Treatment of uncertainties in IPCC Assessment Reports: past approaches and considerations for the Fifth Assessment Report

The assessments of the Intergovernmental Panel on Climate Change (IPCC), since the First Assessment Report, have involved calibrated uncertainty language and other methods aimed towards clear communication of the degree of certainty in findings of the assessment process. There has been a continuing tradition of iterative improvement of the treatment of uncertainties in these assessments. Here we consider the motivations for the most recent revision of the uncertainties guidance provided to author teams of the Fifth Assessment Report (AR5). We first review the history of usage of calibrated language in IPCC Assessment Reports, along with the frameworks for treatment of uncertainties that have been provided to IPCC author teams. Our primary focus is the interpretation and application of the guidance provided to author teams in the Fourth Assessment Report, with analysis of the successes and challenges in the application of this guidance and approaches taken in usage of its calibrated uncertainty language. We discuss the ways in which the AR5 Guidance Note attempts to refine the calibrated uncertainty metrics and formalize their interrelationships to improve the consistency of treatment of uncertainties across the Working Group contributions.