For those who do research on the sustainability of cities, there's a tantalizing puzzle. Per capita, cities are greener than other places; urban residents have a much smaller ecological footprint.
In our current models, we understand only about half of that difference, perhaps even less. To be sure, some of the greenness of cities is not so hard to explain. For example, people drive less in bigger cities because it's harder to drive, and because it's easier to get around without a car. Other factors are small in themselves, but add up: the closer spacing of buildings results in lower transmission losses and pumping energy; there's less embodied energy in roads and other infrastructure; urban residences tend to be more compact and energy-efficient.
But the most intriguing reason may be the one we understand the least: people in cities actually interact and use resources in a more efficient pattern. When we look at individual factors in isolation, we miss the synergetic effects of this network.
There are other models that might help us unravel this, especially those that explain the dynamics of networks. In particular, there is a phenomenon in economics that's known as a "Knowledge Spillover." It is one of the reasons that cities are such powerful economic engines. Within a city, if you are making x, and I am making y, then our combined knowledge might allow us to make z together, but only if we are physically close enough that our knowledge can spill over from one sort of enterprise to another.
In practice, many such spillovers gradually connect and reinforce each other, creating a kind of virtuous circle of economic activity, and whole new industries. (Think of the automotive industry centered in Detroit, or the personal computer industry centered around Palo Alto.) This pattern is a classic kind of "network," familiar to those who work with metabolic processes in biology.
Scientists have long known that a similar dynamic helps to explain how a body or an ecosystem can have high metabolic efficiency, creating new chemicals or structures, and re-using the resources to do so again and again in a sustainable pattern.
It seems very likely that something similar happens in cities, and in what we might think of as resource spillovers. District energy is an obvious example – I generate electricity, then sell to you the waste heat very cheaply, and we both come out ahead. But the same kind of “metabolic efficiency” can be seen in other, less obvious patterns, like walkable networks. If I'm on foot in a neighborhood that offers many of my daily needs, I can easily combine trips, stop to see you, share a meal.
By contrast, if I am in my car, my pattern of movement is much more limited, both by the confining capsule of the car, and by the much larger-scale road pattern. I'm more likely to plug into high-consumption patterns engineered for that more fragmented system (like drive-through restaurants, say). So I am not only burning gas in my car, I'm generating a lot of other forms of high consumption, because I've lost the metabolic network efficiency from these “resource spillovers.”
There are many other connections that promote the efficiency of our settlements: the impact on our bodies' metabolisms and health, the economic vitality of walkable cities, and much more. In each case the lesson is the same: there's power in these self-organizing networks within cities, and there's opportunity in learning to support and enhance them in the way we plan and build.
The corollary is that we have been building a generation of sprawling suburban models that lack this metabolic efficiency, and that rely instead on prodigious inputs of resources to sustain them. In a world of infinite resources, whose consumption had no impact on calamities like resource depletion and climate change, perhaps this would not be a problem. But in our world, it's a problem of the most serious kind.
Top image courtesy Flickr user OiMax