Economic versus Thermodynamic Accounting - Thanatia: The Destiny of the Earth's Mineral Resources - page 54

2.6.2 Ecological footprint
Another very important sustainability indicator is the well-known ecological foot-
print. It represents “the amount of biologically productive land and sea area nec-
essary to supply the resources a human population consumes and to assimilate
associated waste” (Wackernagel and Rees, 1996). It has been used as a standard-
ised measure of demand for natural capital. The calculation consists of converting
equivalent global hectares to the direct and indirect consumption of energy, biomass,
building materials, water and other resources on a population basis. Proponents
estimate that the Earth provides 1.9 hectares of bioproductive space per world in-
habitant. For 2012, humanity's total ecological footprint was estimated at 1.5 planet
Earths 16 .
It should be stressed that the environmental impact on mining is hardly measur-
able with biologically productive area and this indicator is consequently insensitive
to depletion problems. Therefore, whilst it explains well the demand for the regen-
erative capacity of biotic systems, it provides insu cient information when dealing
with abiotic ones.
2.6.3 Energy/exergy indicators
Throughout the chapter the reader has seen that money, mass and land can be used
as indicators for sustainability. They are easy to understand and each has its distinct
advantages and drawbacks when evaluating different environmental impacts.
Energy indicators have also played an important role in this endeavour. Any
form of material manifestation can be measured in energy units. Joules, kWh or
tonnes of oil equivalent are all equally valid. Energy is a thermodynamic property
that whilst not so easy to understand is more scientifically precise: it is measurable,
additive and universal.
However, as will be seen in the next chapter, energy does not distinguish quality
but quantity. When energy indicators make reference to energy, they are in reality
referring to exergy, which is another, as equally precise, thermodynamic property.
Energy and exergy coincide in numerical values in an important number of cases:
electromagnetic and mechanical energies and in practice fossil fuels. However, when
dealing with heat, chemical substances and irreversible phenomena like mixing,
throttling or chemical reactions, the exergy calculation differs greatly and becomes
more complex. Fortunately, in most cases, the orders of magnitude coincide.
A selection of the most prominent energy indicators are explained next. As
regards to the exergy counterparts, they are only briefly explained, since Chap. 3
is entirely devoted to their description.
16 See http : ==www:footprintnetwork:org. Accessed July 2013.