New York is the most crowded city in the United States. Its streets are dirty, the air is polluted, and the traffic is often unbearable. On a purely intuitive level, New York is an ecological disaster. Calculated by the square foot, New York City does generate more greenhouse gases, use more energy, and produce more waste than most other regions of comparable size in the United States. Yet, in terms of its overall impact on the environment, New York can be seen as comparatively green.    The key to New York’s relative environmental benignancy is its extreme compactness. Manhattan's population density is more than 800 times that of America as a whole. Indeed, if the United States were as densely compacted as Manhattan, it would fit into an area roughly the size of the state of New York.    Because densely populated urban centers concentrate human activity, most people automatically think of them as pollution crisis zones. “Conventional wisdom holds that any place with such tall buildings and heavy traffic is obviously an environmental disaster ― but this simply isn’t the case,” says John Holtzclaw, a transportation consultant for the Sierra Club and the National Resources Defense Council. Holtzclaw explains that if New Yorkers lived at the typical American sprawl density of three households per residential acre, they would require many times as much land. They would be driving cars instead of taking the subway, they would have huge lawns and be using pesticides and fertilizers on them, and they would need twice the electricity they use now.    For these reasons, experts now say that cities, which cluster lots of people together, may actually be an important means of limiting environmental damage. This notion has yet to be widely embraced, however, partly because it is counterintuitive on the surface, partly because most people-particularly Americans-would rather live in the leafier suburbs than in built-up cities, and partly because many that do live in cities would like to move out.