Sustainability is often approached from the standpoint of understanding the problems faced by humanity as it considers the possibility of sustainabil-ity. Several years ago, Schellnhuber et al. (2004) identified what were called "switch and choke elements in the Earth system" and illustrated a "vulnerability framework." A "Hilbertian program for Earth system science" was presented to help frame the discussion (Clark et al. 2004), but this was not regarded as a recipe for sustainability. That program, or set of questions, focuses on needed increases in knowledge of the Earth system. However, only one or two of the twenty-three questions address the other half of the sustainability challenge: that of quantifying the present and future needs of a sustainable world, quantifying the limitations to response that the Earth system defines, and understanding how to use that information to encourage specific actions and approaches along the path to sustainability.
Nonetheless, most of the topics related to addressing sustainability have been treated in detail, if in isolation, by the scholarly community. The human appropriation of Earth's supply of freshwater, for example, has been discussed by Postel et al. (1996). Similarly, the limits to energy, and the ways in which energy in the future may be supplied, were the subject of a five-year effort led by Nakicenovic et al. (1998). Mineral resources have been treated, again in isolation, by Tilton (2003). Other research could be cited, but the central message is that the investigations in one topical area related to sustainability do not generally take into account the limitations posed by interacting areas. Engineers like to talk of their profession as one that is centered on "designing under constraint" and optimizing a design while recognizing a suite of simultaneous limitations. For the Earth system, including but not limited to its human aspects, the constraints are numerous and varied, but it is still the integrated behavior that we wish to optimize, not selected individual components.
A challenge in addressing some of these questions in detail involves not only the flows of resources into and from use, but also information on stocks, rates, and trade-offs. The available data are not consistent: the stocks of some resources, those yet untapped and those currently employed, are rather well established, while for others there remains a level of uncertainty that is often substantial. In an ideal situation, resource levels would be known, their changes monitored, and the approaches to the limits of the resource could then be quantified. Consider Figure 1.1a, which could apply, for example, to a seven-day space flight. The stock is known, the use rate is known, future use can be estimated, and the end of the flight established. So long as total projected use does not exceed the stock, adequate sustainability is maintained.
Consider now Figure 1.1b, the "Spaceship Earth" version of the diagram. Here the stock is not so well quantified. The general magnitude is known, certainly, but the exact amount is a complex function of economics, technology, and policy (e.g., oil supply and its variation with price, new extraction technologies, and environmental constraints). This means that stock is no longer a fixed value, but that its amount may have the potential to be altered. Rates of use can be varied as well, as demonstrated so graphically in the scenarios of the Intergovernmental Panel on Climate Change (IPCC 2008a) for future climate change, not to mention changes in commuter transportation with changes in fuel prices. Nonetheless, the starting point for consideration remains the same: How well can we quantify the factors that form the foundation for any consideration about the sustainability over time of Earth's resources?
A major complicating factor in this assessment is that Earth's resources cannot be considered one at a time; there are interdependencies and potential conflicts that must be accounted for as well. A textbook example is water, an essential resource for human life and nature. We use water for drinking, working, and cooking, but it is also required to produce food and to enable industrial
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