A multivariate copula framework quantifies the augmenting joint risks of compound high temperature and dry/wet events under climate change in China

As climate change intensifies, the urgency to accurately quantify the risks associated with compound climate events increases, yet the multi-dimensional integrated risk assessment of these events remains very weak. Here, we assessed the occurrence characteristics of compound high temperature and dry events (CHTDE) and compound high temperature and wet events (CHTWE) in the historical and future using daily data from the Coupled Model Intercomparison Project Phase 6 (CMIP6), and constructed a three-dimensional copula model to investigate their joint occurrence risks in China. In future emission scenarios, the precipitation threshold line are projected to shift northward, and the extent of high temperatures will continuously expand. These changes directly influenced the spatial distribution of two types of compound events (CEs). Under the highest emission scenario, China is likely to experiences an increase in the total/maximum duration, frequency, and magnitude of both types of CEs, with CHTDE and CHTWE rising by 25%-55% and 92%-677%, respectively. If emissions were controlled under the SSP245 scenario, the occurrence of CHTDE could decrease overall by 11.5%-30%. Specifically, 87% and 93% of regions will experience more frequent, larger, and more intense of CHTDE and CHTWE. Historically, CEs were concentrated in July and August, but it is likely to occur more frequently across a broader range of months in future. The three-dimensional copula analysis highlights the complex interdependencies among the duration, frequency, and intensity of CEs under different climate scenarios. Our findings further emphasize the inrease in severity of CEs in future scenarios, with a noticeable reduction in their return periods, compared to two-dimensional copula analysis. We quantitatively assessesed the occurrence risk of CEs, explored the mechanisms of their occurrence and persistence, and emphasized that CHTWE should warrant greater attention.