More than half of the world's expected nine billion people will live in giant urban expanses by 2030 as cities and their hinterlands occupy an additional 1.2 million square kilometers, thereby tripling in size. That's an additional 1.35 billion people living in cities, suggesting that urban areas that currently occupy roughly 3 percent of the planet's surface will continue to expand. By comparison, urban areas increased by just 58,000 square kilometers between 1970 and 2000.
In new work published in Proceedings of the National Academy of Sciences, urban environment researcher Karen Seto of the Yale School of Forestry and Environmental Studies and her colleagues first divided the global land area into discrete parcels and, using predicted gross domestic product growth, population growth and urban land area cover in 2000, they projected which parcels had a high or low probability of succumbing to citification over the next few decades. Using that model, 1.2 million square kilometers of land have probabilities higher than 75 percent of becoming citified and nearly six million square kilometers have some probability of going urban.
Fifty-five percent of that expansion would come from massive urbanization in India and China—a trend that has been growing in recent decades. For example, a megalopolis similar to the urban corridor between Boston and Washington, D.C., in the U.S. is likely to form between Hangzhou and Shenyang in China. But the fastest urbanization is predicted to occur in newly developing regions in Africa, such as the coast of west Africa along the Gulf of Guinea and the shores of Lake Victoria farther south, encompassing Burundi, Kenya, Rwanda and Uganda, among other regions.
This may be bad news, in some cases, for the rich array of plants, animals and microscopic life that also inhabit Earth. The Eastern Afromontane, Guinean forests of west Africa and Western Ghats of India, along with Sri Lanka, are all home to such biodiversity—as well as projected to undergo rapid urban expansion that will encroach on the territory of already endangered amphibians, birds and mammals. Seto and her colleagues project the worst impacts of urban growth to occur in Central and South America.
In addition, such land use change is likely to result in even more of the greenhouse gas emissions that drive climate change, according to the new analysis. An estimated 1.38 billion metric tons of carbon could be released as forest transforms to roads, buildings and homes. As it stands, the world's cities bear responsibility for at least 70 percent of global carbon dioxide emissions. "We need to be more deliberate as a society about how we want urban places to become what they serve for humanity," Seto argues. "Too often, urban expansion is haphazard."
This is not just bad news for animals or the atmosphere, of course. Haphazard urbanization can also ill serve human inhabitants cut off from supplies of clean water or food. And the impact of urbanites is not confined to city limits; a typical Australian from Melbourne or Sydney requires greenhouse gas emissions, water diversion and land use from the entire continent. "Cities have always relied on their hinterlands and other distal places for resources from food and fuel to waste assimilation," the researchers wrote.