Urban density is a guarantor of efficiency. It doesn’t necessarily feel like Manhattan is a terribly green place — not when you’ve got buses belching in your face, not when the air smells like garbage and bad Chinese food — but those facts of life in New York ensure that no one drives, everyone takes mass transit and walks, and that our living spaces are (generally) small and thus inexpensively heated and cooled. That unconscious efficiency is what makes New York City’s per-capita carbon footprint so stunningly small, but the discomforts described above are also the sort of things that send people sprawling towards notably less efficient suburbs and exurbs. Sure, there are lawns there, but by just about every measurable metric, places like Long Island are hugely un-green. The newly released Long Island Index, funded by the Rauch Foundation, has what looks like a counterintuitive suggestion on how to ameliorate that. Which is that Long Island needs to get denser.