In assessing and reporting the state of knowledge about climate change, scientists have devoted serious debate and discussion to appropriate ways of expressing uncertainty to policy makers (Moss and Schneider, 2000). Recent climate change assessment reports have adopted specific procedures and terminology to describe the degree of confidence in specific conclusions or the estimated likelihood of a certain outcome (see, e.g., Manning et al., 2004). For example, a statement that something is “very likely” in the assessments by the Intergovernmental Panel on Climate Change indicates an estimated 9 out of 10 or better chance that a certain outcome will occur (see Appendix D).

In estimating confidence, scientific assessment teams draw on information about “the strength and consistency of the observed evidence, the range and consistency of model projections, the reliability of particular models as tested by various methods, and, most importantly, the body of work addressed in earlier synthesis and assessment reports” (USGCRP, 2009a). Teams are also encouraged to provide “traceable accounts” of how these estimates were constructed, including important lines of evidence used, standards of evidence applied, approaches taken to combining and reconciling multiple lines of evidence, explicit explanations of any statistical or other methods used, and identification of critical uncertainties. In general, statements about the future are more uncertain than statements about observed changes or current trends, and it is easier to employ precise uncertainty language in situations where conclusions are based on extensive quantitative data or models than in areas where data are less extensive, important research is qualitative, or models are in an earlier stage of development.

In this report, Advancing the Science of Climate Change, when we draw directly on the statements of the formal national and international assessments, we adopt their terminology to describe uncertainty. However, because of the more concise nature and intent of this report, we do not attempt to quantify confidence and certainty about every statement of the science.