Climate change is causing global temperatures to rise, leading to droughts, heat waves and wildfires. It is warming the surface of the ocean, intensifying hurricanes and increasing acidity and ecosystem imbalances.

But the climate crisis is also happening beneath our feet, in a phenomenon called “underground climate change.”

The concept has been studied for years surrounding issues of railroad tracks buckling in the heat and groundwater contamination, according to CNN.

However, it was not until recently, in a new study by Alessandro F. Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern University, that the effects of underground climate change — also known as “subsurface heat islands” — on civil infrastructure were examined, a press release from Northwestern University said.

“Underground climate change is a silent hazard,” said Rotta Loria in the press release. “The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations. Although this phenomenon is not dangerous for people’s safety necessarily, it will affect the normal day-to-day operations of foundation systems and civil infrastructure at large.”

The study, “The silent impact of underground climate change on civil infrastructure,” was published in the journal Communications Engineering.

When the ground gets hotter, it can expand, contract and become misshapen, causing the foundations of buildings to move and sometimes crack. This can affect the soundness and performance of the structures.

“Chicago clay can contract when heated, like many other fine-grained soils. As a result of temperature increases underground, many foundations downtown are undergoing unwanted settlement, slowly but continuously. In other words, you don’t need to live in Venice to live in a city that is sinking — even if the causes for such phenomena are completely different,” Rotta Loria said.

Earlier research discovered warming of 0.1 to 2.5 degrees Celsius each decade for cities’ subsurfaces.

“If you think about basements, parking garages, tunnels and trains, all of these facilities continuously emit heat,” Rotta Loria said in the press release. “In general, cities are warmer than rural areas because construction materials periodically trap heat derived from human activity and solar radiation and then release it into the atmosphere. That process has been studied for decades. Now, we are looking at its subsurface counterpart, which is mostly driven by anthropogenic activity.”

Rotta Loria and the research team installed more than 150 above and below ground wireless temperature sensors along the Chicago Loop. The researchers placed them in subway tunnels, basements, subsurface streets and underground parking garages. They also buried some in Grant Park for comparison.

The team found that the temperatures under the Chicago Loop were frequently 10 degrees Celsius warmer than those under Grant Park.

Underground air temperatures can be as much as 25 degrees Celsius hotter than the temperature of intact land, potentially leading to problems of warping, cracking and people using the structures getting heat stroke.

“We used Chicago as a living laboratory, but underground climate change is common to nearly all dense urban areas worldwide. And all urban areas suffering from underground climate change are prone to have problems with infrastructure,” Rotta Loria said.

Rotta Loria completed a 3D computer model after three years of collecting temperature data that was able to simulate how subterranean temperatures had evolved since 1951 — the year the city finished its subway tunnels. The values were comparable to the field data, so Rotta Loria used those to predict the evolution of temperatures until 2051.

Rotta Loria also came up with a model for the deformation of ground in response to rising temperatures. Materials like stiff and soft clay contract when they are heated, while materials such as limestone and hard clay expand.

The simulations showed that warmer temperatures can lead to ground expanding and swelling by up to 12 millimeters. Increased temperatures can also lead to ground contraction and sinking beneath a building’s weight by up to eight millimeters. It may not sound like much, but it is more than many foundations and components of buildings can take and still function.

“Based on our computer simulations, we have shown that ground deformations can be so severe that they lead to problems for the performance of civil infrastructure,” Rotta Loria said in the press release. “It’s not like a building will suddenly collapse. Things are sinking very slowly. The consequences for serviceability of structures and infrastructures can be very bad, but it takes a long time to see them. It’s very likely that underground climate change has already caused cracks and excessive foundation settlements that we didn’t associate with this phenomenon because we weren’t aware of it.”

Rotta Loria emphasized that newer buildings fare better than older ones.

“In the United States, the buildings are all relatively new. European cities with very old buildings will be more susceptible to subsurface climate change. Buildings made of stone and bricks that resort to past design and construction practices are generally in a very delicate equilibrium with the perturbations associated with the current operations of cities. The thermal perturbations linked to subsurface heat islands can have detrimental impacts for such constructions,” said Rotta Loria.

Rotta Loria pointed out that building planners can minimize the amount of heat that goes into the ground by installing thermal insulation.

“The most effective and rational approach is to isolate underground structures in a way that the amount of wasted heat is minimal,” Rotta Loria said in the press release. “If this cannot be done, then geothermal technologies offer the opportunity to efficiently absorb and reuse heat in buildings. What we don’t want is to use technologies to actively cool underground structures because that uses energy. Currently, there are a myriad of solutions that can be implemented.”

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