Comparing IPCC assessments: how do the AR4 and SREX assessments of changes in extremes differ?

Abstract

The assessments of observed, attributed, and projected changes in extremes in two recent reports, the Intergovernmental Panel (IPCC) on Climate Change Fourth Assessment of Climate Change (AR4) and the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) are compared. This comparison focuses on global-scale changes in extremes as summarized in two “summary” tables from the AR4 and SREX, respectively. Many of the compared SREX and AR4 assessments of changes in extremes are essentially identical or in partial agreement, once the different language and different approaches to estimating uncertainty are taken into account. Two main exceptions are tropical cyclones and droughts. In the case of tropical cyclones, a less confident SREX assessment of past changes reflects post-AR4 studies that have improved our understanding of “the uncertainties in the historical tropical cyclone records, the incomplete understanding of the physical mechanisms linking tropical cyclone metrics to climate change, and the degree of tropical cyclone variability”. In the case of droughts, “(d)efinitional issues and lack of data…plus the inability of models to include all the factors likely to influence droughts” have led to overall weaker SREX assessments than was the case in AR4, both for observed and projected changes, although differences in the statements being assessed also explain some of the differences. Increased consistency of approach between assessments would simplify future attempts to compare the assessed uncertainties associated with changes in extremes, although changes in the wording of such assessments also needs to be considered. For instance, some aspects of the SREX assessments were the consequence of the revised IPCC uncertainty guidance, which was prepared mid-way through the SREX process.