Constrained Projection of the Frequency of Compound Hot and Dry Events across Global Land Areas ^※

Global warming has intensified the risk of compound events, among which compound hot and dry events (CHDEs) have drawn considerable attention due to their high frequency and severe impacts. In this study, based on observational data and simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we compare the performance of indirect and direct emergent constraint approaches in projecting the frequency of global CHDEs. The results show that by the end of the 21st century, the frequency of CHDEs increases significantly over most global land areas, with the largest increases occurring in North America, South America, and Europe, while the increase is relatively smaller in eastern and southern Asia. Attribution analysis indicates that temperature rise is the primary driver of increased CHDEs frequency, whereas spatial variations in precipitation contribute to regional differences. The indirect constraint, based on global warming magnitude, effectively reduces projection uncertainty in most regions, with the largest reduction in uncertainty range reaching 37.4% (25.4%) for Shared Socioeconomic Pathways (SSP) 5-8.5 (SSP2-4.5) scenario. The direct constraint, established using the relationship between historical precipitation trends and future CHDEs frequency, can reduce the uncertainty range by up to 15.99% (14.87%) for SSP5-8.5 (SSP2-4.5) scenario. Overall, the indirect constraint demonstrates broader applicability and achieves greater reductions in projection uncertainty compared to the direct constraint.