Emerging evidence of anthropogenic influence on weather extremes at the regional scale: Formalising and iterating the IPCC Hexagon maps

In the sixth assessment report of the IPCC, working group 1 presented regional summaries of changes in hot extremes, heavy precipitation, and agricultural and ecological drought (Figure 3; Summary for Policymakers; IPCC, 2021). In particular, they summarized and classified the observed trends in these hazards to date and the confidence with which these trends could be attributed to anthropogenic forcings. These 'Hexagon maps' have become a useful tool for policymakers and scientists alike, providing a clear overview of current evidence and highlighting where further analysis might be prioritised. However, the number of trend detection and attribution studies, the science of extreme event attribution, as well as the strength of anthropogenic forcing, have all advanced rapidly in the intervening period since this report was published. For instance, according to a database of event attribution studies compiled by CarbonBrief, by late 2020 around 350 extreme event attribution studies had been published (CarbonBrief, 2024). By the end of 2024, an additional 270 studies were published, most of which used more advanced methods and datasets, and studied events occurring at higher warming levels.

Here, we present a framework for iterating these maps as new evidence emerges, with the twin aims of transparency and interpretability for the climate science community and beyond. Expert elicitation is necessary when compiling evidence across fields, analysis methods, experiment conditioning levels, hazard types, and temporal and spatial scales. We therefore seek to provide guidance as part of this formalised framework, both on the interpretation of these regional summaries and on the steps required to come to an overall conclusion that accurately reflects current evidence. To present this framework, we showcase regions in which the evidence base has grown since AR6 and where apparently conflicting lines of evidence may be reconciled through expert elicitation. It is hoped that this will lead to an iterative and open-sourced approach to the communities' knowledge of changes in extremes and their attribution to anthropogenic forcings.