Bay Area Ecological Footprint Calculation Methodology
Overview
Local footprints are calculated using primarily local data. Where local data are not available, national data are substituted. In addition, the conversion factors used to convert local data into Footprint acreages are primarily national figures drawn from previous national Footprint calculations. The one exception to this, noted in the energy section as well, is conversion of electricity use to carbon emissions. Calculation methods for the different sections of the Footprint, as well as for biocapacity are detailed below.
In the Bay Area, local data included:
- Population
- Acreage
- Electricity use by source
- Natural gas use
- Gasoline use
- Number of vehicles
- Road miles
- Type, age, and number of housing units
- Recycling
- Biocapacity (area of different types of land)
Energy
Energy use footprints are based on local consumption of electricity and natural gas. Gasoline for vehicle use is reflected in the Transportation footprint.
Fossil-fuel based electricity generation is converted to carbon emissions using conversion factors developed by Lawrence Berkeley National Lab based on PG&E's mix of sources in 1999 (as cited by the Bay Area Alliance). This was described by a scientist there as a conservative estimate because sources have become more numerous and further flung as energy provision is deregulated in the state. Carbon is then converted to land area using national factors for: tons of carbon per hectare, percent of carbon NOT absorbed by oceans, and global acres per acre.
Nuclear energy (determined by multiplying total electricity by the percent of nuclear used in California) is converted to a carbon equivalent using the same methodology as the national calculations. This converts nuclear energy to a fossil fuel equivalent. The resulting carbon 'emissions' are converted to a footprint acreage using the same method as fossil-fuel based electricity.
The impact of hydro-electric power use is found using a similar method to nuclear energy. It is then converted to a land area (for reservoirs), using a national figure for the water area required to provide a unit of energy.
Natural gas consumption is converted to carbon emissions using emissions factors developed by the Bay Area Alliance. This is then converted to area using the same method as fossil-fuel based electricity.
Housing
The area occupied by housing is reflected as built area occupied by housing. It is converted to global hectares assuming that most built land occupies former cropland. This is a reasonable assumption given global settlement patterns, as well as local settlement patterns in the Bay Area.
Housing results in both an Energy and Forest Footprint, reflecting the manufacture of materials and the wood embodied in homes. The number of board-feet as well as the embodied energy in single-family and multi-family homes starts with figures for both the number of board-feet and the embodied energy in a typical home. Then, the surface-to-volume ratio is calculated to convert from the square-footages that are reported to a volume that can then scale these figures from the standard size to larger or smaller homes. Sizes of units are based on average size units by age. Embodied energy is then converted to carbon and to Footprint area using national conversion factors. Board-feet are converted to meters of wood using national conversion factors and discounted for a standard 40-year lifespan of a home.
Food
The food footprint information is derived from national Ecological Footprint calculations. No methods are currently available to develop local Food Footprints, but further research should make these available in the future.
Goods and Services
The goods and services footprint is also derived from national Ecological Footprint calculations. In some cases they are calculated directly from the results of the national accounts. In other cases, the results from the national accounts have been divided among more than one category based on their relative use within the US economy.
Built area associated with goods and services is equivalent to the built area in each county less the built area used for housing and transportation.
Transportation
Road miles are converted to both their embodied energy and the actual area that they occupy. The embodied energy is based on the amount of energy per unit area of the roadways.
The calculation for embodied energy in vehicles starts with the number of vehicles registered in each county and converts it to their total weight using an average current weight for vehicles. It then converts that weight to energy using embodied energy figures.
Gasoline and diesel consumption are converted to an Energy Footprint using emissions factors developed by the Bay Area Alliance.
Recycling
Recycling footprints actually reduce the overall footprints because they represent footprint area that has been avoided. We could count waste as a Footprint, but this is already reflected in the Goods and Services section of the footprint. Unfortunately, conversion factors are not available for all products that can be recycled. This means that the recycling Footprint estimate leaves out other products that could have further reduced the overall footprint. However, the materials represented do make up a large portion of overall recycling. Disposal rates of the materials are converted to amounts recycled using the recovery rates. These are then converted to energy intensity and multiplied by the reduction in energy use as a result of recycling.
Biocapacity
Biocapacity is based on land use in each county. These are then converted to their productivity using national averages for each type of land use. The residential area is separated out to provide data for the built area footprint of housing.
