Calibrated Language: Communicating Confidence and Likelihood in the Fifth National Climate Assessment

The Fifth National Climate Assessment (NCA5) is the preeminent source of climate information for the United States. All chapters in NCA5 present “key messages,” which are high-level assessment findings that use risk-based framing to lay out key attributes and assets that are vulnerable to climate change. Key messages almost always include calibrated language, standardized terms used to convey information about the degree of scientific confidence and the likelihood associated with assessment findings. Calibrated language serves an important function in assessments such as NCA5, providing transparent information about uncertainty and likelihood that helps decision makers make a nuanced determination of risks, impacts, and possible responses. Chapter authors collectively generate confidence statements by surveying the literature and reaching a group consensus regarding the general strength and agreement of the evidence base. The selection of likelihood statements follows a similar collaborative process but has more rigidly defined probabilistic criteria. Determining this consensus language is a challenging process that involves large amounts of qualitative and quantitative information and varying expert opinion.

Here we present analyses of the use and development of calibrated language in NCA5 and the evolution of topic areas, confidence, and likelihood between NCA4 and NCA5. We also discuss how to make the process of developing and presenting likelihood and confidence more useful and robust.