31 October 2023

Climate change intensifies extreme heat in the soil


According to a German study, extreme heat events occur more frequently in the soil than in the air. Germany, France, Italy and central Europe are the most affected areas

by Matteo Cavallito


The global soil  is increasingly experiencing exceptionally high temperatures, a phenomenon linked to climate change that occurs less intensely in the air, however. This is supported by a study by the Helmholtz Center for Environmental Research (UFZ) in Leipzig, Germany. According to the research, published in the journal Nature Climate Change, this trend is most evident in central Europe.

The findings fill a gap in knowledge about trends in  soil temperatures, which, unlike air temperatures, have reportedly been the subject of little attention for years due to the absence of reliable data, the researchers say in a statement released by Helmholtz calling into question a “considerably more complex measurement.”

The study

Researchers, led by Almudena García-García, a scientist specialized in remote sensing at the UFZ, collected data from meteorological measuring stations, satellites and other sources. The research team then used the collected information to develop an index called TX7d and defined as the average daily maximum temperatures during the hottest week of the year.

The index reflects the intensity of the heat extremes, that is, how high the temperature extremes can be, and was calculated for the upper layer of the ground (first 10 centimeters) and near-surface air (up to 2 meters above the surface).

The analysis focused on data trends for the years 1996 to 2021 and showed that at two-thirds of the 118 stations observed, the trend in heat extremes is strongest in the ground. “This means that heat extremes develop much faster in the soil than in the air,” explains García-García, lead author of the study.

Extremes increase by 0.7 °C per decade.

The phenomenon is particularly evident in Germany, Italy and southern France. “Changes in hot extremes are often determined on the basis of air temperatures; however, hydrology and many biogeochemical processes are more sensitive to soil temperature,” the study reports.

Moreover, “soil hot extremes are increasing faster than air hot extremes by 0.7 °C per decade in intensity and twice as fast in frequency on average over Central Europe.”

This last figure appears particularly relevant. “If there are currently high temperatures in the soil and air on 10% of the days in a month,” García-García explains, “a decade later, there will be high temperatures in the air on 15% of the days and high temperatures in the soil on 20%.” Soil moisture is crucial, as it impacts the soil’s heat exchange with the air.

More frequent heat waves

The detected phenomenon has important consequences. If the ground temperature is higher than the air temperature, for example, the additional heat is released into the lower atmosphere. Thus causing atmospheric temperatures to rise. In addition, explains Jian Peng, co-author and director of the UFZ’s remote sensing department, “Soil temperature acts as a factor in the feedback between soil moisture and temperature and can thus intensify heat periods in certain regions.”

Finally, the researchers used the collected data to estimate the frequency with which extreme soil temperatures could amplify heat waves in the atmosphere.

Under a climate scenario characterized by an average temperature 2 to 3 degrees higher than pre-industrial levels, they explain, the impact on central Europe would be much greater when compared with the picture associated with a more modest rise (+1.5 degrees, in line with international mitigation targets). In particular, “there could be 8% more hot days on which the soil releases heat into the atmosphere.” Which would in turn contribute to an increase in global warming.