Sinking cities: Climate change is warping the ground our cities are built on, study says
Climate change is warming the world above and below ground, and what's happening below could be causing the world's cities to slowly sink, a new study suggests.
“Underground climate change is a silent hazard,” said Northwestern University’s Alessandro Rotta Loria, who led the study. “The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations."
The study is the first of its kind to quantify the effects of underground climate change on civil infrastructure. Using data from sensors placed beneath downtown Chicago and computer simulations, the researchers found that heat is causing the ground to deform, and it could ultimately cause infrastructure to crack.
“Chicago clay can contract when heated, like many other fine-grained soils," he said. "As a result of temperature increases underground, many foundations downtown are undergoing unwanted settlement, slowly but continuously.
“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.”
Subsurface 'heat islands'
According to the study, urban areas increasingly suffer from "subsurface heat islands," which involve an underground climate change responsible for environmental, public health and transportation problems. Soil, rocks and construction materials deform under the influence of temperature variations, and excessive deformations can affect the performance of civil infrastructure, the study reports.
"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," Rotta Loria said.
The study, titled "The silent impact of underground climate change on civil architecture," was published Tuesday in the U.K. journal Communications Engineering.
Key points from the study:
Underground systems, like subways, continuously emit heat into the ground.
As the ground heats up, it deforms.
Ground deformation can cause civil structures and infrastructures to crack.
Researchers based the study on data gathered through computer simulations and a wireless sensing network installed throughout downtown Chicago.
How above-ground climate change is affecting underground
According to the researchers' computer simulations, warmer temperatures can cause the ground to swell and expand upward by as much as 12 millimeters, a Northwestern University news release said. They also can cause the ground to contract and sink – beneath the weight of a building – by as much as 8 millimeters.
Although this seems subtle and is imperceptible to humans, the variation is more than many building components and foundation systems can handle without compromising their operational requirements, according to the study.
Have the researchers found evidence of underground damage in Chicago?
Although researchers haven't yet directly found evidence of structural damage because of underground climate change in the Chicago Loop, "there is indeed evidence of problems for the operational performance of civil infrastructure in Chicago from past reports," Loria said.
He added that some of the damage and cracking seen in Chicago over the years has been "attributed to inappropriate foundation designs and construction methods." But underground climate change "might have exacerbated these issues," Loria said.
How development is making underground climate change worse
In cities worldwide, heat from buildings and underground transportation causes the ground to warm at an alarming rate, by as much as 2.5 degrees Celsius a decade. Known as "underground climate change," the phenomenon has been known to have ecological and health consequences.
But, until now, the effect of underground climate change on civil infrastructure has remained unstudied and little understood, the study said.
“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 (human-caused) activity."
Sensors in Chicago provided data
Over the past few years, researchers put in a wireless network of more than 150 above- and below-ground temperature sensors around the Chicago Loop, an area in the city's downtown.
That included placing sensors in the basements of buildings, subway tunnels, underground parking garages and subsurface streets.
“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.”
An evolving risk
This is the sort of problem that worries insurance agencies.
"We’re seeing more risk evolving because of things that are below ground," said Steve Bowen, chief science officer and meteorologist with Gallagher Re, a global reinsurance broker. Bowen was not involved in the study.
For example, France may have set a record last year for the highest volume of payouts after land sinkage during droughts, Bowen said. Insured losses could be as high as $2.7 billion, according to the insurance broker’s summary of natural catastrophes in 2022.
“Historic heat and significant drought led to a degradation of structures,” Bowen said. “That crumbling has led to a significant increase in costs.”
“France is vulnerable to subsidence because much of the land area contains soils with high concentrations of clay,” the report said.
Source: USA Today