Weird Science | It is indeed time to worry about the Upside Down

Research on climate change has mostly focused on the impact on the atmosphere. But what of the impact underground? A study undertaken in Chicago, with potentially global implications, measured underground temperature variations over time and simulated its impact. The ground beneath Chicago is getting deformed, or shifting, either moving upwards due to the due to the expansion of some materials (such as soft clay) or sinking due to the contraction of other materials (such as hard clay and sand).

The study, published in the journal Nature Communications Biology, flags concerns about the impact of underground climate change on civil infrastructure and also about health; earlier studies have linked extreme air temperatures underground to heat-induced diseases. There is also an ecological impact, such as contamination of groundwater.

“Underground climate change characterises, at least to some extent, all urban areas worldwide. Older cities experience a more intense underground climate change,” study author Alessandro Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern University, said in an email interview.

Similar to heat island effect

While the new study is possibly the first to look at ground deformation as a result of temperature variations, underground climate change itself has been recognised as a hazard for some time now. In 2020, the Universities of Cambridge and California, Berkeley, in partnership with the British Geological Survey, launched a project to model and monitor urban underground climate change.

The phenomenon is somewhat similar to the urban heat island effect. A heat island occurs in a city when it experiences much higher temperatures than surrounding areas that are less urbanised, a result of the different amounts of heat absorbed by various surfaces in the two areas.

Buildings and infrastructure overground, and transport systems underground, are releasing massive amounts of heat continuously into the ground. This is in addition to heat that diffuses into the ground from the atmosphere. The phenomenon of underground climate change, in fact, is widely described as underground heat islands.

“The amounts of heat characterising the surface and subsurface of cities differ because they originate from different causes. Heat at the surface of cities mostly derives from the fact that construction materials absorb heat from solar radiation and anthropogenic activity, releasing such heat at night in the atmosphere. This phenomenon yields so-called surface or meteorological urban heat islands,” Rotta Loria said.

“Heat in the subsurface partly derives from the one at the surface of cities, but mostly comes from heat sources including building basements, parking garages, tunnels, sewage networks, and district heating lines. As the ground warms up over time, it approaches a thermal saturation state.”

How they did it

Rotta Loria and his team installed a network of temperature sensors over and below the ground at two locations in Chicago. One was the Chicago Loop, marked by buildings and underground railway tunnels, and the other was Grant Park, a green space. The sensors, wirelessly connected, were placed in basements, parking garages and tunnels and monitored the temperature in those environments.

“Based on these data and a 3D model of the considered urban area, which reproduces the myriad heat sources within, the heat released in the underground was simulated over 100 years, starting from the 1950s till the 2050s. The model was used to further explore what likely happened in the past and what will likely happen in the future, not only in terms of temperature variations but also ground deformations,” Rotta Loria said.

Underground temperatures were found often 10°C warmer beneath the Loop than beneath Grant Park. Also, air temperatures in underground structures were up to 25°C higher than the undisturbed ground temperature.

“Larger temperature variations lead to larger ground deformations. Variable heating rates yield variable rates of ground deformations. The magnitude of soil and rock deformations strongly depends on aspects such as geological origin, stress state and stress history, and other variables,” Rotta Loria said.

The implications

Rotta Loria said the ground deformations caused by underground climate change do not threaten to result in the collapse of structures. “But,” he added, “on a case-by-case basis, they are of sufficient magnitude to affect the day-to-day function and long-term durability of civil structures and infrastructures.” Over the long term, the deformations can affect the aesthetic requirements of structures and infrastructures and their durability, he said.

This goes for all cities packed with civil infrastructure. “These phenomena should be analysed with care and on an ad-hoc basis depending on the considered city. They develop slowly but continuously, and likely with a larger magnitude for denser and older urban areas across the world,” Rotta Loria said.

Rotta Loria’s team, in fact, noted a silver lining in their findings: an opportunity to capture the heat emitted underground from the Chicago subway, parking garages and basement facilities.

This would help urban planners not only mitigate the effects of underground climate change, but also reuse the heat into an untapped thermal energy resource. Rotta Loria suggested that policymakers can “determine which buildings would benefit the most from retrofit interventions aimed at enhancing energy efficiency, or geothermal installations to reutilise waste heat”.

Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but it isn't.