Which is dirtier? 11 million people living in 16 square miles or 11 million people spread out all over 'nature'? I would argue that greater efficiencies can be achieved in cities in terms of environmental degradation AND quality of life and that a given number of people pollute less living in close proximity. A single pig farmer can have a far greater impact on the environment than a brooklyn grocer, for example. And that grocer doesn't require acres of parking surrounding his store so all the country folk can park when they come to buy; his customers walk to his business. One poster mentioned how lousy transportation is in cities; he must have been high. I live in the country and you can't walk anywhere unless you don't have much to do that day. So you are a slave to your car. In a city you have buses, trains, subways, taxis, hydrofoil ferries, all powered by clean energy, you can easily bike anywhere and many cities encourage and facilitate use of public and clean modes of transportation.
My small town on the prairie has a heroin and meth problem, underemployment, contaminated groundwater from agriculture and livestock operations, etc etc.
rant over.