New York City produces nearly 1 percent of the nation's greenhouse gas emissions - an amount that puts it on par with Ireland or Portugal - according to a city study.
The article does mention that although NYC emits 1% of the greenhouse gas, it also has about 3% of the population, which means "the average New York City resident contributes less than a third of the emissions generated by a typical American."
It also doesn't mention that NYC, at ca. 8m, contains twice the population of all of Ireland, at ca. 4m, making it twice as "green" per capita as Ireland. It is also only 20% less "green" per capita than Portugal, with ca. 10m people. A side consequence is that without cities like NYC, USA as a national average would likely have its per capita greenhouse gas emission rise! To some extent, the per-capita measure doesn't seem intuitive, either- I haven't been to Portugal, but having seen how mind-numbingly green-colored Ireland is, it's hard to think of NYC as "more green", less *twice* as "green".
Obviously, when talking about resources such as energy and transportation, the closer together you can pull your population, the more "green" the land is going to become. Density forces economy to some extent. This is obviously at odds with the seeming glorification on the part of "greens" of an agrarian society, which would force a greater distribution of population in order to ensure life "close" to your food source.
Density also exposes the weakness of certain philosophies- for example, in an agrarian setting no one would care if they pee randomly behind a bush, because "the land" just absorbs it. There are a huge number of sins that appear to be washed clean by dilution. By concentrating population, we're forced to manage waste, and this can lead to energy recycling (generated methane from decomposition, e.g. from compost piles, can be recycled into energy if the waste is funneled into centralized facilities).