By the middle of this century, there will be as many people living in cities worldwide as there are alive on the planet today. Sustainability, then, is first and foremost an urban project, and I’m always a little surprised to find that there’s a lingering divide between hardcore cleantechies and urban design geeks. You still meet renewable energy obsessives who obsess over the next generation of solar technology but have never given much thought to mixed-use development, and there remain complete-street fans and bike-lane zealots unaware that solar power’s now vering on cost-competitive with coal and nuclear. (And don’t get me started on the hardcore climate activists who don’t pay any attention to cities and how they work at all.) Welcome to the urban (sustainability) century Welcome to the urban (sustainability) century