Beneath the soaring Art Deco towers of downtown Chicago, the multi-level roads, and the bustling subway and rail lines, the land is sinking, and not just for the reasons you might expect.
Since the mid-20th century, the ground between the city’s surface and bedrock has warmed an average of 5.6 degrees Fahrenheit, according to a new study from Northwestern University. All that heat, mainly from basements and other underground structures, has caused the layers of sand, clay and stone under some buildings to subside or swell a few millimeters over decades, enough to exacerbate cracks and defects in walls and foundations .
“You have heat sources all around you,” said study author Alessandro F. Rotta Loria, as he backpacked through Millennium Station, a commuter rail terminal below the city’s Loop district. “These are things that people don’t see, so it’s like they don’t exist.”
It’s not just Chicago. In major cities around the world, the burning of fossil fuels by humans raises the surface mercury. But heat also flows from basements, parking garages, train tunnels, pipes, sewers and power lines and into the surrounding earth, a phenomenon that scientists have come to call ‘underground climate change’.
Rising underground temperatures are leading to hotter subway tunnels, which can lead to overheated tracks and steamy commuters. And over time, they cause small shifts in the ground under buildings, which can create structural stress, the effects of which are not noticeable for a long time until suddenly they are.
“Today you don’t see that problem,” said Asal Bidarmaghz, a senior lecturer in geotechnical engineering at the University of New South Wales in Australia. “But in the next 100 years there is a problem. And if we sit there for the next 100 years and wait 100 years to fix it, that would be a huge problem.”
Dr. Bidarmaghz has studied subsurface heat in London, but was not involved in the Chicago study.
To assess subsurface climate change in Chicago, Dr. Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern, installed more than 150 temperature sensors above and below the Loop’s surface. He combined three years of measurements from these sensors with a detailed computer model of the district’s basements, tunnels and other structures to simulate how the ground has warmed at different depths between 1951 and now, and how it will warm from now to 2051.
Near some heat sources, the ground under the feet of Chicagoans has warmed by 27 degrees Fahrenheit over the past seven decades, he found. As a result, the earth layers under some buildings have expanded or contracted by up to half a centimeter.
Warming and deformation of the soil are now slower than they were in the 20th century, he found, simply because the Earth is almost as warm as the basements and tunnels buried within it. More and more, those structures will stay warm instead of dissipating heat to the ground around them.
Dr. Rotta Loria’s findings were published Tuesday in the journal Communications Engineering.
The most effective way for building owners and tunnel operators to tackle the problem, he said, would be to improve insulation so that less heat leaks to Earth. They could also put the heat to work. Dr. Rotta Loria is chief technology officer for Enerdrape, a start-up in Switzerland that makes panels that absorb the ambient heat in tunnels and parking garages and use it to power electric heat pumps, reducing energy bills. The company has installed 200 of its panels in a supermarket car park near Lausanne as a pilot project.
Dr. Rotta Loria purposely left out one factor in his estimates of subsurface warming in Chicago: climate change at the city’s surface.
Warm weather warms the top layers of the soil. But the calculations of Dr. Rotta Loria assumes that air temperatures in Chicago will remain at their average recent levels through 2051 — that is, his estimates do not account for climate scientists’ projections for future global warming. Nor do they explain the fact that, as we continue to warm the planet, large buildings will most likely use more air conditioning and pump even more waste heat into the ground.
The reason for these omissions, said Dr. Rotta Loria, is that he’s trying to find a conservative lower bound for subsurface warming, not a worst-case scenario. “It already shows that there is a problem,” he said.
The office of Chicago Mayor Brandon Johnson did not respond to requests for comment.
On a recent morning, Dr. Rotta Loria and Anjali Thota, a Northwestern doctoral student in civil engineering, a reporter and a photographer take a tour of their network of temperature sensors, which trace out a kind of invisible city beneath the city.
Dr. Rotta Loria said the Chicago Transit Authority wouldn’t allow him to install sensors in subway stations for fear people would mistake them for bomb detonators. But he and his team have managed to get sensors in plenty of other well-known and lesser-known places: on commuter rail platforms and at service entrances behind high-rise buildings, in leafy Millennium Park and along Wacker Drive, the cavernous concrete lair made famous by car chases. movies “Blues Brothers” and “Dark Knight”.
The sensors themselves are unremarkable: a white plastic box with a button and two indicator lights. They cost Dr. Rotta Loria $55 each. The temperature information they collect — one reading every minute or one every 10 minutes, depending on location — is downloaded to a phone via Bluetooth, meaning Dr. Rotta Loria and his students have to visit them in person periodically to collect their data, around 20,000 total records per day.
Many of the sensors have been wiped or gone over the years, leaving 100 in use. At Millennium Garages, an underground parking complex, one is zip-fastened to a tube behind a column.
“That’s all, isn’t it?” said Admir Sefo, a manager at the garage, peering at the widget. “And nobody found them?”
“Even for us it’s hard to find them,” Mrs. Thota said. She has saved their locations on Google Maps, but underground there is often no cellular reception, forcing her to hunt around.
Another sensor, at the Blackstone hotel, is in a basement room filled with chairs and bags of melting ice pellets. There’s one in the boiler room of Chicago’s Union League Club with temperatures as high as 96 Fahrenheit. A sensor in the Grant Park South parking garage registered 97 degrees in September 2021.
Just behind the walls in each of these places, out of sight and out of mind, this heat quietly does what heat does: spread.
