Beneath the towering Art Deco towers of downtown Chicago and its multi-level avenues, subways and busy rail lines, the earth is sinking, and not just for the reasons you might expect.
Since the mid-20th century, the ground between the city’s surface and the shale has warmed by an average of 5.6 degrees Fahrenheit, according to a new study from Northwestern University. All that heat, mostly coming from basements and other underground structures, has caused layers of sand, mud and rock beneath some buildings to subside or swell by several millimeters over decades, enough to exacerbate cracks and defects in walls and foundations.
“All around you, you have heat sources,” said study author Alessandro F. Rota Lauria, as he walked with a backpack through Millennium Station, a commuter rail station 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, humans burning fossil fuels cause more mercury to surface. But heat also flows from basements, parking lots, train tunnels, pipes, sewers, and electrical cables and into the surrounding ground, a phenomenon scientists have taken to calling “underground climate change.”
Higher underground temperatures result in warmer subway tunnels, which can lead to overheating of tracks and steam bath conditions for passengers. Over time, they cause small shifts in the ground beneath buildings, which can cause structural stress, the effects of which go unnoticed for a long time until they suddenly appear.
“Today, you don’t see this problem,” said Asal Bidarmages, 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 for the next 100 years and wait 100 years for it to be resolved, that will be a big problem.”
Dr. Pedermages studied geothermals in London but was not involved in the research in Chicago.
To assess climate change underground in Chicago, Dr. Ruta Lauria, assistant professor of civil and environmental engineering at Northwestern University, installed more than 150 temperature sensors above and below the surface of the ring. He combined three years of readings from these sensors with a detailed computer model of basements, tunnels and other structures in the area to simulate how the Earth would warm at various depths between 1951 and now, and how warm it will become between now and 2051.
It found that near some heat sources, the ground under Chicago residents’ feet has warmed 27 degrees Fahrenheit over the past seven decades. This caused the earthen layers to expand or contract by half an inch under some buildings.
He found that the warming and deformation of the Earth is happening more slowly now than it was in the 20th century, simply because the Earth is as close to being warm as the cellars and tunnels buried within. More and more, these structures would stay warm rather than dissipate heat into the ground around them.
Dr. Ruta Luria’s findings were published Tuesday in the Journal of Communications Engineering.
He said the most effective way for building owners and tunnel operators to address the problem is to improve insulation to reduce heat escape to the ground. They can also turn on the heat. Dr. Ruta Lauria is the chief technology officer of Enerdrape, a startup in Switzerland that makes panels that absorb ambient heat in tunnels and parking garages and use it to power electric heat pumps, reducing utility bills. The company installed 200 panels in a parking garage near Lausanne as a pilot project.
Dr. Ruta Luria intentionally did not include a single factor in his estimates of global warming for Chicago: climate change on the city’s surface.
Hot weather warms the upper layers of the soil. But Dr. Ruta Luria’s calculations assume that air temperatures in Chicago are still at their most recent average levels throughout 2051 — meaning his estimates don’t include climate scientists’ projections of future global warming. They also don’t account for the fact that as we continue to warm the planet, large buildings are likely to use more air conditioning and pump more waste heat into the ground.
The reason for these omissions, said Dr. Ruta Luria, is that he is trying to figure out a conservative bottom line on underground global warming, not the worst case scenario. “It really shows that there is a problem,” he said.
Chicago Mayor Brandon Johnson’s office did not respond to requests for comment.
One morning, Drs. Ruta Lauria and Anjali Thuta, a PhD candidate in civil engineering at Northwestern University, take a reporter and photographer on a tour of their network of temperature sensors, which is tracking a kind of invisible city beneath the city.
Dr. Ruta Lauria said the Chicago Transit Authority did not allow him to install sensors in subway stations for fear that people would mistake them for detonators. But he and his team managed to insert sensors in plenty of other well-known and lesser-known locations: on commuter rail platforms and service entrances behind tall tower blocks, in leafy Millennium Park and down Wacker Drive, the cavernous concrete lair made famous by car chases in the “Blues Brothers” movies and “Dark Knight”.
The sensors themselves are not described: a white plastic box with a button and two LEDs. They cost Dr. Ruta Lauria $55 each. The temperature information they collect — one reading every minute or one every 10 minutes, depending on the location — is downloaded to a phone via Bluetooth, which means Dr. Ruta Luria and his students must visit them periodically to collect their data, about 20,000 records per day in total.
Many sensors have passed or disappeared over the years, leaving 100 in service. At Millennium Garages, an underground parking complex, one of which is piped behind a column.
“That’s all there is to it, isn’t it?” said Admir Sefo, the executive at the garage, as he stared at the lot. “And no one found them?”
“It’s even hard for us to find,” Ms. Thuta said. They have their locations saved on Google Maps, but oftentimes there is no cellular reception underground, forcing them to dig around.
Another sensor, at the Blackstone Hotel, is in a basement room filled with chairs and bags of de-icing pellets. One in the boiler room of the Union League club in Chicago has recorded temperatures as high as 96 Fahrenheit. A sensor in the Grant Park South parking garage recorded 97 degrees in September 2021.
Beyond the walls in each of these spots, out of sight and out of mind, this heat silently does what heat does: diffuse.