Researchers studying supplies of copper, zinc and other metals have determined that these finite resources, even if recycled, may not meet the needs of the global population forever. According to the study, if all nations were to use the same services enjoyed in developed nations, even the full extraction of metals from the Earth's crust and extensive recycling programs may not meet future demand. The researchers suggest the environmental and social consequences of metals depletion become clearer from studies metal stocks--those in the Earth, in use serving people and lost in landfills--instead of tracking the flow of metal through the economy in a given time and region. Yale University researchers Robert Gordon and Thomas Graedel and their colleague Marlen Bertram of the Organisation of European Aluminum Refiners report their findings in the Jan. 17, 2006, Proceedings of the National Academy of Sciences.