The streets, sidewalks and rooftops of cities all absorb heat during the day, making some urban areas up to six degrees Fahrenheit hotter than rural areas during the day — and 22 degrees F hotter at night. These “urban heat islands” can also develop underground as heat from the city diffuses down below the surface. And basements, subway tunnels and other underground infrastructure constantly vent heat to the surrounding earth, creating hotspots. Now that underground heat is building up as the planet warms.
Underground hotspots could pose a threat, according to a new study from downtown Chicago the same structures which give off the heat in the first place. Such temperature changes cause the soil around it to expand and contract enough to cause potential damage. “Without [anyone] when he realized it, downtown Chicago was warping,” said study author Alessandro F. Rotta Loria, a civil and environmental engineer at Northwestern University.
The findings, published July 11 in Communication technology, expose a “silent danger” to civilian infrastructure in cities with softer soil — especially those near water — says Rotta Loria. “There may have been structural issues caused by this subterranean climate change that has taken place, and we didn’t even know it,” he adds. While not an immediate or direct threat to human life, this previously unknown effect highlights the impacts of a lesser-known component of climate change.
“For a lot of things in the subsurface, it’s kind of ‘out of sight, out of mind,'” said Grant Ferguson, an engineering geologist at the University of Saskatchewan, who was not involved in the new study. However, the underground world teems with life. It is home to animals that have adapted to life underground, such as worms, snails, insects, crustaceans and salamanders. These creatures are used to “very static conditions,” said Peter Bayer, a geoscientist at Martin Luther University Halle-Wittenberg in Germany, who was also not involved with the paper. Above-ground temperatures often fluctuate throughout the year, but the underground remains around the annual average temperature, he explains. In Chicago, that’s about 52 degrees F.
The subsoil has “a memory that air temperatures don’t,” says Ferguson. As these stable temperatures rise due to climate change and subsurface urban development, scientists such as Ferguson and Bayer keep an eye out for the potential implications for subsurface ecosystems. For example, if groundwater gets too warm, it can kill or scare away animals, cause chemical changes in the water, and become a breeding ground for microbes.
But the question of how underground hotspots might affect urban infrastructure has gone largely unstudied. Because materials expand and contract with temperature changes, Rotta Loria suspected that heat seeping from basements and tunnels could contribute to wear and tear on various structures.
He collected temperature data over three years from more than 150 sensors installed in basements, train tunnels and parking garages under the Loop district of downtown Chicago. For comparison, sensors were also installed in the soil beneath Grant Park, which is located in the Loop, along the shore of Lake Michigan.
General ground temperatures in Chicago are rising by 0.25 degrees F each year, with readings in specific underground locations reaching as much as 27 degrees F hotter than undisturbed ground. Temperatures under Loop buildings are often 10 degrees F hotter than those under Grant Park. To understand how this large difference has affected the soil’s physical properties, Rotta Loria used a computer model to simulate the subsurface environment from the 1950s to the present – and then to predict how conditions will change from now to 2050.
He found that by the middle of this century, some areas under the Loop could rise as much as 12 millimeters (0.47 in) or sink as much as eight millimeters (0.31 in), depending on the soil composition of the area involved. . While these may sound like minor displacements, Rotta Loria says they can cause cracks in the foundations and walls of some buildings. This can lead to water damage or the overturning of buildings. In recent decades, this hidden factor could have contributed to some of the ongoing challenges and costs of maintaining these structures, he says.
Kathrin Menberg, a geoscientist at Germany’s Karlsruhe Institute of Technology who was not involved in the Rotta Loria study, says these displacement predictions are orders of magnitude higher than she guessed and could be linked to Chicago’s soft, clay-heavy soils . “Clay material is particularly sensitive,” she says. “It would be a major problem in any city around the world built on such material.” This includes many cities near oceans and rivers – London, for example, is built on a layer of clay. By contrast, cities built largely on harder rocks (like New York City) wouldn’t be as affected by this effect, says Ferguson.
As with climate change above the surface, these subsurface changes take place over long periods of time. “These effects have taken decades, a century to develop,” says Ferguson, adding that it would also take a long time for elevated underground temperatures to subside on their own. “We could actually turn everything off, and the temperature signal will be there for quite some time.”
But Ferguson and the other researchers interviewed for this story all say this wasted energy can also be reused, presenting an opportunity to both cool the subsurface and save energy costs. Subway tunnels and basements can be retrofitted with geothermal technologies to recover the heat. For example, water pipes can be installed to run through underground hotspots and capture some of the thermal energy. While that energy wouldn’t be hot enough to turn the water into steam and generate electricity, it could still be used to heat buildings and other civilian infrastructure. This approach may or may not be worthwhile because it would require a high upfront cost and, in the case of the Loop district, could represent less than 1 percent of local energy demand.
Still, this calculus is subject to change as above-ground climate change continues to amplify underground warming. In a warming world, buildings will require more electricity to stay cool, generating more wasted energy in the form of heat. However slowly, this heat will build up under our feet. “It’s like climate change,” says Rotta Loria. “It happens. Maybe we don’t always see it, but it happens.”