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The world's metropolitan carbon footprints have distinct geographies that are not well understood or recognized in debates about climate change, partly because data on greenhouse gas emissions is so inadequate. This article describes the results of the most comprehensive assessment of carbon footprints for major American metropolitan areas available to date, focusing on residential and transportation carbon emissions for the largest 100 metropolitan areas in the United States. These findings are put into the context of efforts across the country and the globe to characterize carbon impacts and policy linkages.

IntroductionGreen buildings and smart growth strategies are key to reducing global greenhouse gas (GHG) emissions in the future. In the United States, as in most developed countries, the energy requirements of buildings account for more than one third of GHG emissions. Approximately 43% of US carbon dioxide (CO 2 ) emissions result from the energy services required by residential, commercial, and industrial buildings, while transportation accounts for 32% and industry accounts for 25% (Brown et al, 2005). Thus, assessments of GHG reduction have itemized the many ways that building services could be provided in a more energy-efficient manner. It is also important, however, to consider other GHG impacts affiliated with the built environment, including: the effects of alternative urban designs; the use of on-site power generation; and the life-cycle GHG emissions from building construction, materials, and equipment. This broader perspective leads to climate change strategies that address not only how buildings in the future are to be constructed and used, but also how they will interface with the electric grid and where they will be located in terms of urban densities and access to employment and services.The United States has made remarkable progress in reducing the energy use and carbon intensity of its building stock and operations (figure 1). These improvements are largely the result of advances in the energy efficiency of US buildings following the 1973^74 OPEC oil embargo. Since 1972 primary energy use in buildings has overall increased at less than half the rate of growth of the nation's gross domestic product (GDP). Since the late 1970s, when detailed energy-use data first became available through the Energy Information Administration's (EIA) Residential and Commercial Building Energy Consumption Surveys, residential energy use per capita has declined by 27%, while energy use per square foot of commercial building space has declined by 25% (Brown et al, 2005, page 2). These energy intensities have decreased despite the fact that the size of homes has increased and the range of electric equipment provided in buildings has expanded, especially air conditioning (in the South) as well as electronic equipment, televisions, and other`plug loads' in buildings nationwide.