I am an urbanist. I grew up in a medium size mill city northwest of Boston, I moved to Pittsburgh for college, explored Paris for five months of bliss, and went to graduate school to gain a greater understanding of the complexities of urban economies.
Cities fascinate me as they are a melange of interacting chaos upon which we try to impose an understandable degree of order and indexing. I am also amazed at the continuous stream of externalities that cities generate; art, innovation, societal arrangements, experimentation and efficiencies.
The trend for the past six hundred years of Western history and the past hundred and fifty years of world history has been towards ever greater urbanization as the rural population first moved to the cities for increased economic opportunity, and then the urban populations started surviving due to improved public health measures. This trend will continue and on global warming it is a good thing.
New York City just released its evaluation of its CO2 footprint [big PDF] and the raw numbers are very large, which is to be expected as New York City has 3% of the country's population. However New York City is extraordinarily carbon efficient compared to the rest of the United States. The typical NYC resident directly emits 33% of the CO2 compared to the typical US resident.
Most of the efficiency is due to the high density of the city which allows for a massive and highly used public transit system. However building efficiency is also present as heating and cooling costs are reduced due to the number of multi-unit buildings. Row houses, townhouses, co-ops are all more efficient to use for internal climate control purposes than detached single family homes.
The CO2 intensity of the city is even higher when compared to the US average because NYC has a higher Gross City Product/capita than US GDP. New York City's economy and urban geography is a unique geography, but it is an existence claim that stabilizing and reducing CO2 emissions can occur without compromising economic viability.