Reducing doubt about uncertainty: Guidance for IPCC’s third assessment

It is usually necessary to apply incomplete and uncertain information to inform policy and decision making, creating the need to characterize the state of knowledge and identify when more certain information may be available. After all, some information is better than none and conversely, even perfect information is of no use if it is available only after a decision has been made. In scientific assessments for global change, the challenges are particularly acute because of scientific complexity, long time horizons, and large political and economic stakes, among other factors. Moss and Schneider prepared uncertainty guidelines for the Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) that recommended a process to make expert judgments of levels of confidence and uncertainty more systematic and transparent. The guidance provided calibrated uncertainty terms to improve communication of findings to users and urged preparation of a traceable account of the authors’ assessment of the evidence for each major finding. This article reviews the recommendations and their effectiveness and highlights ensuing critiques and the evolution of uncertainty guidance for subsequent assessment reports. It discusses emerging challenges in providing science for decision making in the era of increasing model resolution and complexity and burgeoning interest in information to inform adaptation and mitigation at regional and finer scales.