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slowly realises the importance of conserving its resources. And maybe in a not very
distant future, humanity will have to take “Nature into account” by correcting the
economic indices, as stated by van Dieren's topic of the same name (van Dieren,
1995).
13.5 The depletion of the exergy reservoir of the Earth's principal
minerals in the 20th century
Exergy is an accounting tool that allows one to assess the single or aggregated
commodities of a region, nation or even across the globe. In Sec. 13.3 the authors
evaluated the exergy degradation of the mineral capital of Australia. The aim now
is to extrapolate it for use across the entire planet.
This ambitious task is not free of di
culties. The first and most important
problem is the lack of current and historical data available for many commodities. A
few geological institutions, such as the USGS or BGS, compile world production data
of the most important minerals but provide few estimates regarding non-fuel mineral
reserves. Furthermore, with little in the way of exception, no ore grade trends have
been compiled. Australia is unusual in this respect, thanks to the initiative of G.M.
Mudd in his papers Mudd (2004, 2007a), which provide historical information of
the ore grades of the main metals produced. Further to these limitations, whilst
unit exergy replacement costs are available for many important non-fuel mineral
commodities, there are many more that remain inaccessible. In order to overcome
such issues the authors are obliged to make assumptions at the expense of accuracy.
Firstly, it is assumed that the ore grade of a given non-fuel mineral commodity
remains constant and equal to the average ore grades estimated (Table 6.10). This in
turn implies that specific concentration exergy will not change over time. Moreover,
the application of the Hubbert Peak Model, in terms of exergy, ignores any change
in concentration factor, which is quite significant for many non-fuel minerals.
As with the previous case studies, unit exergy replacement costs are assumed to
be constant. In reality however, such costs are a function of the state of technology
and hence vary with time. Hence, the year of the peak obtained when applying
the Hubbert Model will generate the same result regardless of the parameter used:
tonnage, exergy, or exergy replacement costs. The only advantage of using exergy
replacement costs over tonnage is that the curves can be compared and aggregated.
The exergy countdown diagram can also be obtained, as will be seen later in this
chapter.
13.5.1 Non-fuel minerals
With the help of historical data on world production compiled by USGS (2010)
and introducing the aforementioned assumptions, the authors have estimated the
following variables:
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