Model cities are ranked by a combination of criteria. These include urban planning and environmental statistics. They encompass energy sources, consumption and emissions, as well as transportation options and habits. Most lists also make note of green living (such as the availability of public parks, green jobs and sustainable buildings) and green perspective (such as recycling).

It's uniquely challenging for urban areas to be green. They have a high volume of people, traffic congestion, trash and air pollution to name just a few obstacles. Seventy-five percent of the world's energy is consumed by the world's cities [source: ThomasNet Industrial Newsroom]. Green cities have to strike a balance of managing their current needs without compromising the city's (and environment's) future.