A coherent set of indicators is needed to determine the rate and extent of climatic change and to provide a yardstick for measuring progress in controlling the greenhouse effect. Indicators are also indispensable to detect instabilities in the climate system. Current estimates are essentially based on simple extrapolations of linear representations of a dynamic system with numerous surprises, introducing even greater risks that should be considered when defining targets.

Although important scientific uncertainties remain, they should not keep us from implementing policies that would help achieve the targets identified here. Rather, the uncertainties should be used as a reason to periodically review and adjust targets.

The authors of this report provided drafts of their contributions. These drafts were discussed at a review workshop in Rotterdam, April 25-27, 1990, in which several additional reviewers participated. The participants in this workshop examined the results, and developed a consensus on the recommended indicators and targets for climatic change. The Executive Summary of the report was written at the workshop. After the workshop the various contributions were revised, and edited for consistence and coherence.