IEEE Spectrum's special issue on Megacities
Cities need to import the materials, food and energy they consume, and have a footprint which is much larger than the area they occupy. For example, Tokyo's footprint is 60% larger than all productive land available in Japan.
Paradoxically, because of their concentration, cities offer a potential to rething transport, eliminate cars, use cogeneration, recycling and remanufacturing. Compact enclosed dwellings near a place of work can be heated efficiently. But we're not yet using cities in this manner.
Cities' inefficient design, combined with the relatively higher incomes of urbanites increase the footprint of the urban environment well beyond what it could be.
The perfect city could very well be sustainable. Since 3 decades, Paolo Soleri is promoting the concept of the lean linear city, which aims to combine quality of life with a low footprint.
In China, where major greenfield cities are mushrooming, the concept of a sustainable city design is being developed with the city of Dongtan, a new urban development which will eventually be home to half a million people.