“You don’t need to live in Venice to live in a city that is sinking.”

A study unveiled a hidden danger that could wreak havoc on major cities around the world.

What happened?

Researcher Alessandro Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern University, published a paper in Communications Engineering last July titled “The silent impact of underground climate change on civil infrastructure.”

You may have heard of the urban heat island effect, in which overabundances of concrete and other constructed materials exacerbate the consequences of warm weather, trapping heat from solar radiation and human activity and releasing it into the atmosphere.

With subsurface heat islands, heat from buildings and underground transportation warms the shallow layer beneath cities by 0.1 to 2.5 degrees Celsius per decade, according to previous research, per a news release about the study.

Rotta Loria noted the Chicago underground had been warming at 0.49 C per year but has slowed to 0.14 C annually.

He and a team of scientists installed more than 150 above- and below-ground temperature sensors across the Chicago Loop. They used basements of buildings, subway tunnels, underground parking garages, subsurface streets, and Grant Park, a greenspace absent of buildings or underground public transportation.

Data showed subsurface temperatures in the Loop were “often” 10 degrees warmer than those in Grant Park and up to 25 degrees warmer, per the release.

Why is this concerning?

Rotta Loria also simulated how ground temperatures have changed since 1951 when subway tunnels were completed in the Loop, and how they will change through 2051. He posited that cities with older buildings, such as those in Europe, could be more susceptible to subsurface heat islands, which can cause swelling of the ground up to 12 millimeters and shrinking up to 8 millimeters.

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

Subsurface heat islands can contribute to ecological issues such as contaminated groundwater and health hazards such as asthma and heatstroke. The study stated an underground heat island wouldn’t cause buildings and infrastructure to collapse or rupture but that their durability, aesthetics, and operational requirements would be compromised.

“Chicago clay can contract when heated, like many other fine-grained soils,” Rotta Loria said. “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.”

What can be done?

Rotta Loria determined that subsurface heat island effects can be “predicted realistically” and using geothermal technologies to absorb waste heat and retrofitting buildings with envelopes and enclosures could offset the issue. He cautioned against trying to cool underground structures, which would require the use of energy.