During the summer, cities can get really hot compared to surrounding rural areas—just look at New Orleans and New York City compared to nearby areas with fewer impermeable surfaces. This urban heat island effect happens because buildings and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes. Daytime temperatures in urban areas may end up being 1 to 7 degrees Fahrenheit higher than that in outlying areas.
But it’s not just the surface and air temperatures that can rise—the ground also warms up. Rising air temperature, combined with the effects of human activities and infrastructure, can cause subsurface heat islands under urban areas. This “underground climate change” is also affected by indoor heating and operating appliances in buildings that inject heat into the ground.
Since soils, rocks, and construction materials can deform when subjected to temperature variations, a recent study published in Communications Engineering sought to assess whether subsurface heat islands can cause ground deformations that would affect the performance of civil infrastructure.
“The results of this study support that the ground deformations caused by underground climate change can be of sufficient magnitude to affect the day-to-day function and long-term durability of civil structures and infrastructures,” says Alessandro Rotta Loria, study author and assistant professor of civil and environmental engineering at Northwestern University.
Potentially excessive angular distortion, tilting, and/or cracking of structural members may affect the aesthetic and operational requirements of infrastructure. Luckily, these changes don’t necessarily represent an impact on their performance and don’t threaten people’s safety, says Rotta Loria.
How underground climate change affects the soil
Extreme changes in underground temperature under or near infrastructure impose temperature gradients that can promote pore water movement. The drying and wetting of soil is responsible for strains and deformations that can cause damage to structures, says Claudia Zapata, geo-engineer and associate professor in the School of Sustainable Engineering and the Built Environment at Arizona State University.
“While this is not a new issue that geotechnical engineers have to deal with, longer periods of high temperature can promote more significant changes in moisture content,” says Zapata. “The unsaturated condition will extend to deeper areas, causing larger deformations than those allowable by building codes.”
The impact is generally related to the soil type and the extent of drying or wetting, among other factors. For instance, sandy materials are not as prone to large deformations under climatic change conditions, unlike clay-heavy soils, says Zapata.
When analyzing the potential impact of structures, Rotta Loria says the distinct features of different cities and their infrastructure should be considered. Older and denser cities may generally experience a more intense underground climate change, which can translate to more significant effects on civil infrastructures.
“Major cities like New York City, which are densely built and rich in underground structures and heat sources, exhibit a particularly intense underground climate change,” says Rotta Loria. “For this reason, these cities may be particularly prone to structural and infrastructural operational issues in the long-term.”
Ground deformations caused by underground climate change develop slowly, but continuously, therefore it should be mitigated in the coming years to avoid unwanted effects on civil structures and infrastructures, he adds.
Mitigating underground climate change in a warming world
Underground climate change presents an opportunity for urban planners and policymakers to “enhance the sustainability of urban areas worldwide,” says Rotta Loria.
For example, applying thermal insulation to underground building envelopes and enclosures can minimize the amount of waste heat that would be injected into the ground. Installing shallow geothermal technologies to absorb at least part of the heat from basements, parking garages, and tunnels to reutilize it in buildings and infrastructures for space heating and hot water production is also a major possibility.
A 2022 study published in Nature Communications said that recycling subsurface heat, which accumulates due to climate change and urbanization, is a sustainable alternative to conventional space heating methods for various sites. Subsurface heat recycling makes it possible to capitalize on warming climates while helping society move to a low-carbon economy at the same time.
Rotta Loria says that retrofit interventions aimed at enhancing energy efficiency and geothermal installations to reutilize subsurface waste heat are “two concrete and relatively straightforward mitigation strategies” that would hamper underground climate change and its effects on civil infrastructure in a warming world. With all the impacts that climate change is having, and will soon have, on cities, it’s best to act sooner versus later.