Underground climate change is helping sink the land beneath us
If you think its hot walking around a city, its worse underground. Beneath the high rises in downtown Chicago, the ground has been heating up significantly for decades. In some locations, the excessive heat is causing deformations in the land and destabilizing buildings, according to a study released Tuesday . Scientists are calling this subsurface heating underground climate change, the counterpart of what people experience above ground. Except this subterranean warming is much more intense than above the surface, especially in densely built cities. Over the past 70 years, ground beneath the Chicago Loop in the citys downtown has warmed by 5.6 degrees Fahrenheit on average, according to the study author. Its actually more significant than what we are observing at the surface because of climate change, said Alessandro Rotta Loria, author of the study and architectural engineer at Northwestern University. Ground deformations triggered by underground climate change can be significant, and they can represent an issue for the performance of civil infrastructure. The underground heat in cities can be blamed on both human activity and meteorological forces. First, buildings use a lot of energy to regulate their temperatures or run appliances, exuding heat downward. Second, building materials tend to absorb more sunlight and increase the temperature in cities, known as the urban heat island effect; energy from the warm air can transfer to below the surface. The excess heat is often obvious in basements, underground parking garages and busy subways . Previous research has investigated the effects of the rising underground temperatures on plants, groundwater flows, transportation systems and human health. But the new study is the first to quantify how underground climate change is affecting the integrity of buildings in a city. Soils, rocks and concrete deform under temperature variation, Rotta Loria said. The overarching question was ... what are the associated deformations and what is the impact of these deformations on the performance of city infrastructure. To investigate this question, Rotta Loria and his colleagues measured the temperature in several locations above and below ground around the Chicago Loop. They placed more than 150 temperature sensors in basements of buildings, underground parking garages and streets, subway tunnels, and green spaces like Grant Park, for example. They found that underground temperatures beneath the building-heavy Loop were often 10 degrees Celsius warmer than beneath Grant Park, according to a news release . Air temperatures in underground structures were also up to 25 degrees Celsius warmer than undisturbed ground temperatures. After collecting measurements for three years, Rotta Loria then created a 3D computer simulation to study how the underground temperatures have grown since 1951 and also predict how they will continue to grow until 2051. He then looked at how that affected land movements. The analysis showed ground temperatures under the Loop have increased over the past 70 years, but warming rates have slowed in recent years. According to the study, the ground down to 100 meters warmed on an average of 0.49 degrees Celsius per year in the past. Today, the ground is warming at about 0.14 degrees Celsius per year. He said the warming has probably slowed down because the soil layers below are hitting a thermal saturation point. He explained when the temperatures of the air and underground are very different, more heat can be transferred below surface. But as the ground has warmed up more in recent decades and air temperatures have increased, the temperature differential has decreased, so the transfer of heat has slowed. In other words, this implies that the worse [underground heating] has already happened, Rotta Loria said. The models projections do not show major increases in warming rates to 2051. Climate scientist Hugo Beltrami, who was not involved in the study, was not surprised to see intense warming over the past few decades in Chicago. He said these underground temperatures have been warming mostly since the postindustrial revolution because of climate change. Thats an area in the continental United States where the ground has warmed quite a bit, said Beltrami, a research chair in climate dynamics at St. Francis Xavier University in Nova Scotia. Its difficult to say what is coming from the ... climate itself changing and what is coming from the actual activities of the city. Different soils and depths warm up at different rates. For instance, shallower depths experience the most temperature variations. Yet limestone layers further below ground also experience significant warming as the shallower soils, if not more. The warming also can affect structures above ground. The land contracted over soft, stiff clay layers but expanded at hard clay layers. Shallower and deeper sand layers and bottom limestone layers expanded as temperatures rose, as well. In areas, the model showed land could rise as much as 12 millimeters and sink as much as 8 millimeters. Millimeters might not sound like much, but he said those relatively small displacements can have significant effects in civil engineering. It can cause unwanted sinking on building foundations, angular distortion in slabs and beams and tilting. It can also cause cracking, which could allow water to seep in and corrode the structure. Even these relatively small displacements can lead to problems, Rotta Loria said. He said that these small displacements probably wont topple over a building at once and pose imminent danger because the movements are slow. But they are continuous ... so they accumulate over time, he said. These movements could help explain past building issues in this part of Chicago, which have been historically blamed on poor construction work or building design, he said. Historical buildings with outdated or inappropriate building designs are likely contributing the most to these subsurface warming and are also the structures at most risk for damage and deformation. Rotta Loria said this underground warming could be deterred. One approach is to harvest some of this energy for power and increase the total amount of heating energy that could be supplied to buildings ... mitigate underground climate change in Chicago and other cities, the study said. Or its also worth better insulating buildings. That can be done by installing thermal insulation on underground walls or basement ceilings. Underground climate change is a silent hazard, he said, but it can also be seen as an opportunity, as a resource.