The use of indicators in integrated assessments faces the same challenges as with other uses of indicators: selection of appropriate indicators, data availability, comparisons between disparate topics and forms of measurement, weighting, and total and relative numbers of indicators selected (which often implies an inherent weighting). In addition, there is the challenge of integration itself: finding indicators that reflect the whole and not just the parts. In the present state of the art of integrated assessment, this question has not yet been resolved. One approach will be through complex computerized system models that mathematically reproduce the structure and dynamics of the system. As assessments come to be based on such models, as is now at least partly the case for climate change assessments, indicators can be derived from the models to reflect system resilience, susceptibility to perturbation, and ability to maintain basic functions and outputs over long time periods. Once these new indicators of system performance and sustainability have been validated by such models, they can be implemented with models driven by real data streams.
Some specific types of indicators have an integrating aspect useful for integrated assessments, such as indicators of material flows (Adriaanse et al. 1997), energy intensity, and decoupling of resource inputs from outputs. Indicators that show vectors of trends toward or away from a sustainable state or convergence with a target can also be helpful (Dahl 1997b).
One of the most difficult aspects to treat in a methodologically and scientifically rigorous way is the underlying assumptions guiding the assessment and therefore the selection of indicators. Different individuals, organizations, sectors of society, and cultural groups have their own worldviews, visions of the future, perspectives, and values. There is an inherent tendency to select indicators and make assessments that validate a preconceived view of the world or confirm inherent biases. Such assessments tend to be more popular and influential and receive acceptance in policy circles not because they are scientifically valid or right but because they say what people want to hear. An indicator set that reflects the views of corporate leaders in a materialistic, free enterprise economic system will be very different from one prepared by environmental groups or social activists in undeveloped countries.
The methodological challenge is first to make these different perspectives and biases transparent and then to separate the normative dimension of sustainability from the scientifically verifiable trends in that particular context. Integrated measures of the sus-tainability of a system for warfare or development assistance should be possible without moral judgments about the goals of the activity. After all, the integrated index of gross national product was first developed to measure the American war effort. Once indicators of system behavior and sustainability have been developed, it will be necessary to try to step outside the context of the various dominant worldviews and to judge sustainability with respect to planetary limits, at least for the factors that can be established scientifically. This scientific perspective on sustainable limits can then be reintegrated transparently with value judgments about the choices to be made to keep the human economic and social system within those limits.
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