How does the ecological footprint work?  Understanding the spreadsheet calculations

Consumption of food, housing, transportation, goods, services, and the generation of waste all require energy and land.  These are represented in columns G-L in the spreadsheet.  For example, vegetables (row 21) require arable land on which to grow (column H).  However, they also require fossil fuels for fertilizer, tractors, and trucks to ship produce to markets.  Because the ecological footprint area is measured in hectares, we need a method for converting fossil energy used into land area.  Wackernagel and Rees (1996) assume that to live sustainably, all CO2 emitted from the combustion of fossil fuels must be fully absorbed by vegetation (e.g. forests).  The greater the amount of fossil fuels used, the greater the land area required to support the trees needed to absorb the CO2.  Without this CO-absorbing capacity, atmospheric CO2 will rise and possibly lead to climate warming. By definition, this is not sustainability.

The amount of land needed to support fossil fuel used (column G) can therefore be calculated using the following formula (for this and subsequent equations, each term is defined in words on the first row, and the units of each term are shown in the second row):

Land area
=
Carbon sequestration ratio
*
Energy intensity ratio
*
Consumption quantity in metric or US standard
*
Metric conversion factor, if needed
*
Waste factor, if needed
m2
=
m2
*
Gj
*
Kg
*
 
*
 
Gj
Kg

where m2 = a square meter of land; Gj = gigajoules or 1015joules, which is a unit of energy; and Kg = kilograms of material consumed.  The carbon sequestration ratio is the amount of land area covered with forests needed to sequester the amount of CO2 released from the production of one Gj of energy.  Wackernagel and Rees (1996) estimate this quantity to be 0.141.  The energy intensity ratio represents the amount of energy required to produce and transport a kilogram of material, and this quantity changes for each material.  This ratio is sometimes called the "embodied energy" of a product, reflecting all of the energy that went into making it.  Notice how the units cancel by multiplying through the right hand side of the equation.         

This approach can be extended to all of the other materials in the spreadsheet in terms of converting quantities of material consumed (items listed in the rows) into land area needed to support sustainable fossil fuel consumption, or "fossil energy land" (column G), arable land (column H), pasture (column I), forest (column J), built-up land (column K), and ocean (column L).  The spreadsheet then adds up all of these land areas for all of the materials you consume and reports a total ecological footprint (cell G 148).

Please examine the links for each of the six consumption categories to see, specifically, how land area is calculated.