01/27/2011 06:40 pm ET Updated May 25, 2011

10 Sustainable Cities

With widely reported statistics stating that anywhere between one-half and two-thirds of the world's population will live in urban areas by 2030, there's no question that cities are swelling. Estimates suggesting that there will be 5 billion city dwellers by this landmark year paint a somewhat frightening picture: Where will cities get the resources needed to keep up with such rapid growth?

The rise of sustainable cities -- or eco-cities -- cropping up across the globe may hold the answer. Whether they are sophisticated urban plans, futuristic, utopian housing communities or simply abstract concepts dreamed up by innovative thought leaders, this trend ensures that issues like air and water quality, pollution and energy use -- to name just a few -- are addressed before we hit crisis mode. (Though if that happens, perhaps we can all retreat to the LilyPad Floating Cities?)

Top 10 Sustainable Cities

Looking at the varied examples of sustainable cities, several themes emerge. Gwanggyo Power Centre, a winning contest entry from MVRDV, is an urban center of green hill-shaped retail, housing and business "nodes" not far from Seoul, South Korea. The effect of this abundance of green surfaces -- a vertical park -- is increased ventilation and reduced energy and water usage.

With its unparalleled urban expansion, Asia and the Middle East are both hotbeds for sustainable cities. When Masdar City in Abu Dhabi is completed in 10 to 15 years, it will be home to not only 40,000 residents but also 1,500 "clean technology" companies. In addition to focusing on renewable power resources and instituting a car-free lifestyle (a tenant shared by many of the cities featured here), Masdar emphasizes sustainability through technology and industry. Similarly, PlanIT Valley, a partnership between Cisco and Living PlanIT, will be a living showcase of sustainable technologies across 4,000 acres in Paredes, Portugal -- a prototype for future "Smart+Connected Communities."

Innovative -- or even age-old -- transportation alternatives factor into many sustainable cities. In addition to its state-of-the-art Transmileneo public transit, Bogotá, Columbia transformed its city landscape -- not to mention its air quality -- with bike paths, even instituting regular ciclovias to encourage people to forego car transportation. Beyond just reducing CO2 emissions, there are social benefits. It begs the question: Which other cities in warm climates would be wise to follow Bogotá's pioneering example?

As with Bogotá, not all the cities on this list are new planned communities. Kalundborg, Denmark instituted innovative cradle-to-cradle policies decades ago. The city's industries practices "industrial symbiosis" or resource sharing, an idea from the past worth integrating into the future.