Asphalt, population density and tree canopy aren’t the only factors that are leading to the urban heat island effect, and it’s the rising heat below the streets that may drive long-term threats to buildings in global cities, according to new research from the United States.

Scientists from Northwestern University placed more than 150 temperature sensors across the Loop, a core district in the city of Chicago that’s the second-most densely populated district in the nation. They monitored the temperature changes underground for three years and modeled how those changes occur from 1951 (the year Chicago added its subway trains) into 2050.

They also compared the results with data from monitors in Grant Park, a green space along nearby Lake Michigan that’s comparatively free of traffic and buildings.

“Data from the wireless sensing network indicated that underground temperatures beneath the Loop are often 10 degrees Celsius warmer than temperatures beneath Grant Park,” the university said in a statement. “Air temperatures in underground structures can be up to 25 degrees Celsius higher compared to the undisturbed ground temperature. When the heat diffuses toward the ground, it puts significant stress on materials that expand and contract with changing temperatures.”

The results, published Tuesday in the journal Communications Engineering, showed an average ground warming rate of 0.49°C per year in the Loop, down to 100 meters of depth, across the past 70 years. That rate appears to be slowing and is currently an annual rate of 0.14°C.

And with those temperature changes, the risk to building safety rises with foundation damage and structural cracks. Europe’s older buildings may be even more susceptible to threats than younger buildings in the U.S., as the study notes that the science behind their findings in Chicago may apply beyond that city alone. The news isn’t all bad, though, since planners can integrate geothermal technologies into the urban landscape to harness the power of below-ground urban heat.

There are a lot of sources that cause the “silent hazard” of underground heating. Buildings, trains, cars, sewers and high-voltage cables all contribute to heating the ground beneath a city.

“If you think about basements, parking garages, tunnels and trains, all of these facilities continuously emit heat,” said study author Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern.

“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.”

Previous studies of the underground urban effect linked it to health risks, like asthma, and environmental damage like groundwater contamination. But in the Loop, the amount of urban heating underground is enough to cause subsidence that means the buildings on sinking land may be at risk. Subsidence often occurs due to weight and water, but the author says this is the first study to demonstrate the effect due to underground heat.

“As a result of temperature increases underground, many foundations downtown are undergoing unwanted settlement, slowly but continuously,” Loria said. “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.”

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