It's that density — the sheer number of people living in such a small area, often literally on top of each other — that makes Manhattan, and New York City as a whole, so green. Manhattan's population density is 800 times the national average. Density comes with negatives, certainly — small living spaces, air pollution, lots and lots of concrete — but it also enables amazing efficiencies. More than 80% of Manhattanites travel to work by public transit, by bike or on foot — compared to an average of about 8% everywhere else in the country. The vertical apartment buildings that Manhattanites live in are far more energy-efficient than single-dwelling housing in the suburbs. "Most Americans, including most New Yorkers, think of New York City as an ecological nightmare, a wasteland of concrete and garbage and diesel fumes and traffic jams," wrote David Owen in his 2009 book Green Metropolis. "But in comparison with the rest of America it's a model of environmental responsibility."Density is the key to sustainability. While there are a lot of people who don't want to live in dense cities, at the very least, it would be nice if the government would incentivize density rather than sprawl. Of course, dense cities create public health hazards and those are important. But these problems are easier to deal with than our sprawl and the massive environmental disasters this causes.