Living With Climate Change: Cities generate dangerous underground heat. It could be an energy source instead.

But there’s also a smart reuse for that trapped underground heat: it could be captured as a renewable thermal energy source, say the Northwestern University environmental engineers behind the findings, which were published Tuesday in the journal Communications Engineering, part of the scientific journal Nature. 

Led by Alessandro Rotta Loria, assistant professor of civil and environmental engineering at Northwestern, the team found that since the mid-20th century, the ground between Chicago’s street surface and the bedrock below has warmed by 5.6 degrees Fahrenheit on average.

The underground heat, which leaches from basements, parking garages, train tunnels, pipes, sewers, electrical cables and other structures, has caused the layers of sand, clay and rock beneath some buildings to subside or swell by several millimeters over the decades. That may sound small, but it’s been enough to worsen cracks and defects in walls and foundations for buildings not necessarily designed with underground heat in mind. Heat can also pose risks to the reliability of subway train tracks, said Rotta Loria and team.

“Underground climate change is a silent hazard,” says Rotta Loria.

Not designed with underground heat in mind

“The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations,” he said, according to a news release from the school. “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.”

Read:Here’s why there is still so much lead pipe in Chicago

The Northwestern team installed more than 150 temperature sensors above and below the surface of Chicago’s Loop, its primary business center so named because the elevated train tracks loop around this section of the city. Subway stations, where the team added sensors, complete the downtown’s local and commuter train lines. The city made for a conducive real-world laboratory because it also features urban greenspace. For comparison, the team also buried sensors in Grant Park, near Lake Michigan and away from buildings and underground transportation systems.

The team combined three years of readings from these sensors with a computer model of the district’s basements, tunnels and other structures to simulate how the ground at varying depths has warmed between 1951 and now, and how it will warm from now through 2051.

Read: The top U.S. cities labeled as dangerous ‘heat islands’ include a few small-population surprises

An urban challenge around the globe

It isn’t just Chicago’s concern, of course. In many urban areas around the globe, heat continuously diffuses from buildings and underground transportation, causing the ground to warm at an alarming rate. Previous researchers have found that the shallow subsurface beneath cities warms by 0.1 to 2.5 degrees Celsius per decade. The Northwestern research is unique in that it marks the first study to quantify ground deformations caused by subsurface heat and its effect on civil infrastructure. 

Known as “underground climate change” or “subsurface heat islands,” this phenomenon has been known to cause ecological issues (such as contaminated ground water) and health issues (including asthma and heatstroke). But, until now, the effect of underground climate change
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on civil infrastructure had remained unstudied and little understood.

“If you think about basements, parking garages, tunnels and trains, all of these facilities continuously emit heat,” Rotta Loria said. “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.” 

Read: The cost of extreme heat in the U.S.? 235,000 ER visits and $1 billion in healthcare bills this summer alone.

Data from the wireless sensing network indicated that air temperatures in underground structures can be up to 25 degrees Celsius higher compared to the undisturbed ground temperature in greenspaces. When the heat diffuses toward the ground, it puts significant stress on materials that expand and contract with changing temperatures, the research shows.

“In the United States, the buildings are all relatively new,” Rotta Loria said. “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.” 

Smart idea? Harvest and use the heat

The Northwestern engineers — like many who look at recycling carbon waste and other creative solutions for handling the very gasses that are raising Earth’s temperature since the Industrial Revolution — believe that trapped heat could be put to good use.

Going forward, Rotta Loria said future planning strategies should integrate geothermal technologies to harvest waste heat and deliver it to buildings for space heating. Planners also can install thermal insulation on new and existing buildings to minimize the amount of heat that enters the ground. 

“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. “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.” 

Read more of MarketWatch’s Living With Climate Change coverage.