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Emptying the Bucket in the Next 100-250 Years In the graph shown here, a professor of mechanical engineering at the Center of Research for Energy Resources and Consumption (CIRCE) at a leading Spanish university has used Hubbert curves to estimate when the main fossil fuels are likely to be used up and when the natural sources of three economically important metals are likely to be exhausted. Great amounts of these metals can, unlike fuels, be recycled, and this is true also of the more costly metals such as gold, platinum and the rare earths. Indeed, the fact that the latter corrode (oxidizing into substances that are useless like rust) far less readily than iron corrodes into rust is one of the reasons why they cost more. In presenting this graph at an international economics conference in Barcelona in 2010,* Prof. Valero mentioned that (a) for each item, if the Earth really has twice as much to offer as she has estimated, then the time of the predicted peak of production would shift only 30 years into the future, and (b) it is the presence of the chemicals in fairly concentrated natural caches that makes them capable of being mined. To make eco o ic use of the does ’t destro the (e cept, i a certai se se, whe nuclear power plants convert one element into another); it only changes their locations. But if and when their economic uses cause them to become so dispersed that retrieving them for recycling becomes economically unfeasible, then what has happened is analogous to entropy (i.e. to the conversion of exergy into uselessly dispersed heat). On the strength of this analogy, a leading textbook in ecological economics relies on a broade ed co cept of e trop as the degree of 'used-up-ness' or randomization of the structure or cap- acity of matter or energy to be useful to us. It may perhaps be the case that some of the rare earths which are essential to certain modern devices – solar-energy panels, for instance – are so rare that the demand for those devices could outstrip the capacity of the Earth to provide as much of those ingredients as humankind would like to get. (Please note also that all these worries are about depletion only, not about pollution and waste-disposal.) _______________ *See http://barcelona.degrowth.org. The title of Prof. Valero’s book on this topic (written in collaboration with her father) is Exergy Evolution of the Mineral Capital on Earth.