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.

To improve the reliability of future summer precipitation projections over eastern China, this study uses outcomes of climate model from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and four initial-condition large ensemble simulations. Through the perfect model test framework, the constraint variables and their spatial scales in the Climate model Weighting by Independence and Performance (ClimWIP) scheme are optimized. Results indicate that using the historical global-scale mean temperature trend as a constraint significantly improves the projection reliability compared to using smaller-scale constraints. Moreover, the ClimWIP scheme, which considers both historical global temperature trend and regional precipitation mean state, provides the most reliable projection. The optimal projection indicates that under the SSP5-8.5 scenario, summer total precipitation in eastern China will respectively increase by 6.84% (2041—2060, mid-21st century) and 12.91% (2081—2099, end-21st century) relative to the 1995—2014 baseline. More notably, summer extreme precipitation exhibits stronger responses, increasing by 16.94% and 28.66% respectively during the corresponding periods. Northern China shows even more pronounced changes by the end of the 21st century, with summer total and extreme precipitation increasing up to 19.09% and 35.53%, respectively. Compared to the multi-model ensemble mean (MME), the optimal projection scheme enhances the interannual variability of summer precipitation, indicating that future fluctuations between droughts and floods will become more pronounced. It also reduces the uncertainties of future projections for summer total and extreme precipitation, primarily for the upper bound. The largest uncertainty reduction occurs in northeastern China (about 40%) by the mid-21st century and in northern China and the Yangtze river basin (about 50%) by the end of the 21st century.