Surprised by that last one? Here's writer Jonah Leher in Seed on the topic:
When most of us think about environmentally friendly places, we imagine rural landscapes and bucolic open spaces. We picture a terrain untouched by concrete. Cities, in contrast, seem like ecological nightmares. They are densities of pollution, artificial environments where nature consists of cockroaches, pigeons and florist shops. But, according to Bettencourt and West, the conventional wisdom is exactly backwards. Cities are bastions of environmentalism. People who live in densely populated places lead environmentally friendly lives. They consume fewer resources per person and take up less space. (On average, city dwellers use about half as much electricity as people living outside the city limits.) The typical Manhattanite generates 30 percent less CO2 than the typical American.