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As global warming intensifies and populations continue to grow, the likelihood of extreme high-temperature events is increasing. Hot–dry compound events, in particular, pose a direct threat to human health. High temperatures can have prolonged and delayed effects on people's health; and under conditions of low relative humidity, the mortality associated with extreme heat rises significantly.

Previous climate risk analyses have largely focused on the abnormal state of a single variable, but concurrent extreme events, i.e., compound extreme events, tend to have more severe impacts on the environment and human systems than individual extremes. Therefore, the study of compound extreme events related to both is urgently needed.

A recent study by a Chinese joint team from the Institute of Atmospheric Â鶹ÒùÔºics, Chinese Academy of Sciences, Nanjing University of Information Science and Technology, Tsinghua University, and the National Climate Center, China Meteorological Administration, reveals a decadal-scale increase in hot–dry events with a high risk of mortality over the past 20 years. The findings have recently been in Atmospheric and Oceanic Science Letters.

The study integrates meteorological and mortality data across China to identify high-risk mortality thresholds for hot–dry events. Based on these thresholds, the frequency of the compound extreme events was quantified.

According to the study, hot–dry events with a high risk of mortality occur frequently in the northwestern–western and southwestern–southern regions in China, exhibiting an interdecadal upward trend. This type of compound event exhibits interdecadal variability, with an abrupt change occurring in the late 1990s.

"The interdecadal variability of hot–dry events with a high risk of mortality is associated with phase transitions of the Atlantic Multidecadal Oscillation," says one of the corresponding authors, Dr. Zhao, a researcher from the Institute of Atmospheric Â鶹ÒùÔºics, Chinese Academy of Sciences.

"In the late 1990s, the Atlantic Multidecadal Oscillation shifted from a negative to a positive phase, inducing anomalous circulation over the North Atlantic. These anomalies propagated eastward, leading to an increase in the frequency of this type of compound event."

The researchers demonstrate that anticyclonic circulation over northern China facilitates a reduction in moisture transport to the Yangtze River Basin, thereby weakening the East Asian summer monsoon, which leads to an interdecadal increase in hot–dry events with a high risk of mortality in the regions south of the Yangtze River.

Overall, when dealing with extremely high-temperature and low-humidity environments, it is not only necessary to strengthen the protection of human health in the cities of Northwest and North China, but also to closely monitor the populations of cities close to the south of the Yangtze River basin, to prevent harm being caused by such hot–dry compound extreme events.