The most robust Ecological Footprint accounts exist at the national and international scales. Although subnational footprint calculations are both possible and common, because of limitations on the availability and accuracy of data sources, the quality of subnational footprint calculations can be more variable. The international community of footprint practitioners is currently in the process of developing standards and certification procedures for subnational footprint applications.
The National Footprint Accounts are currently maintained by Global Footprint Network, a nonprofit organization headquartered in Oakland, California, and its over 65 partner organizations throughout the world. This high-level analysis relies heavily on data published by the FAO, the United Nations Statistics Division, the Intergovernmental Panel on Climate Change (IPCC), and the International Energy Agency (IEA). Other data sources, including meta-analyses, scientific publications, and thematic collections, are used to fill in the gaps between these international sources.
Yield Factors and Equivalence Factors
Two conversion factors, yield factors and equivalence factors, are used in calculations of footprint and biocapacity.
Yield factors are calculated as the ratio of national, country-specific yields for a given land type to the average world yield for that same land type. This ratio describes the extent to which a biologically productive area in a given country is more (or less) productive than the global average of the same land type. Differences in national and global yields can be due to a wide variety of factors, including variation in climate, soil conditions, available technology, and management regimes. Yield factors are specific to individual land types, countries, and years.
Equivalence factors relate the average productivity of a given land type to the world-average productivity of all biologically productive land types. Cropland, for example, has a higher productivity than world average land. Grazing land is, on average, less productive than the average of all land types. Equivalence factors are currently calculated using Global Agro-Ecological Zones (GAEZ) data, which provide a spatial model of potential agricultural yields. The equivalence factor for a land type depends on its level of potential agricultural productivity relative to other land types.
Calculating Footprint and Biocapacity
The general formula for calculating the Ecological Footprint associated with the consumption of a quantity of product is given as
where EF is the Ecological Footprint of a given product flow (in global hectares), M is the mass of the product flow (in tonnes per year), NY is the national yield of the country in which that product was produced (in annual tonnes per hectare per year), YF is a yield factor calculated as the ratio of national yields to world yields for a given product, and EQF is an equivalence factor reflecting the relative productivity of a given land type compared to world-average productivity.
This formula can be applied directly to all products harvested directly from the first four major productive land types: cropland, grazing land, fishing ground, and forest land. The footprints of secondary products (e.g., flour) that are created from primary products (e.g., wheat) are calculated by converting them back into primary-product equivalents.
The footprint of built-up land is calculated by using the physical extent of the area occupied in built area (in hectares) instead of the product of mass (M) and national yield (NY). Yield and equivalence factors for cropland are applied, reflecting the assumption that most built land occupies former cropland, unless more accurate data are available. The footprint of 'carbon land' is calculated using the total mass of carbon dioxide emissions released from a given activity and the world-average sequestration rate of forested land in place of the ratio of national yield (NY) and yield factor (YF).
To calculate the footprint of a nation, the footprint of all products consumed within that country is calculated using the formula above and then summed. The global Ecological Footprint is calculated as the sum of all national footprints.
The biocapacity associated with a given productive land or sea area is calculated in a similar fashion:
where BC is the useful biocapacity of a given area (in global hectares), A is physical extent of the area under analysis (in country-specific hectares), YF is a yield factor for that country and land type, and EQF is an equivalence factor for that land type. Note that this formula is identical to that for Ecological Footprint, except here area substitutes for the ratio of mass and national yield. This formula can be applied equally to all five major productive land types on the planet: crop land, grazing land, fishing grounds, forest land, and built-up land.
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