What’s the greenest place in America? If you answered something like the granola-crunchy, Rocky Mountain-high town of Boulder, you’d be wrong. If you guessed the sea breezes and warm sunlight of Santa Barbara, you’d be wrong again. The greenest place in America is almost devoid of nature — the buildings outnumber the trees — and the air isn’t all that great. But what it has is density and efficiency — the twin qualities that ultimately define green in the global warming era. Applying those standards, the greenest place in America is New York City — specifically, the overcrowded, overpriced and sometimes overwrought island of Manhattan, which has a per-capita greenhouse gas footprint less than 30% that of the national average.