If it isn’t already abundantly clear, I am a big fan of cities. If there’s one thing that gets me a little concerned about them, though, it’s the fact that they turn over so fast. According to Stewart Brand’s recent lecture, cities replace at least 2-3 percent of their fabric every year, so every forty years or so they have been completely remade. Where does all that sheer mass go? And where do we get all that sheer mass? Regardless of where it goes, this doesn’t strike me as a particularly sustainable way to go about things. The suburbs probably aren’t much better in this department, but this is an issue that a good urban planner should have on her radar.

It’s a minor quibble, really. I’ll be back to the regularly scheduled praising of all things urban by tomorrow, most likely. And the Stewart Brand talk is just packed with great stuff; more on that surely to come as well.