questions must be considered: Effective according to whom? Effective in achieving which goals over what time frame?

Despite this diversity, the record of past assessments indicates certain specific categories of contributions that assessments can make to policy debates or to decision making. Illustrative examples of such contributions are given below (Mitchell et al. 2006). While this list is not exhaustive, it captures the most important categories of contributions that are evident in the record of global change assessments of the past 30 years. Note that the ability of an assessment to make any of these types of contributions depends on the state of both the scientific and the policy context in which the assessment is conducted.

  1. Assessments have the potential to establish the basic significance or importance of an issue and elevate it onto the decision-making agenda. If an issue is not yet on the agenda of decision makers, which have with the authority and resources to address it, an assessment that assembles an review of evidence can make the case that it is serious or urgent enough to deserve their attention. The stratospheric ozone trends panel (WMO 1990a) and the Villach report (WMO 1986b), for example, exerted a decisive influence over the policy debate by showing the seriousness of ozone depletion and climate change, respectively. In fact, when the policy context for an issue is immature, this may be the only contribution that an assessment can make.

  2. Assessments have the potential to provide authoritative resolutions of policy-relevant scientific questions. Sometimes particular scientific questions come to be widely perceived as important, perhaps even decisive, for policy decisions. Important examples include the significance of an environmental change (e.g., how much ozone depletion is required to significantly impact the skin cancer rate?), the significance of the human contribution to a naturally occurring change, or discrepancies in data records or observational techniques. If the policy debate on an issue is characterized by controversy or deadlock because conflicting claims are being made about key scientific questions, an assessment can inform and advance the policy debate by authoritatively resolving these questions. Such a contribution requires both that available scientific knowledge is able to support a clear resolution and that there is a policy-making body with the issue on its agenda.

  3. Assessments have the potential to link actions to consequences. When the policy context is even more advanced—in that a decision forum and agenda have been established, a specific set of options is being considered, and actors broadly agree on the consequences of these choices—assessments can inform decisions by making specific, scientifically founded



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