The review of Green Metropolis kicked off a discussion that illustrates some of the confusion that crops up in discussions of density. Hoisting a representative snippet up from comments:
> I think people can be extremely green in either rural or urban settings, it’s a matter of intent. Hard to measure the difference in impact – an urban setting allows for a much more communal way of using and reusing resources, but on the other hand, if the concentration of people is greater than the city can naturally sustain, the expense and difficulty of importing food and fuel and exporting waste may mitigate what is saved by sharing.
This was followed by some further speculation that maybe the ideal density is some village type thing where people are reasonably close together, but still able to sustain themselves from the surrounding land.
Partly this line of thinking reflects the ongoing myth that sustainability is best measured by proximity to a farm. I recently read that a freight train can move a ton of goods 460 miles on a single gallon of diesel. Your car can move a bag of groceries about 20 miles on a single gallon of gasoline. Sustainability is best measured by proximity to a *supermarket*, not a farm.
The bigger conceptual problem, though, is the conflation of population size and population density as determinants of environmental impact. Any population of a given size requires a certain amount of agriculture to support it. If the attendant farming places excessive pressure on natural ecosystems or otherwise threatens to exhaust the land itself, then it might be said that the population is “unsustainable.” But that issue has little to do with *where* those people live in relation to the farmed land. Generally speaking, greater density is always going to come out ahead from an environmental perspective.
Take a state with a lot of agriculture like, say, Nebraska. Imagine two extremes: in the first scenario, the population of 1.7 million Nebraskans are distributed evenly throughout the state, so that all the lawyers, accountants, school teachers, and dentists (because even in Nebraska relatively few people work as farmers) are scattered amongst the corn fields; in the second scenario, the entire non-farming population of the state is placed in downtown Omaha.
Which is likely to be more sustainable? Scenario 2, by a mile. Even in scenario 2, Omaha wouldn’t be amazingly dense — it would be considerably more spread out than, say, Brooklyn — but people would likely be driving less, living in smaller houses, and making better use of mass transit. The very small (or, more likely, non-existent) environmental impact of food transportation will be more than offset by the inherent efficiencies of city living.
Green Metropolis illustrated this point nicely with a discussion of water use. A common complaint lodged against cities is that they “steal” water from neighboring watersheds. Indeed they do: New York siphons a vast amount of water from upstate New York. But from an environmental perspective, it is far preferable to concentrate people in cities and have them draw water from the surrounding area than to sprinkle them across those watersheds. Suburbia tends to bring with it lawns and all sorts of other things that would only despoil sparsely settled areas and place even heavier pressure on water resources.
A final point, and then I’m done (for now) belaboring this issue: the comment up top suggests that either rural or urban residents can lead sustainable lives, as long as the intent is there. But that’s just it: intent doesn’t really enter into the picture for the majority of folks. Cities have a lower per-capita footprint not because their residents are more virtuous, but because efficiency is built into their basic fabric.