What's the greenest city in the United States? Boulder, Colorado? Burlington, Vermont? Bellingham, Washington? How about New York City?

According to author David Owen, America's largest urban place, with a population density far greater than most of the country's other cities, is the true model of environmentalism.

While most of us think of green cities as those with lots of open spaces, leafy boulevards and little or no traffic, Owen argues in his 2009 book "Green Metropolis" that density equals sustainability. By concentrating people in such a small area, New York City – and the borough of Manhattan in particular – achieves an environmental efficiency not seen elsewhere in the U.S. He points out that largely due to the city, New York state residents use less gasoline on average than people in any other state – about half as much as the average Wyoming resident. And, that the intensely developed city avoids converting surrounding countryside to urban development (Phoenix, Arizona, by comparison, has about twice the population of Manhattan, yet covers more than 200 times the land area).

The idea of density as being green runs counter to what so many of us think, especially when we recall the gray, sun-blocking, and sometimes littered streetscapes of big cities. But what Owen argues makes sense.

One of the book's central ideas is that the true path to sustainability is not to rely on encouraging or cajoling people into living greener, but to create metropolitan areas that are inherently efficient. For example, New Yorkers have the lowest per capita energy consumption in the U.S. – and a resulting small carbon footprint per person – not because they are environmentally aware, but because the place they live creates its own efficiencies. The reasons include that New York mixes dwellings, shopping, and workplaces in close proximity, making walking feasible, and that the small apartments caused by astronomical real estate costs require less energy to heat and cool than suburban homes and the limited storage space helps reduce purchases of non-essential items.

He boils New York's environmental lessons down to three points:
·       Live smaller
·       Live closer
·       Drive less

The city offers another, more far-reaching lesson: Sometimes the greenest actions run counter to common assumptions.