There was some confusion last week about a recent Brookings Institution study that calculated the carbon footprints of 100 metropolitan areas in the United States. Mainly, it revolved around the news that metro Atlanta had shown improvement in 2000-2005 and actually lessened its impact on the environment whereas other metropolitan areas did not. This was surprising because the region hasn't expanded its public transit system, made sweeping advances in energy-efficiency, or seen drastic changes in land-use planning.
After the jump, view the methodology with emphasis added.
Sent by Brookings:
To produce comparable carbon footprints for the 100 largest metropolitan areas, the authors examined national databases for passenger and freight transportation and for energy consumption in residential buildings. These estimates are as current as data sources will allow across metro areas, yet at the same time they are incomplete. Major omissions are the carbon emissions from commercial buildings, industry, and other modes of transportation such as planes, transit, and trains. These sources account for roughly half of national emissions. For this reason, results for any particular metropolitan area should be treated with caution. Still, the majority of commercial buildings are powered by electricity derived largely from coal, and their spatial arrangement would be expected to follow the general compactness and density characteristics of residential developments in a metro area. Thus, their footprints are likely to resemble those reported here for residential buildings, although this remains to be seen.
Personal and freight transportation. Information on the amount of energy used for transportation is unavailable at the metropolitan level. Instead, the authors derived estimates based on VMT data from the Highway Performance Monitoring System for both personal and freight transport. They followed a three-step process:
- Estimate the annual VMT within each metro area using highway traffic count data
- Convert these VMT estimates to gallons of fuel consumed, by major fuel types, but principally gasoline and petro-diesel
- Convert this fuel consumption into a) its equivalent energy content, and b) its equivalent carbon content
The results estimate the energy and carbon footprint created by each metro areas auto and truck travel.
Residential buildings. The authors obtained data on electricity sales from Platts Analytics, including the total residential electricity sales and the total number of residential customers of utilities whose service territories include all or a portion of the 100 metropolitan areas. They followed a five-step process:
- Estimate the average electricity consumed per residential customer of each utility serving the metropolitan area
- Estimate the number of households each utility serves within the metropolitan area by mapping the utilities service districts at the ZIP code level
- Adjust county estimates to account for landlord electricity payments, based on county-specific data on types of housing and region-specific data on how utilities are paid by housing type
- Sum the final estimates by county across all of the counties within each metro area to produce metrowide estimates
- Convert to carbon emissions estimates using statewide averages of the carbon content of electricity generation
The authors also estimated the magnitude of residential fuels (natural gas, fuel oil, kerosene, liquid propane gas, and wood) consumed in residential units in each metropolitan area, using the Energy Information Administrations (EIA) state data on fuel consumption in the residential sector and EIAs Residential Energy Consumption Survey data on fuel-specific consumption of different types of housing. The results estimate the energy and carbon footprint created by each metro areas stock of residential buildings.
The authors also generated combined but partial carbon footprints for all 100 metro areas by summing the transportation and residential buildings footprints. Appendix A includes full data tables by metro area, with ranks, and Appendix B discusses limitations of the available data.
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