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

High temperatures and droughts pose a great threat to the human health, social economy and ecosystems. A large number of previous studies have focused on meteorological hot-dry events (based on temperature and precipitation), but there is a lack of comprehensive studies about hydrological hot-dry events (based on temperature and runoff). Here, using the ensemble empirical mode decomposition method and Copula function, we assess spatio-temporal evolution of global compound hot-dry events from temperature and runoff, and quantify their drivers based on monthly temperature and runoff data during 1902-2019. We find there is a significant warming at an unprecedented pace over the past 118 years, especially in the mid-latitudes of the Northern Hemisphere. However, changes in accumulated trends in precipitation and runoff show complex patterns globally. Probabilities of meteorological and hydrological hot-dry events both have been increasing significantly, but hydrological events are more likely to occur with higher spatial homogeneity, wider coverage and more severe damage. To analyze its underlying driving mechanism, we estimate quantitatively the contribution of high temperature, low runoff and the dependence between high temperature and low runoff to the compound event. High temperature plays a dominant role in the driving mechanism. In several regions, such as Australia, Europe and South America, hot-dry events could be considered as a potential hazard caused by increasing temperatures. Runoff deficit and dependence between the two, together with high temperature, exacerbate the occurrence of compound hot-dry events. Our findings provide a promising direction to predict joint probability of hot-dry events. Hydrological hot-dry events have seldom been considered, so far, in strategic policy formulation and risk assessment. Our results offer a powerful tool to improve planning and strategies to adapt to climate change.