How cities around the world will sprawl in the next 20 years

September 19, 2012

In China last year, roughly 51 percent of the population lived in urban areas. In Mexico and Turkey, that figure hovered in the mid-70s. That sounds like a lot of people—until you realize that rate of urbanization is still well below that of the United States, where about 82 percent of the people live in urban areas. Cities in the developing world still have a lot of room for growth.


(ZHOU JUNXIANG - IMAGINECHINA)

A new paper in the Proceedings of the National Academy of Sciences tries to figure out what, exactly, will happen as these countries become increasingly urbanized between now and 2030. The amount of total urban land area will swell by 185 percent—an area the size of South Africa. And that could have a fairly sizable impact on both the environment and the Earth's climate.

First, here's a map showing how urban areas in three select countries—China*, Mexico, and Turkey—are expected to swell by 2030. The study incorporated 1,000 different urban growth models and expressed the results in terms of probabilities. So the potential amount of expansion in Mexico is relatively small, but it's very likely to happen (the red spots). With Turkey, there's a lot more uncertainty about where cities will sprout up and swell (that's the sea of yellow):

If cities around the world grow as they're supposed to by 2030, the total amount of land taken up by urban areas will nearly double, adding an additional 1.2 million square kilometers.

Nearly half of that growth will occur in Asia—mostly in China and India. But Africa is expected to see a large boom as well, particularly in five key areas: the Nile River in Egypt; the coast of West Africa; the northern shores of Lake Victoria; the northern regions of Nigeria; and Addis Ababa in Ethiopia.

This rapid growth, the authors point out, could have significant environmental impacts. As cities expand and sprawl, they tend to disrupt natural habitats, reduce local biodiversity and release the carbon stored in vegetation and soils that are cleared. Right now, land use changes account for around 6 percent to 17 percent of man-made carbon emissions and that's expected to swell in the next two decades. The PNAS study estimates that emissions from the coming expansion in tropical regions alone will likely amount to around 5 percent of that for tropical deforestation—not a major factor, but not negligible, either. Similarly, hundreds of endangered species will likely see their habitats destroyed unless care is paid.

This largely counts as progress—an urbanized world is likely to be a wealthier world. But the authors, Karen Seto, Burak Güneralp and Lucy Hutyra, point out that it's worth getting a handle on the potential side effects now, before trillions of dollars worth of concrete gets paved. "Recent estimates suggest that between $25 and $30 trillion U.S. dollars will be spent on infrastructure worldwide by 2030," the authors note.

P.S. By the way, for a more skeptical take that China's frenetic pace of urbanization will really continue at its current rates, see Kate McKenzie's excellent recent post at FT Alphaville. She argues that China's hukou system, which essentially places restrictions on urban migration, could slow the rate at which China's urban middle class balloons.

(For the PNAS link, thanks to Nate Berg, who has some excellent additional charts.)

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