Each resident of the 100 largest metropolitan areas is responsible for 2.47 tons of carbon dioxide in energy consumption on average, compared to the US average of 2.87 tons. Typically, a city dweller will use public transport more often and live at higher population density, both factors which can decrease carbon footprint.

There was a large variation in footprint between cities, most likely due to factors such as the weather, availability of public transport, electricity prices and the carbon intensity of local electricity generation. Carbon dioxide emissions tended to be higher in the eastern US, where a large proportion of coal is used to produce electricity. In the west, on the other hand, the weather is more favourable and electricity, petrol and diesel prices have been higher. The Seattle-Portland region, which gains much of its electricity from hydropower, also had a low carbon footprint.

The study looked at data for 2005.