Scientists have long understood the urban heat island effect, a term used to describe the way that cities, with their miles of roads and acres of cement covered lots, tend to be hotter than their surrounding areas. But now, thanks to an ongoing study in Chicago, scientists are learning that it isn’t only the air that is hotter in cities, the ground beneath them as well. The study started in 1951, the year that Chicago got its first subway, and is slated to end in 2050. But even before the study is complete it is already yielding results. By placing around 150 temperature sensors throughout “The Loop,” Chicago’s most densely populated area, and comparing it to a nearby greenspace the team at Northwestern University behind the study has found that the ground under the urban area is about ten degrees warmer than the rural areas nearby.
There are plenty of reasons for this temperature difference. One is the traditional heat island effect, since the air above cities is hotter it pushes more heat into the ground as it diffuses. But that doesn’t account for all of the difference, especially since the temperature changes can be felt even 100 meters underground. The rest of the temp difference comes from underground structures like subway tunnels and basements that push heat directly into the earth around them.
There are consequences to this newfound underground heat, both good and bad. The worry is that the higher temperatures will cause some unwanted expansion and settling of the dirt beneath structures in a way that could damage them. “As a result of temperature increases underground, many foundations downtown are undergoing unwanted settlement, slowly but continuously,” the report author said. The settling can be even more severe in places that are already shrinking or dealing with the additional moisture caused by the record rainfall that climate change has brought on. While the authors of the study don’t think that the damage would be severe enough to cause structures to collapse, they will certainly increase the repair bills needed to keep them in safe condition.
The benefit of underground heat islands is that they might help us reduce our energy consumption. Many cities are already seeing a lot of geothermal technology being used to help heat and cool buildings. This is done by pumping air underground to take advantage of the consistent temperature down there. Now cities in cold climates will be able to heat their buildings even more effectively with geothermal installations. The heat can also be harnessed to generate electricity. Hudson Yards, the massive new development in Manhattan, already does this by using heat from the rail station below it to run a number of generators giving the complex cheap, clean energy and resilience against blackouts.
More than anything this study shows that there are many effects of climate change and our growing urban population that we have not even considered yet. As we learn more about the man-made changes to our environment we must remember how important it is to find ways to reduce our impact before it poses an existential threat to us and our buildings. Luckily we will be able to use some of these changes for good, like the underground heat islands, but only if we prevent them from becoming such big problems that they lose their silver linings.
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