This year, for the first time in human history, more people live in urban areas than do not. As this trend increases, it raises a question: What kind of city is better for the environment?

Your mental image of a green city might resemble designer fantasies such as Ebenezer Howard's Garden City or Frank Lloyd Wright's Broadacre City—sleek towers nestled in lush forests, where a stroll down Main Street would feel like a walk in the woods. Or maybe you're thinking of small towns such as Hastings, Neb. (population 25,000), which Yahoo! just named “the greenest city in America.” Or you might have in mind something more nostalgic, like Grover's Corners, the fictional hamlet made famous in Thornton Wilder's Our Town, New Urbanism's literary ancestor. Modest developments with lots of green space must be the answer, right?

Wrong. New research reveals that bigger is better. This spring, a groundbreaking study led by Geoffrey West of the Santa Fe Institute showed that cities conform to the phenomenon known as “biological scaling.” All organisms operate in similar ways, regardless of size—metabolically, an elephant is a lot like a mouse, just bigger. More important, the larger the animal, the more efficiently it uses energy. Cities are the same—the larger they are, the more economical. Analyzing various data including electrical use, gas consumption, and lengths of roads, West and his team found that “regardless of size and location, cities conform to certain universal dynamics—just like biological organisms.”

  • The metabolic efficiency of animals is directly proportional to body mass. Cities are the same. Consider New York as the elephant. The number of people on one square mile in Manhattan is 2.5 times the entire population in Hastings, Neb. Hastings was recently named the greenest city in America, but, compared with New York, its efficiency is akin to that of a mouse.

    Credit: Jesse Jensen

    The metabolic efficiency of animals is directly proportional to body mass. Cities are the same. Consider New York as the elephant. The number of people on one square mile in Manhattan is 2.5 times the entire population in Hastings, Neb. Hastings was recently named "the greenest city in America," but, compared with New York, its efficiency is akin to that of a mouse.

In terms of per capita consumption, New York is much greener than Hastings. Although the Big Apple didn't make Yahoo!'s list, it is in fact more energy efficient per person than any other American city—and even many states. The reason is density; more people per square foot equals lower average waste. Carbon emissions in NYC are less than a third of the national average, and typical electricity use is 75 percent lower than in Dallas. Because walking and public transit are popular, gasoline consumption approximates U.S. levels from the 1920s. When it comes to saving the planet, it doesn't take a village—it takes a metropolis.

More people migrating to cities can provide an effective antidote to sprawl and therefore promote the health of both people and environment. Denser, mixed-use communities encourage walking and discourage greenfield development. The suburbs originally developed as an alternative to urban and rural life that gave convenient access to both. But in the last half century, unchecked suburban growth has threatened to eliminate the countryside by displacing it. Reversing the trend is good conservation because it alleviates the pressure to develop natural areas. If municipalities create greater incentives for clean, renewable energy, the future may lie in keeping town and country separate.

Lance Hosey is a director at William McDonough + Partners.