Compound Drought–Heat Wave Events Under-Recognized in Global Soils, Finds Study

By Liu Jia

Soil is essential for life and plays a crucial role in the Earth's ecosystem, providing support for plant roots and hosting countless microorganisms. In a warming world, it is important to understand how soil hydrothermal conditions, particularly the dry-hot extremes, have changed and will respond.

In a study published in the Proceedings of the National Academy of Sciences, researchers led by Prof. Zhang Yunlin from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, along with collaborators from the Helmholtz-Center for Environmental Research (UFZ) and Bangor University, have quantified global soil compound drought–heat wave (SCDHW) events from 1980 to 2023, and predicted their evolution until the end of this century.

Combining three state-of-the-art reanalysis datasets and four Earth System Model datasets, the researchers analyzed global SCDHW trends and variabilities in the past and future. They also incorporated long-term observational data to enhance their conclusions.

The researchers revealed a global increase in the occurrence rate, duration, extremeness, and severity of SCDHWs, as well as the affected area over the past 44 years. "We observed a notable escalation of SCDHWs from 1980 to 2023, particularly in this century. Global warming, beyond all doubt, took the major responsibility, and the situation worsened in El Niño years.

"More importantly, the escalation of SCDHWs was concentrated in summer, posing a significant challenge to water security," said Prof. Zhang, corresponding author of this study.

Additionally, the researchers revealed that CDHWs in soils were stronger and increased faster than those in the air. "For the sake of data accessibility, we used to express CDHWs in terms of meteorological measures such as air temperature. However, this common practice might underestimate the severity of SCDHWs, and the adverse impact on the carbon cycle," said Dr. Fan Xingwang, first author of this study.

Furthermore, the researchers found that in general, SCDHWs were more intense in the northern hemisphere and longer-lasting in the southern hemisphere. The severity of SCDHWs increased rapidly in the northern high-latitudes, where soil temperatures were typically low and warming was highly pronounced. These events may jeopardize carbon neutrality goals in the northern hemisphere and food security goals in the southern hemisphere.

Based on a space-for-time substitution approach, the researchers project that the degradation of forests and conversion of wetlands to croplands will worsen the severity of SCDHWs.