Green Manhattan - Why New York is the greenest city in the U.S. is an article (by David Owen, The New Yorker, October 18 2004) explaining the environmental efficiencies of living in New York City.
we lived, quite contentedly, in circumstances that would strike most Americans as austere in the extreme: our living space measured just seven hundred square feet, and we didn't have a dishwasher, a garbage disposal, a lawn, or a car. We did our grocery shopping on foot, and when we needed to travel longer distances we used public transportation. Because space at home was scarce, we seldom acquired new possessions of significant size. Our electric bills worked out to about a dollar a day.
Densely packed city dwellers have a smaller ecological footprint than those in leafier, more natural looking settings. The author explains several fundamental principles of design and enforced habits that lead to this smaller footprint.
Population density actually facilitates sustainable design in housing and transport. With their apartments backing against each other (great insulation), nowhere to park a car (trains, buses and feet can work well in a dense city) and using elevators as a regular form of transport, their daily energy use is greatly reduced.
Tall buildings have much less exposed exterior surface per square foot of interior space than smaller buildings do, and that means they present relatively less of themselves to the elements, and their small roofs absorb less heat from the sun during cooling season and radiate less heat from inside during heating season.