But wait. This is not necessarily a prediction of a grim future, but
. rather an indication of future costs and adjustments necessary to maintain
industrial economies. How so? First, note that Table3.2 has different years
to exhaustion for reserves and for resources. Reserves are mineral sources
: currently identified and profitable or al least marginally profitable to
extract by existing technologies. Resources are reserves plus known mineral
sources that are "subeconomic" -that is, not profitable to extract with
existing technologies-plus hypothetical resources in known districts plus
"expert" speculation about what minerals might be available' in undiscov-
ered districts or forms. Mineral reserves and resources are calculated
according to a standard formula developed by the US, Geological Survey
and the Bureau of Mines.' As you might guess, as more minerals are dis-
covered or as technology improves, the amounts of both reserves and
resources can increase. Second, we will never actually run out of minerals,
because the earth's crust is like a sponge saturated with minerals than a
cup to be emptied of them. You can always (economically and technologi-
cally) squeeze harder and get more. The question is how much are you
willing to pay for minerals when they become scarce (and often subeco-
nomic, meaning that the cost of their extraction exceeds the value of their
use)? Third, the depletion curves ("years to depletion") estimated by experts
vary greatly because they are made with different assumptions, Some esti-
mates are only of consumption (as those above), but some assume con-
sumption with significant conservation efforts, and some assume
significant conservation efforts and recycling (Keller, 1992: 354). Fourth, and
more significant, substitutes for minerals can be discovered that effectively
change consumer demands, cancel all such calculations, and require calcu-
lation of depletion curves for the new resource. Estimates of mineral