2023-09-29 05:00:07
Sunflowers burned during a heat wave, in the suburbs of the village of Puy-Saint-Martin (Drôme), August 22, 2023. The temperature there reached 43°C. JEFF PACHOUD / AFP
Soil temperature is a still relatively unexplored area of global warming. For several decades, scientists have mainly compiled and analyzed data from the air and oceans thanks to thousands of meteorological stations scattered on land, most often at a height of a few meters, but also on ships, buoys, probes. marine, also thanks to satellite measurements. But how do the surfaces of continents endure extreme heat? To answer this question, a team of scientists from the German Helmholtz-Centre for Environmental Research (UFZ) compiled data from 118 stations. Their study, published Thursday September 21 in the journal Nature Climate Changeis alarming.
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In two thirds of the 118 measuring points, the increase in extreme temperatures is greater in the ground than in the air. More precisely, the intensity of extreme heat in Central Europe is increasing per decade by 0.7°C faster under the surface than in the air. “We were the first surprised by this differencecomments climatologist Almudena Garcia-Garcia, head of the study. This can have a real influence because, under certain conditions, the soil accentuates heatwaves by releasing energy towards the lower atmosphere at night, and even sometimes during the day. »
To carry out this work, scientists deciphered data from stations measuring temperature at a depth of 10 centimeters and up to 2 meters in height, satellite observations and climate model simulations. Taking as a reference the statistical average calculated over the period 1996-2021, they compared the average maximum temperatures of the seven hottest days of the year in the soil and in the air, as well as the number of heat waves over ninety days: 66% of the readings show a higher average increase in the soil than in the air. A trend that is particularly marked in the south of France, Germany and Italy.
A worrying feedback
According to their results, these extreme temperatures are also twice as frequent in soils. “In a context of global warming, these differences and the distribution of energy on the earth’s surface will be important factors in explaining future changes, particularly in the frequency and intensity of heatwaves”write the researchers.
Because the study also shows that soils can release heat to the atmosphere, with an amplifying effect on drought. On wetter land, such as in the shelter of a forest, the energy released tends to increase the water content of the atmosphere, leading to higher precipitation rates. On the other hand, on drier soil, net radiation increases the surface temperature more strongly.
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