A study by the Brookings Institute concluded that people living in the most populated cities in the US have almost 15% lower carbon footprints (2.47 tons per year) than the national average. (3.87 tons per year). The study compared transportation, and density, indoor climate control among the cities:

-Fuel prices: higher fuel prices encourage more fuel efficiency; the highest prices were found farthest from sources and where the most taxes and environmental regulations existed.

-Common fuel types: coal heavy states had large carbon footprints and was most often used by states with coal heavy economies in the East.

-Weather: more extreme weather required more heating and cooling, resulting in higher energy usage.

-Sprawl: density helps to lower carbon footprint by reducing building size and inefficiently used space found in suburbs.

-Transportation infrastructure: mass transit, related to high fuel prices and density, tends to help consumers lower their carbon footprint. Mass transit is economically practicable where there is a high density of customers concentrated along certain corridors, as in cities.

An unmentioned but intimately related factor is politics. The worst carbon per capita was found in ‘red states’, where Republican party ideals fight environmental protection, as can be seen by the recent death of the Lieberman-Warner Climate bill in the Senate.

The highest emitters were in Kentucky, Indiana and Ohio. Cities that had the lowest scores were in the West, where fossil fuels are becoming prohibitively expensive, with California, the Pacific Northwest and Hawaii leading in reduced carbon output.