Operationalising the IPCC uncertainty guidance for climate policy evaluation

The rigorous treatment of uncertainties in global environmental assessments is essential to characterise the scientific state of the art and to inform policy. Recognizing this, the Intergovernmental Panel on Climate Change (IPCC) has adopted a standardized assessment framework to promote transparency and consistency in reporting the level of confidence in its key findings — most prominently those featured in the Summary for Policymakers. However, applying this framework is challenging and often leads to confidence statements based on expert judgment rather than a transparent and replicable process. Here we recommend an updated framework for assessing uncertainties that is suitable for lines of quantitative ex-post policy evaluation evidence that inform Working Group II and III reports. The framework uses evidence synthesized from systematic reviews to support robust and transparent confidence statements and to provide a clear and traceable rationale for conclusions. Where such synthesized evidence is lacking, we offer practical guidance and outline intermediary steps for improving confidence assessments. Our approach provides a concrete and replicable pathway to enhance the level of transparency and the reliability of scientific assessments that inform climate policy.