able with aggregating these unique values and trading them off against mundane things such as guns or butter.
Aggregation of data in different physical units requires weighting the various measures in order to convert them to a common unit of account. Often this aggregation is accomplished by using a common physical unit of measure, such as weight, volume, or energy content. However, this approach is seldom sensible because the environmental impacts per unit of physical measure differ by orders of magnitude according to the substance and the pathway of human exposure. Compare, for example, the impact of 20 kilograms of plutonium and sulfur in different delivery vehicles.
The Dutch NAMEA system converts dissimilar pollutants to common units on the basis of their contribution to environmental themes such as global warming and acid rain. Thus, emissions of greenhouse gases today are commonly measured in terms of their CO2 equivalent or global-warming potential. While this approach is often sensible, it embodies hidden assumptions that may be highly controversial on close scrutiny. For example, the conversion of greenhouse gas pollutants to a common unit requires detailed scientific knowledge about the relative contributions of different gases, and if the contribution is nonlinear (as is almost always the case), the aggregation will be inaccurate. Sometimes, the aggregation includes hidden economic assumptions. For example, the usual approach to aggregating greenhouse gases is to take their contribution to global warming over 100 or 200 years, but not to discount them; this approach is generally flawed and may lead to inappropriate decisions (Reilly and Richards, 1993).
The above examples suggest that physical indicators are subject to many of the same pitfalls and difficulties that plague economic measures of nonmarket and environment activities. Physical accounting systems are most valuable for policy and scorekeeping purposes when overall environmental objectives and targets are clearly established. When the physical systems are highly complex and heterogeneous and are less closely linked to policy objectives, physical accounting is less useful for policy or accounting purposes. It is essential to emphasize, however, that detailed physical information remains an essential component of both economic accounts and environmental policy making.
At present, the United States places less emphasis on developing a comprehensive set of environmental indicators than do many other nations, especially in Europe. Recently, the need for a set of policy-relevant and scientifically based environmental indicators has received high-level attention. This point has been emphasized by the President's Council on Sustainable Development and the National Performance Review.