Soil is essential for life and plays a crucial role in 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 dry-hot extremes, have changed already and how they will respond to further warming.
In a study published in PNAS , 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 Centre for Environmental Research (UFZ) and Bangor University, have quantified global soil compound drought-heatwave (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 the study.
In addition, 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 the 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. Integrated watershed management requires sustainable policies and actions to protect soils from the risks of desiccation and overheating.
"We must continue our efforts to preserve natural ecosystems," said Prof. ZHANG.
If no action is taken, formidable global SCDHWs will occur by the end of this century, with a mean duration exceeding 70 days and a soil temperature anomaly reaching 10 °C under the SSP5-8.5 emission scenario.
"The mean duration of individual SCDHWs will increase substantially, which means that soil biota and plant root systems will struggle to recover from extreme water and heat stresses," said Dr. FAN.
