Increasing likelihood of global compound hot-dry extremes from temperature and runoff during the past 120 years

Compound hot-dry extremes pose great threats to human health, ecosystems, and food production. A lot of attention has been paid to compound meteorological hot-dry extremes (based on high near-surface air tem-perature and low precipitation); however, there is a gap on the compound hydrological hot-dry extremes (based on high near-surface air temperature and low surface runoff) which is more directly related to damages on social and natural systems. Here, using the ensemble empirical mode decomposition method and Copula function based on multiple global gridded datasets during 1902-2019, we find that the globe is dominated by accumulated increases in zonal average of warm-season (i.e., May-September in the Northern Hemisphere and November -March in the Southern Hemisphere) mean precipitation (WMP), while zonal average of warm-season mean runoff (WMR) mainly shows accumulated decreases. This discrepancy is possibly due to the modulation effect of zonally accelerated warming trends in warm-season mean temperature (WMT). Despite of the discrepancy in changes of individual WMP and WMR, compound meteorological hot-dry extremes (from WMT and WMP) show increased occurrence probability, expanded affected area, and enlarged spatial homogeneity (i.e., connected-ness), and these features are worse for compound hydrological extremes (from WMT and WMR). Both high WMT and low WMP can lead to negative anomalies of WMR; however, the negative anomalies of WMR can be largely amplified by additional low WMP under high WMT conditions. The increased occurrence probability of com-pound hydrological hot-dry extremes is directly related to WMT, WMR, and their dependence. Although a strong dependence is found between WMT and WMR especially at mid-latitudes in the Northern Hemisphere, attri-bution results show that increased WMT plays a dominant role in reducing return periods of compound hy-drological hot-dry extremes at the global scale. Our results provide a deeper understanding of changes in compound hydrological hot-dry extremes at the global scale from a longer-term period.