For decades, Americans have fled to the suburbs in search of clean air and open space. Ironically, some of the “greenest” places in America are its dense, vibrant cities. New York City, seen by many as an urban jungle, was recently called the “greenest city in the U.S.” because so many of its residents live in efficient multi-story buildings, and use public transit, bicycles, or their own feet to commute to work. City residents also consume about half as much electricity as those who live in more spread-out suburban areas. The Apollo Alliance report has the details, and the policy lessons for America.