The patron saint of American environmentalism, Henry David Thoreau, was no fan of cities. At Walden Pond he became so “suddenly sensible of the sweet and beneficent society in Nature” that “the fancied advantages of human neighborhood” became “insignificant.” Thoreau’s like-minded heirs, including the urbanist, Lewis Mumford, praised the “parklike setting” of suburbs and denigrated the urban “deterioration of the environment.”

Millions of Americans proclaimed their love of nature by moving to leafy suburbs while denigrating New Yorkers for living in the most man-made of places. In the eyes of the pseudo-environmentalist suburbanites, anyone who didn’t care enough about nature to flee Manhattan’s great glazed brick towers seemed worthy of both pity and disdain.

Now we know that the suburban environmentalists had it backwards. Manhattan, not suburbia, is the real friend of the environment. Those alleged nature lovers who live on multiacre estates surrounded by trees and lawn consume vast amounts of space and energy. If the environmental footprint of the average suburban home is a size 15 hiking boot, the environmental footprint of a New York apartment is a stiletto-heeled Jimmy Choo. Eight million New Yorkers use only 301 square miles, which comes to less than one-fortieth of an acre a person. Even supposedly green Portland, Ore., is using up more than six times as much land a person than New York.

New York’s biggest environmental contribution lies in the fact that less than one-third of New Yorkers drive to work. Nationwide, more than seven out of eight commuters drive. More than one-third of all the public transportation commuters in America live in the five boroughs. The absence of cars leads Matthew Kahn, in his fascinating book, “Green Cities,” to estimate that New York has by a wide margin the least gas usage per capita of all American metropolitan areas. The Department of Energy data confirm that New York State’s energy consumption is next to last in the country because of New York City.

Is there any reason beyond civic pride to care that New Yorkers are true friends of the environment? I think so. Environmental benefits are one of the many good reasons that New York should grow. When Manhattan builds up, instead of Las Vegas building out, we are saving gas and protecting land. Every new skyscraper in Manhattan is a strike against global warming. Every new residential high rise means a few less barrels of oil bought from less than friendly nations belonging to the Organization of the Petroleum Exporting Countries.

Given how valuable development in New York is for the environment, one might think that environmentalists would be fighting the preservationists on Madison Avenue to ensure that New York builds taller buildings. But the ground troops of the environmental movement haven’t yet got themselves around to being pro-development, even in places, like New York, where development makes the most environmental sense. All of those years of opposing new development have made many activists reflexively anti-growth. Almost every act of neighborhood anti-development Nimbyism, or Not In My Back Yard, gets wrapped up in a mantle of environmentalism.

The great problem with being reflexively anti-growth is that development in America is close to being a zero-sum game. New homes are going to be built to meet the needs of a growing population. If you stop development in some areas, you are ensuring more development elsewhere. A failure to develop New York means more homes on the exurban edges of America.

In the 1960s, an environmental anti-growth movement grew strong in California. This group proclaimed that it was saving the San Francisco Bay, and group members managed to shut off development in their region. Some bay counties have enacted 60-acre minimum lot sizes. The environmentalists did stop development in their own increasingly unaffordable back yards, but they did not stop development spreading to eastern California, Las Vegas, and Phoenix.

In many cases, development occurred in places that were less dense and that had less public transit than the older places that the environmentalists had protected. The environmental consequences of this kind of environmentalism are far clearer, since the net effect seems to redirect development to places where it would do more harm.

Good environmentalism requires a national perspective, not the narrow outlook of a single neighborhood trying to keep out builders. As a nation, we need to think clearly about where new housing causes the least environmental damage, and we need to make sure that our land-use policies help that happen. A local approach can do more harm than good because dense areas are rich in protesters who push new housing out to where there are fewer people to oppose it.

With this broader view, we see that Mayor Bloomberg’s vision of an even bigger New York City is a great environmental vision. We also see that those preservationists who have arrogated to themselves the power to stunt New York’s growth are not only enemies of affordable housing but also abettors of the environmental damage done by urban sprawl.

Mr. Glaeser is the Glimp professor of economics at Harvard, director of the Taubman Center for State and Local Government, and a senior fellow at the Manhattan Institute.