The reserves used in Table 31.1 are defined as the demonstrated resources that are economically recoverable at today's prices and extraction costs. They are by no means a definitive measure of available primary resources. Also there is another stock of recoverable metals that has been extracted in the past and still remains in the technosphere (secondary resources). Moreover, the diffusion of a technology may be constrained, not only by the available stock of resources, but also by the rate at which these are recovered and by competition for metals from other end uses. All of these factors must be considered.
There has been considerable controversy over assessments of future supplies of primary resources; see, for example, Tilton (1977, 1996). Even if there is no consensus on how to construct reliable economic indices of scarcity (Cleveland and Stern 1999), no economic indices, such as real prices or recovery costs, have clearly indicated an increasing scarcity of metals (Barnett and Morse 1963; Krautkraemer 1998). Over a large part of the 20th century, technological progress, new discoveries and substitution have compensated for the depletion of high grade ores, and kept prices from rising.
However, this by no means implies that metals are not scarce. Of prime concern for technology evolution is that the abundance of metals varies by many orders of magnitude (Figure 31.1). Iron, for example, is half a billion times more abundant in the Earth's crust than ruthenium. This difference in abundance is reflected in the industrial metabolism. The global refinery production of iron in 1999 was 90kg per capita or 50 million times that of ruthenium, which had a refinery production of less than 2mg per capita. Clearly, scarcity militates against building bridges of ruthenium.
The availability of the metals considered for batteries, fuel cells and thin film PV may be constrained in different ways. Different factors govern the availability of metals that are mined primarily as main products or high-value co-products versus those mined as low-value by-products. We may also distinguish between metals that have low and high extraction-to-reserve ratios (Table 31.2). Some main product metals such as lead, copper, zinc and nickel are mined in large quantities and have reserve-to-extraction ratios well below 100 years. These appear to be constrained by the available stock of economic resources rather than by the extraction rate.
Historically, new discoveries of metal deposits have kept adding to known resources. However, as pointed out by Skinner (1976, 1979, 1987) there is a distinct difference between abundant metals such as aluminum and iron and rare metals such as copper, zinc, nickel and lead. The former have an average crustal abundance high enough to form separate minerals in common rocks. They are mined from deposits where they have been enriched (by geological processes) by factors of two to 10.
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