From Erica C. Barnett at The Stranger's Slog Blog:
An exhaustive new report (via Sightline) shows that carbon emissions for people who live in dense cities are far lower than those who live in suburbs. And the denser a city is, the less carbon each of its inhabitants emits. Why? Because people who live in cities drive less (producing lower transportation emissions) and use less energy overall (because sharing walls reduces energy consumption).