When do probability distribution functions (PDFs) about future climate misrepresent uncertainty? How can we recognise when such misrepresentation occurs and thus avoid it in reasoning about or communicating our uncertainty? And when we should not use a PDF, what should we do instead? In this paper we address these three questions. We start by providing a classification of types of uncertainty and using this classification to illustrate when PDFs misrepresent our uncertainty in a way that may adversely affect decisions. We (...) then discuss when it is reasonable and appropriate to use a PDF to reason about or communicate uncertainty about climate. We consider two perspectives on this issue. On one, which we argue is preferable, available theory and evidence in climate science basically excludes using PDFs to represent our uncertainty. On the other, PDFs can legitimately be provided when resting on appropriate expert judgement and recognition of associated risks. Once we have specified the border between appropriate and inappropriate uses of PDFs, we explore alternatives to their use. We briefly describe two formal alternatives, namely imprecise probabilities and possibilistic distribution functions, as well as informal possibilistic alternatives. We suggest that the possibilistic alternatives are preferable. -/- . (shrink)

An uncertainty report describes the extent of an agent’s uncertainty about some matter. We identify two basic requirements for uncertainty reports, which we call faithfulness and completeness. We then discuss two pitfalls of uncertainty assessment that often result in reports that fail to meet these requirements. The first involves adopting a one-size-fits-all approach to the representation of uncertainty, while the second involves failing to take account of the risk of surprises. In connection with the latter, we respond to the objection (...) that it is impossible to account for the risk of genuine surprises. After outlining some steps that both scientists and the bodies who commission uncertainty assessments can take to help avoid these pitfalls, we explain why striving for faithfulness and completeness is important. (shrink)

Les décisions politiques sur les risques environnementaux complexes font fréquemment intervenir des éléments scientifiques contestés. Il n’y a généralement pas de « faits » qui conduisent à une politique correcte unique. Les éléments de preuve qui sont intégrés dans les avis scientifiques destinés à une décision politique nécessitent une évaluation de leur qualité. En 2003, l’Agence néerlandaise d’évaluation environnementale a adopté une méthode standardisée, désignée sous le nom de « guide », dans le cadre de laquelle les principaux aspects de (...) la production et de l’utilisation des connaissances sont présentés grâce à une liste de contrôle visant à l’évaluation et à la communication des incertitudes. Dans cet article, nous présentons des résultats de l’application de ce guide à la controverse sur les risques des particules en suspension. La délibération active sur l’incertitude dans un contexte d’expertise entraîne un processus d’apprentissage commun entre les experts et les décideurs politiques, ce qui conduit à une meilleure prise de conscience du phénomène d’incertitude et de ses implications politiques.Policy decisions on complex environmental risks often involve disputed science. Typically, there are no “ facts” to support a single unequivocally correct policy. The evidence provided in scientific studies for policy-making decisions requires high-quality assessment. In 2003, the Netherlands Environmental Assessment Agency adopted a standardized method, referred to as “guidelines”, whereby key quality aspects of knowledge production and use are presented in the form of a checklist for uncertainty assessments and communication. In this article, we present the outcomes of applying the guidelines in controversies on the risks of ambient particulate matter. In the policy–advisory context, active deliberation on uncertainty produces a common learning process for advisors and policy makers, which leads to a deeper understanding and increased awareness of the phenomenon of uncertainty and its policy implications. (shrink)