Confident, likely, or both? The implementation of the uncertainty language framework in IPCC special reports (2020)

The uncertainty language framework used by the Intergovernmental Panel on Climate Change (IPCC) is designed to encourage the consistent characterization and communication of uncertainty between chapters, working groups, and reports. However, the framework has not been updated since 2010, despite criticism that it was applied inconsistently in the Fifth Assessment Report (AR5) and that the distinctions between the framework’s three language scales remain unclear. This article presents a mixed-methods analysis of the application and underlying interpretation of the uncertainty language framework by IPCC authors in the three special reports published since AR5. First, I present an analysis of uncertainty language term usage in three recent special reports: Global Warming of 1.5°C (SR15), Climate Change and Land (SRCCL), and The Ocean and Cryosphere in a Changing Climate (SROCC). The language usage analysis highlights how many of the trends identified in previous reports—like the significant increase in the use of confidence terms—have carried forward into the special reports. These observed trends, along with ongoing debates in the literature on how to interpret the framework’s three language scales, inform an analysis of IPCC author experiences interpreting and implementing the framework. This discussion is informed by interviews with IPCC authors. Lastly, I propose several recommendations for clarifying the IPCC uncertainty language framework to address persistent sources of confusion highlighted by the authors.