These findings may be most relevant for developing countries, which are urbanizing at a faster rate than the developed world. In Africa, a continent that’s expected to add some 2 billion people by 2100, urban land expanded by 193,000 square kilometers in the middle-of-the-road scenario.
“The African continent has a huge advantage,” Ibrahim said. The places that are just beginning to urbanize now don’t have to build or grow on top of infrastructure that was built with fossil fuels in mind. “They can just skip right to low carbon options,” said Ibrahim. “They exist commercially, and they’re more affordable than they used to be a couple decades ago, if they existed at all.”
But developing countries weren’t the only hot spots of urbanization. Developed countries were projected to add the same overall amount of new urban land as developing countries. Europe, for example, added 275,000 square kilometers under the same scenario.
Existing metropolises don’t have the ability to start from scratch, but they can retrofit in ways that help cut down on wasted resources: electrifying public transportation, upgrading leaky water or gas pipes, and building skyward to pack more people into urban spaces as our population grows. The global population could top 10 billion by 2100.
“The solution to population is densification,” Ibrahim says. In a 2015 survey of resource use in the globe’s 27 megacities—those with more than 10 million people—Ibrahim and her colleagues found that denser cities make public transit more accessible, combat urban sprawl, and use water and energy more efficiently. But they also tend to perform worse in terms of waste management and gasoline use.
“We have a whole suite of low-carbon options to choose from for our current activities,” said Ibrahim. Many cities in North America and Europe have been relying less and less on coal-fired power and more often on renewables. “But that won’t get us there, in terms of all our climate targets,” Ibrahim said. “To get us there, we have to do more than has been done, even tapping into technologies that have not been invented yet.”
—Kate Wheeling (@katewheeling), Science Writer