Geology Reference
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Finally, it is remarkable that the global behaviour has continued to be
exponential-like, despite the shifts in extraction trends among different commodi-
ties. In 2007, the degradation velocity of the natural bonus exceeded 500 Mtoe/yr
(around 18% of the world's current oil consumption). And presumably, it will
continue to increase exponentially at least for another 20 to 40 years, until peak
production is reached.
13.3.11 The exergy countdown
In Fig. 13.29 and Fig. 13.30, the authors have represented Hubbert's bell-shaped
curves for all mineral commodities considered in terms of their exergy replacement
costs. This type of representation, named “Exergy countdown”, shows in a very
schematic way the amount of exergy resources available and the possible exhaustion
behaviour in a future scenario. It should be noted that representing B vs t or B
vs
t generates similar results for the peaking year, as unit replacement costs have been
considered constant throughout the analysed time. The use of exergy replacement
costs allows a comparison between the exergy content of fuels and non-fuel minerals.
However, as stated previously, a better fit should take into account any relevant
technological developments, thereby using the appropriate unit exergy costs as a
function of time.
In Fig. 13.29, the bell-shaped curves of all fuels and the metals iron and copper
are represented. As can be seen, in exergy terms, coal is the most abundant resource,
followed by iron and natural gas. Fig. 13.29 also shows the significantly lower
amount of oil and copper reserves with respect to other commodities.
In order to better visualise the case of copper, Fig. 13.30 is representative of the
exergy countdown of the remaining minerals: copper, zinc, lead, nickel, gold and
silver. Compared to copper and obviously iron, the exergy reserves of the others
are significantly lower. It is also interesting to notice that although copper is the
most abundant commodity in terms of exergy cost, its greater extraction rate will
provoke a faster depletion than that of nickel, for example.
A given country's exergy countdown diagram facilitates a future mineral pro-
duction and a degree of depletion forecast. In this way, one can say for instance,
according to the results published, that about 64% of the total considered mineral
reserves in Australia could be depleted by 2050, should they not increase and should
production follow the Hubbert bell-shaped curve. Gold would be particularly de-
pleted at 99.6%, as would oil at 99.9%, zinc at 91% and lead at 89% followed by
copper at 83%, natural gas at 80%, iron at 75%, nickel at 64% and coal at 54%.
Again whilst such figures are only estimative and serve to predict behaviour pat-
terns only as long as “business as usual” reigns, (which would not necessarily be the
case) they are accurate enough, in terms of order of magnitude, to depict the rate
of depletion of Australian reserves due to mineral extraction.
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