Assuming that an integrated assessment is intended to report on sustainability, the most important and difficult definition is that of sustainability itself. Sustainability is not a goal to be achieved at some point in time but a characteristic of a dynamic human—environmental system able to maintain a functional productive state indefinitely (Dahl 1996, 1997a). Integrated indicators of sustainability therefore should measure the functional system processes that best represent its capacity to continue far into the future. Defining sustainability in terms of durability over time avoids the problem of specifying the characteristics of the system or entity to be maintained, which can be very subjective and specific, and where political, philosophical, and cultural differences can prevent any wide consensus. The optimal sustainability indicators are those that best show a scientifically verifiable trajectory of maintenance or improvement in system functions. Although the choice of indicators depends on the system in question, it is not their substance but their dynamic change over time that is important for measuring sustainability. Science cannot always validate the goals set for the system, but it can validate the ability of the indicators chosen to measure the trajectory toward those goals or the reduction in damaging factors threatening the system's sustainability.
Scientific approaches can also help us understand or model the complex operation of the system and thus ensure that the indicators selected reflect its most essential characteristics and are able to measure its sustainability within the limits of predictable system behavior.
Although sustainability assessment is needed most often in the context of sustainable development, and most integrated assessments specifically aim to do this, the concepts are not synonymous. The term development is often erroneously equated with growth, which by definition is not infinitely sustainable in a finite system. Sustainability requires the redefinition of development to mean improvements in human welfare and prosperity, including poverty reduction but respecting planetary limits, which may entail limited growth in some areas and perhaps reductions in consumption in others.
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