While emissions of metals are decreasing, their input into the economy is still increasing. What happens to this input, now and in the long run? These questions can be addressed through integrated modeling of metal flows within both the economy and the environment. Several case studies applying such integrated modeling showed that the general feeling that the metals problem is solved is premature.
Three main approaches for enhanced metals management now seem feasible: (a) the input into the economy can be lowered; (b) the output can be delayed; and (c) the output can be controlled or sequestered. The first main approach, lowering of the input, can be achieved by replacement of metals in functional applications (for example, PVC for zinc gutters), by recycling or increasing the lifespan of metals with an elastic supply (for example, Cu, Pb or Zn) and by reducing inflows as contaminants, for example in phosphate fertilizer or fossil fuels. The second main approach, delaying the output, can be achieved by keeping non-functional metals within the economy (for example, fly ash in concrete or roads). This option offers time for further development of the third main approach: control of the output, which can be achieved by physicochemical immobilization of the waste flow (for example, vitrification), by waste disposal outside the biosphere, and by bypassing the sensitive environmental routes (for example, reburial in old mines). A number of scenario calculations have been performed in order to assess the effect of several possible policy measures for zinc and copper (van Oers et al. 2000). Although the dynamic model used for these calculations is too unreliable for 'realistic' results, the results of the scenario calculations for zinc and copper indicate that very stringent measures are needed (for example, significant change of agricultural practice, replacement of building materials, adoption of not yet existent waste management techniques) for a sustainable metals management (van der Voet and van Oers 2000).
Although the models used include the full spectrum of flows and accumulations in the economy and environment, the results presented above for the Netherlands are merely indicative. Besides general uncertainties attached to economy environment modeling, a basic limitation is that resource availability has not been taken into account in the steady-state model used; this is in fact assumed to be infinite. So the high-risk ratios will probably not be reached because of enforced declines in resource extraction. However, this is by no means certain, given continually rising estimates of resource availability. Consequently, the results at least imply a warning signal as to the sustainability of current metal metabolism.
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