Climate change is moving underground – and causing cities to sink

Human activity has already caused significant changes in Earth’s atmosphere. Now, climate change is moving underground.

A new study has shown that the heat emitted by subsurface infrastructure such as transport systems, tunnels and basements is causing the ground beneath urban areas to move – risking damage to building foundations.

The research, led by Northwestern University’s Professor Alessandro Rotta Loria, used Chicago as a ‘living laboratory’, deploying a network of more than 150 temperature sensors across the city centre and in nearby Grant Park.

‘If you think about basements, parking garages, tunnels and trains, all of these facilities continuously emit heat,’ said Professor Rotta Loria. 

‘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 activity.’

Previous research has shown temperatures beneath cities can increase by 0.1C to 2.5C per decade.

In Chicago, data showed temperatures beneath the city centre, known as ‘the Loop’, were often 10C higher than beneath Grant Park, which is removed from waste heat sources.

Using 3D computer modelling, Professor Rotta Loria found the data collected was consistent with simulations of how ground temperatures have changed since 1951, when the city’s subway system was completed.

He also used the model to predict temperatures up to 2051.

‘Underground climate change is a silent hazard,’ said Professor Rotta Loria. ‘The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations. Although this phenomenon is not necessarily dangerous for people’s safety, it will affect the normal day-to-day operations of foundation systems and civil infrastructure at large.

‘Chicago clay can contract when heated, like many other fine-grained soils. 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.’

Professor Rotta Loria’s modelling suggested surface materials that contract when heated, such as soft and stiff clay, could sink by as much as 8mm. Substances such as sand and limestone expand when heated, perhaps by as much as 12mm upwards.

‘Based on our computer simulations, we have shown that ground deformations can be so severe that they lead to problems for the performance of civil infrastructure,’ said Professor Rotta Loria, speaking to EurekAlert. 

‘It’s not like a building will suddenly collapse. Things are sinking very slowly. The consequences for serviceability of structures and infrastructures can be very bad, but it takes a long time to see them.

‘It’s very likely that underground climate change has already caused cracks and excessive foundation settlements that we didn’t associate with this phenomenon because we weren’t aware of it.’

Other negative effects of underground climate change, or subsurface heat islands, include changes in plant growth and the pollution of groundwater.

However, it may not be all bad news. Professor Rotta Loria recommends future city plans include methods to harvest waste heat emitted by underground infrastructure and use it for heating, while also incorporating better insulation into new buildings to reduce the amount of heat released.

‘The most effective and rational approach is to isolate underground structures in a way that the amount of wasted heat is minimal,’ said Professor Rotta Loria. ‘If this cannot be done, then geothermal technologies offer the opportunity to efficiently absorb and reuse heat in buildings.

‘What we don’t want is to use technologies to actively cool underground structures because that uses energy. Currently, there are a myriad of solutions that can be implemented.’

The study is published in the journal Communications Engineering.

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