4.  GOODS CALCULATIONS

Goods that we consume include a wide variety of natural and synthetic products. It is not surprising then that our footprint includes just about all land use types.

Fossil fuel land

Fossil energy is used to make each of the 11 products listed in the spreadsheet (cells B99-B109). The standard formula for estimating fossil fuel land applies here, but note that the energy intensity ratio changes for each product—from a low of 15 Mj/kg for porcelain and glass up to 200 Mj/kg for medicine. This reflects the fact that making goods like medicine and tools require significant inputs of energy to run machinery and labs.

Arable land

Arable land is required for people consuming cotton clothes and tobacco products. The productivity ratio for cotton is 10,000m2 required to grow 636 kg (cell H99). 10,000m2 of arable land will grow 1599 kg of tobacco (cell H109).

Pasture

Pasture land is required to raise sheep for wool and cattle for leather. The productivity ratio for wool is 10,000m2 required to grow 10 kg (cell I100). 10,000m2 of arable land will grow 57 kg of leather (cell I104). Note that animal products are much less productive per unit area of land because a farmer can grow significantly more plant crops than animals on a piece of land. This is a good example of the concept of ecological efficiency, where energy is lost at each step up the food chain.

Forest

Our direct consumption of paper products requires the harvest of trees.

Land area
=
Roundwood productivity
*
Paper conversion efficiency
*
Consumption quantity in metric or US standard
*
Metric conversion factor, if needed
*
Waste factor, if needed
m2/yr
=
m2 land
*
m3 wood
*
kg paper
*
*
kg wood harvested
m3 wood
kg paper
kg wood used to make paper

As we saw with the construction of housing, 10,000m2 of land can support approximately 1.99 m3 roundwood. 1 m3 of roundwood can generate about 1000 kg of paper. 1.72 is the ratio of roundwood needed per unit of paper. This reflects a waste factor because not all of the parts of the wood are used in making paper.

Built-up land

This is one of the more difficult quantities to estimate because it incorporates many indirect sources of consumption. For example, when you buy a metal hammer, there are many factories that are required to mine the iron ore, convert the iron to steel, process the steel into hammers, ship the hammers to warehouses, and then ship the hammers to retail outlets. All of these steps required built-up land in terms of factories and transportation networks. We therefore need a way to convert the direct consumption of your hammer into land area required for all of these activities.

You can see how incredibly difficult this would be for any single person to accomplish, so Wackernagel et al. (2000) use several assumptions based on aggregate built-up industrial and commercial land in the US.

First, they assume that the amount of built-up land is directly proportional to the fossil energy area needed to manufacture the good. Thus, the calculation of built-up area for goods is going to involve a factor that multiplies the fossil energy land column (column G).

Built-up land area
=
fossil energy land
*
built-up land
fossil energy required for built-up land
÷
bioproductivity of land
m2
=
m2 FE land
*
m2 BU land
÷
3.5
m2 FE land

To calculate this factor, they use aggregate data for the entire US and the total US population. They estimate that 1100 m2 is the per-capita built-up land footprint component of goods (which also includes wastes, sinces wastes are non-durable goods and the byproducts of durable goods). They divide 1100 m2 by the sum of the fossil fuel energy required to support this built-up land (commercial and industrial energy use) and the waste land (landfills) generated by the built-up land. The values 1324 m2 and 1196 m2 are the fossil fuel areas of goods and waste, respectively. To standardize for the average productivity of land (see equivalence factors link), they also divide by 3.5, which reflects the fact that most built-up land is located on fairly bioproductive land comparable to arable land.

Note how the units cancel on the right hand side of the equation. Although many assumptions go into a calculation like this, it is valuable for seeing how indirect consumption can be quantified.