Photograph from AP

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Known as China's Dead Sea, this saltwater swimming pool is part of a massive indoor water park in China's Sichuan Province.

Photograph from AP

Sustainable Earth: Cities

Rio+20 shows how our rapidly urbanizing world presents challenges and opportunities for sustainable development.

The world's concrete jungles don't have the best reputation when it comes to being environmentally friendly. Think "city," and most of us picture unappealing vistas: gridlocked cars, smoggy horizons, and landfills overflowing with urban debris.

Yet demographers say the majority of the world's population already lives in urban areas; by 2050, three-quarters of the people on Earth will live in cities. Most of the action toward urbanization is taking place in the developing world. Urban areas occupy just 2 percent of the Earth, but consume 60 to 80 percent of the world's energy and produce 75 percent of carbon emissions. (See the National Geographic magazine feature "City Solutions" from December 2011.)

Rethinking Cities

Does the trend towards urbanization spell disaster? Maybe not. Recent research suggests those numbers may be deceptive. In fact, the typical urban resident produces less CO2 than average for the country they live in. The average American is responsible for 23 tons of CO2 per year; the average Washingtonian, just 19.7—and New Yorkers generate only 7.1 tons apiece.

That's because cities offer opportunities for efficiency in a way sprawling suburbs and scattered rural areas simply can't. Small apartments are easier to heat and cool than suburban homes, dense cities make public transit an attractive option, and stacking offices makes them more efficient to manage. In other words, the "concrete jungle" is actually a lot greener than it looks.

At the upcoming United Nations Conference on Sustainable Development (Rio+20), the debate over managing this urban transition will focus on how smart urban planning—from efficient and clean public transport to a realistic appraisal of how migration from the countryside to cities works—can make the most of these efficiencies.

China alone is undergoing one of the most dramatic demographic revolutions the world has ever seen. In the past 30 years, more than a quarter of the Asian giant's population has moved from farms and small villages in the countryside to booming cities like Shenzhen, Beijing, Shanghai, and Wuhan. Over the next 20 years, 15 to 20 million people each year will flood into Chinese cities.

All those new city dwellers will need places to live and work, making China the world's biggest de facto urban planning laboratory. Experts estimate that more than 1,500 skyscrapers will be built in China every year for decades to come; dozens of Chinese cities will need mass transport systems built from scratch. To put that all in context, China must build the equivalent of one Chicago-sized city each year for the next two decades.

China's far from alone. With their booming populations, countries like India, Brazil, and Indonesia are at the forefront of the urban future.

Looking Ahead

The potential is tremendous, but that doesn't mean the future of the city will be smooth. Urban planners must overcome challenges like traffic congestion, a shortage of adequate housing, and declining infrastructure. While cities are more efficient, they also concentrate demand for resources such as power and water, making it vital for planners to look beyond the city limits.

And authoritarian, centrally planned city designs like China's—efficient, perhaps, but often far from democratic—may not be a model many countries want to emulate.

There's no doubt the world of the future will be urban. The challenge confronting us today is whether we can make that world an efficient and sustainable one.