What is an EI? What are its desirable properties? What is(are) its goal(s)? We think that the answers to these questions can help us in bridging the undeniable gap between the scientists who construct EI's and the policymakers that (want to) use them. We will try to approach each question in an analytical way.
'An EI is an aggregate, quantitative measure of the impact of a ''community'' on its surrounding environment'. This is a very broad definition, but it is in no way ambiguous: the EI must be applicable to any 'community', from the bacterial colonies that generated our atmosphere 3.8 billions years ago to the future manned Mars exploration stations. The EI is by its own essence aggregate, because it cannot be limited to a single individual, whose impact is noticeable for the environment only in limit cases. The only 'impact' that is of essence for an EI is that produced on the 'environment' that surrounds the community under examination: although the inclination of many environmentalists is to adopt a 'globally holistic' approach, in practice a reasonable amount of determinism is necessary, and the 'environment' of the community under examination must be reasonably limited.
We see here already that the definition suggests we treat the 'community' and the 'environment' as two separate but interacting systems, and we shall return to this point in sections titled 'The concept of exergy' and 'Exergy destruction as an ecological indicator?' here below.
Desirable Properties of an Ecological Indicator
The properties of an EI descend directly from the above definition:
1. The EI must be expressed by a, possibly simple, numeric expression that produces results that can be ordered in an unambiguous way (from 'bad' to 'good').
2. The EI must be calculated on the basis of intrinsic properties of the 'community' and of the 'environment'.
3. The EI must be normalized in some sense (for instance, by expressing it as a ratio of the actual calculated value to an 'average' value calculated for all similar communities that interact with that environment, or to an 'ideal' measure of impact). This is important if we wish to compare similar but slightly different communities.
4. The EI must be calculated on the basis of an unambiguous, reproducible method under a well-defined set of fundamental assumptions.
5. The EI must comply with the accepted laws of physics.
Since the EI measures the impact of a community on its environment, its only proper use is to measure such an impact. EIs ought not to be used to establish value rankings (in the sense in which 'value' is used in economics) nor to provide guidance in ethical or social issues. It can, and should, be used to assess an individual community (a societal sector, an industrial process, an economic strategy applied to a society or one of its parts). Within these bounds, it may be used to compare alternative scenarios. An exergetic critique of some existing EIs (material throughput analysis, embodied energy, emergy, and environmental footprint) is presented by Sciubba.
In the following sections, we shall critically analyze the possibility of defining an EI based on another thermody-namic function, exergy.
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