Boston has mandated the nation’s first green building code for private projects.Photo courtesy Willem van Bergen via FlickrAdapted and cross-posted from Climate Progress. Humans have officially made their home in the concrete jungle. Ours is the first generation in which most of the world's population lives in cities. With 6 billion people on the planet, and 2 billion more expected within 20 years, the race to our cities and the slums and vast sprawl surrounding some of them will only accelerate. Already, our metropolises -- 21 already have populations of 10 million or more -- consume about three-quarters of the …