grams and tons can hide variables such as volume, land disturbance, toxicity,4 and other environmentally important qualities associated with materials that weight measures do not reflect. Nevertheless, weight does provide a reasonable starting point for appreciating the structure and scale of major activities affecting national environmental quality.
National material metrics do not obviate the need for monitoring environmental variables locally. Rather, they complement smaller-scale metrics that underscore the spatial distribution of problems and needs. In this respect, they resemble national economic indicators, such as gross domestic product (GDP). In addition, national materials metrics offer the prospect of capturing environmentally significant trends and relations not captured in the current regulatory framework, which tends to emphasize reporting by media, especially air and water, rather than along the functioning of the economic system.
We propose eight general classes of metrics to indicate the current status and salient trends in national materials use as they influence environmental performance (Table 1). Most address either the productivity or the efficiency of resource use. Others indicate trends in the size and composition of materials use. Some metrics offer a means for quantifying aggregate environmental changes resulting from current national activities. Although some of the metrics are novel, others are already employed but gain meaning from the more systematic context. Although imperfect, this initial classification is intended to stimulate subsequent inquiry into the development of material metrics and the logic sustaining them.
The total mass of materials consumed by a nation, or individual members of its population, offers an indicator that tangibly values resource use. The components of the total differ in kind (and often in the accuracy of the supporting data), but their sum provides a benchmark for environmental management.
In 1990, each American mobilized on average about 20 metric tons of materials, or over 50 kg/day. The breakdown in Figure 2 equates with Figure 1 on national flows at the level of the individual American. This sum may be similar in other industrial nations. For example, estimates of Japanese materials use in 1990 total 52 kg per capita per day, a number closely comparable to the U.S. estimate (Gotoh, 1997).
The dynamics of per capita resource use as well as the efficacy of various policy initiatives aimed at affecting it could be gauged by comparing this number over time and across nations. More detailed metrics would look at consumption of classes of materials, such as energy fuels or agricultural minerals, and environmentally significant individual materials, such as lead.