Earth’s air is warming. In Europe, for example, the environment is the driest it has been in 400 years. Oceans and lakes are also warming. The ice in Greenland, Antarctica and glaciers are also shrinking in parallel. All these processes underlie global warming because human-generated gases trap an increasing portion of solar radiation that once bounced back into space. But there is one last component of the Earth’s climate system which for its scholars is “the great forgotten one”: the earth under our feet. A work published today in Advances in science shows how the planet’s soil is warming at a rate not seen for millennia.
Ambient heat, the surface air temperature, is only a fraction, and a very small one, of the warming taking place. Most of the excess energy resulting from the imbalance between the radiation arriving from the Sun and that re-emitted into space remains in the water. The oceans and seas store almost 90% of that heat. Meanwhile, the entire atmosphere contains just 1.3%. The soil, the layer immediately below the surface, stores between 5% and 6%.
“The atmosphere is made up of gases and its heat capacity is not very high,” recalls Francisco José Cuesta, researcher at the University of Leipzig (Germany) and first author of this research on soil warming. “A gas can increase its temperature with little energy, but the ocean, the water, needs more to change one degree, which is why the oceans store 89% of the accumulated heat. The subsurface comes next, because the heat capacity of the rock is greater than that of air, but lower than that of water,” explains Cuesta.
Combining the various sources of satellite data, above ground and underground, they saw that the heat accumulated in the Earth’s crust has increased since 1960 between 16 zettajoules and 21 zettajoules (one zetta equals one sextillion, one thousand trillion or 1021) (and one joule per second is equal to one watt). To grasp the dimension of soil warming, the researcher of the Institute of Geosciences (IGEO), a joint center of the Superior Council of Scientific Research and the Complutense University of Madrid, Félix García, co-author of the study, makes a double comparison: “It would be equivalent to the global energy consumption for 30 years (currently it is 0.6 ZJ/year), or the energy of 400 major hurricanes,” he calculates.
To measure environmental heat there are thousands, perhaps millions, of thermometers in as many weather stations. Additionally, they are aided by sensors placed on the growing constellation of satellites. Without the same capillarity, sea temperatures are also widely monitored. But recording thermal changes in the ground is a little more complicated. You have to open a narrow hole and descend hundreds of meters, occasionally taking measurements to create a profile based on the depth. Geophysicists know that temperature increases with depth. It is the basis of geothermal energy and the hopes placed in this energy source.
“This data consists of temperature profiles with depth,” explains Cuesta. “Someone made a narrow hole in the ground, usually it was mining companies looking for minerals. And then, what the others did was go behind them with a thermometer and measure how the temperature changed,” adds the researcher. These other people are usually geophysicists like the Spanish researcher. “From inside the Earth we have a constant heat flow on a scale of millions of years. If you only took this flow into account, you would get a profile in the shape of a straight line,” explains Cuesta. “But when you go to measure, you don’t see this. You have a distortion of that profile that should be in equilibrium, which is greater the closer you are to the surface. This tells you how the temperature on the surface has changed,” he concludes.
Indeed, geothermal data shows how the ground is warming. The problem is that the data series stops at the end of the century. The implementation of stricter environmental legislation on mining companies, which, for example, forces them to seal the holes they create once prospecting has been discarded, has left scientists without holes. For this reason, Cuesta, García and other researchers combined available geothermal information with surface temperature records and that obtained from satellites (used with weather sensors since the 1970s) to infer heat beneath the surface.
“Soil and subsoil, so to speak, are the great forgotten in the study of climate,” explains García, from IGEO. “But it is a very important, if not essential, part of the Earth’s climate system, because of its interaction with the atmosphere and its role as a reservoir of water, carbon and heat,” he adds. What lies behind global warming is a change in the planet’s energy balance. “Every day, every month, every year, a small part of the energy remains, it doesn’t escape,” explains García. “This radiative imbalance not only affects the atmosphere, warms the atmosphere, but warms the ocean, warms the cryosphere, and warms the earth,” he concludes. Although the specific values vary depending on the type of substrate, for a typical soil, “the daily temperature would take about six hours to reach the first 20 centimeters of depth, fifteen hours to reach 50 centimeters and you would no longer see it at ten meters because it would be completely attenuated”, calculates García. But the long-term changes go much deeper.
Soil works like a thermometer, but on much larger time scales. Hugo Beltrami, also a co-author of the research and a geophysicist at the University of San Francisco Javier (Antigonish, Canada), has spent decades studying it. “In the long term, the upper part of the Earth’s crust is in thermal equilibrium. This means that the temperature increases predictably with depth, driven by the planet’s internal heat and the long-term average surface temperature,” he explains in an email. But, he adds, “as surface temperatures rise or fall due to climate change, this balance is altered.”
The result is that the ground gains or loses heat and this thermal anomaly slowly propagates downward. “A secular warming, for example, will leave its mark as a deviation from the expected temperature up to about 150 meters depth,” continues Beltrami. The results of the new work show that between 1960 and 2024 the accumulation of heat underground will not only grow, but accelerate. “Because the subsurface attenuates short-term climate variability, these underground records often reveal long-term trends more clearly than meteorological data alone,” he concludes.
The consequences of soil warming are many. The terrains of the higher latitudes of the Northern Hemisphere have been frozen for hundreds of thousands of years, but now this permafrost It’s melting. “The Earth’s shallowest subsoil contains a global reserve of carbon three times greater than that present in the atmosphere, of which approximately half is sequestered in the permafrostrecalls Beltrami. But its heating provides the thermal energy needed to activate microbial metabolism, releasing this carbon from the soil.
Further south, in unfrozen lands, the increased heat they contain could accelerate the global aridization already underway. And as Cuesta from Leipzig reminds us, in cases of extreme heat, warm ground helps make the air even warmer. “What we are seeing is that heat waves, measured as ground temperature, are increasing faster in frequency and intensity than those in the air,” he points out. Another consequence that is already being observed is the impoverishment of subsoil life and soil ecosystems, with which, concludes the Spanish researcher, “communities of microorganisms must migrate, adapt or even disappear”.
