The continued increase in the use of metals over the 20th century has led to the phenomenon of a substantial shift in metal stocks from the lithosphere to the anthroposphere. Such a shift raises social, economic, and environmental issues that cannot be addressed without quantifying the amount of stock of "metal capital" utilized by society. Estimation of the in-use stock of metals has occurred for at least 70 years, with over 70% of the publications occurring after the year 2000. Despite the long history, this is the first critical review to consolidate current findings, critique methods, and discuss future avenues of research. Only aluminum, copper, iron, lead, and zinc have been studied to any extent Nonetheless, it is clear that for the more-developed countries, the typical per capita in-use metal stock is between 10 and 15 t (mostly iron). Comparison of the per capita stocks in more-developed countries with those in less-developed countries suggests that if the total world population were to enjoy the same per capita metal stock levels as the more-developed countries, using a similar suite of technologies, the amount of global in-use metal stocks required would be 3-9 times those existing at present.

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A key aspect to achieving long-term resource sustainability is the development of methodologies that explore future material cycles and their environmental impact. Using a novel dynamic in-use stock model and scenario analysis, I analyzed the multilevel global copper cycle over the next 100 years. In 1990, the industrialized world had an in-use copper stock about twice as large as the developing world and a per capita in-use stock of about six times as large. By 2100, the developing world will have an in-use copper stock about three times as large as the industrialized world, but the industrialized world will maintain a per capita stock twice that of the developing world. Under a scenario of no material substitution or technological change in copper products, global in-use stock in 2100 will be about as large as currently known copper resources. However, current scrap recycling trends and exploration will alleviate absolute supply pressure but not environmental impacts from decreasing copper are grades. Additionally, unexpected emergent properties of dematerialization are observed from the in-use stock model that arise solely from the properties of stock dynamics, an infrequently discussed cause of dematerialization in the literature.

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Humanity confronts a daunting double challenge in the 21st century: meeting widely-held aspirations for equitable human development while preserving the bio-physical integrity of Earth systems. Extant scientific attempts to quantify futures that address these sustainability challenges are often not comprehensive across environmental and social drivers of global change, or rely on quantification methods that largely exclude deep social, cultural, economic, and technological shifts, leading to a constrained set of possibilities. In search of a broader set of trajectories, we combine three previously separate streams of inquiry: scenario analysis, planetary boundaries, and targets for human development. Our analysis indicates there are plausible, diverse scenarios that remain within Earth's safe bio-physical operating space and achieve a variety of development targets. However, dramatic social and technological changes are required to avert the social-ecological risks of a conventional development trajectory. One identified narrative, which is predominant in the scenario literature, envisions marginal changes to the social and cultural drivers underlying conventional growth trajectories. As a result, it requires unprecedented levels of international cooperation, alignment of powerful conflicting interests, and political willpower to bend technological change in a sustainable direction. We posit that a more viable and robust scenario might lie in the coupling of transformative social-cultural and technological changes, which set the necessary conditions for a transition to a resilient global future. While clearly a first step, our analysis points to the need for more in-depth exploration of the mechanisms and determinant forces for such unconventional futures.