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tivities on the natural world. The signals they produce sound alarms, define challenges, and measure progress. For example, measurements of carbon dioxide levels in the atmosphere warn of possible climate change, population statistics show trends in the rate of growth of the human species, and Gross Domestic Product statistics attest to a nation's prosperity. Generally, indicators are most useful when obtained over many intervals of observation so that they illustrate trends and changes. Their calculation requires concerted efforts and financial investments by governments, firms, nongovernmental organizations, and the scientific community.

Indicators are essential to inform society over the coming decades how, and to what extent, progress is being made in navigating a transition toward sustainability. Numerous efforts are under way to collect, analyze, and aggregate the information needed to form sets of indicators of environmental, societal, and technological change. These efforts range on an ecological scale from watersheds to the whole planet, and on a political scale from municipal to international institutions and activities. For reporting on a sustainability transition, however, it is clear that multiple indicators are needed to chart progress toward the goals for meeting human needs and preserving life support systems, and to evaluate the efficacy of actions taken to attain these goals. Indicators will be needed to monitor and report on human welfare and planetary life support at global, regional, and local scales to catch the appropriate signals. These signals will tell us if societies are on track or if they are headed toward unsustainability. If such indicators focus on different levels of human-environment interactions, it should be possible to measure the directions in which humanity is headed. Another set of indicators will be needed to aid navigation, and thus, help humanity steer a course toward sustainability.

We begin with an overview of the current use of indicators, and then outline an approach formulated on the basis of the Board's normative objectives; finally, we address the role of indicators in navigating the uncharted waters of a transition toward sustainability.

The Use of Indicators

Humans have made repeated, precise measurements of some phenomena since ancient times. As archeologists have deciphered the use of astronomical observations in agrarian societies, it has become clear that environmental indicators have long been used to guide human behavior. The use of indicators has expanded with efforts of industrial societies to measure and manage a widening variety of environmental and societal parameters.

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