The ecological footprint research approach emphasizes looking at ecological limits, or the relationship between demand and supply or biocapacity. All footprint results are expressed in a standard unit of "global acres" or "global hectares" of bioproductive land. A global acre is an acre of biologically productive land that has been adjusted in area so that it reflects global average biomass productivity. This standardization is important to allow for meaningful comparisons of the footprints of different countries or regions, which use different qualities and mixes of cropland, grazing land and forest. For example, arable land in the US is significantly more productive than that in the desertifying Sahel region of sub-Saharan Africa.
Equivalence factors allow us to add up different categories of bioproductive land (cropland, grassland, forest) in a meaningful way, since these land uses have, on average, different inherent productivities. So we say that, on average, cropland is 3.2 times more productive than world average land, whereas forest is 1.8 times more productive, pasture is 0.4 times the average, etc. Cells L137-142 provide an estimate of the relative productivity of different land use types.
Built-up area is a bit of a strange category because it represents a use of nature that leads to the destruction of its biocapacity. In this way, it is distinct from the other footprint categories. Since most built-up area is on what was once high quality arable land, it is counted as if it were arable land. Thus, one acre of built-up area leads to the loss of a significantly larger surface when expressed in global acres.
Information Credit: This information was provided by Diana Deumling, Redefining Progress.