The secret to efficiently heating some buildings might lurk beneath our feet, in the heat that humans have inadvertently stored underground. 

Just as cities warm the surrounding air, giving rise to urban heat islands, so too does human infrastructure warm the underlying earth (SN: 3/27/09). Now, an analysis of groundwater well sites across Europe and parts of North America and Australia reveals that roughly a couple thousand of those locations possess excess underground heat that could be recycled to warm buildings for a year, researchers report July 8 in Nature Communications.

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What’s more, even if humans managed to remove all this accumulated thermal pollution, existing infrastructure at about a quarter of the locations would continue to warm the ground enough that heat could be harvested for many years to come. That could reduce reliance on fossil fuels, and help mitigate climate change.

This work showcases the impact that underground heat recycling could have if harnessed on a large scale, says hydrogeologist Grant Ferguson of the University of Saskatchewan in Saskatoon, Canada, who was not involved in the study. “There’s a lot of untapped potential out there.”

Heat leaks into the subsurface from the warm roots of structures such as buildings, parking garages and tunnels, and from artificial surfaces such as asphalt, which absorb solar radiation. In Lyon, France, for example, researchers in 2016 found that human infrastructure warmed groundwater by more than 4 degrees Celsius.

Scientists don’t fully understand how heat pollution alters underground environments. But warming of the subsurface can cause contaminants, such as arsenic, to move through groundwater more readily.

Extracting the thermal pollution could be accomplished by piping groundwater to heat pumps at the surface. The water, warmed underground by all that trapped heat, could then warm buildings as it releases heat into their cooler interiors, says Susanne Benz, an environmental scientist at Dalhousie University in Halifax, Canada.

Harnessing underground heat in this way could provide some communities with a reliable and low-energy means to warm their homes, Benz says. “And if we don’t use it, it will just continue to accumulate,” she says.

Benz and her colleagues analyzed the population size, heating demand and groundwater temperature at more than 6,000 locations, most of which were in Europe. The researchers found that at about 43 percent of the locations — mostly those near highly populated areas — enough heat had accumulated in the top 20 meters of earth to satisfy a year’s worth of the local heating demand.

Curious about sustainability, the researchers also identified places where the continuous flow of heat into the underground — and not just the stockpiled thermal pollution — was high. Their calculations show that if all of the accumulated heat was first extracted, the heat that continued leaking from existing infrastructure could be harvested at about 25 percent of the 6,000 locations. At 18 percent of locations, this recycled heat could satisfy at least a quarter of the heating demand of the local population.

Constructing systems to take advantage of human heat pollution today could one day help residents harvest heat from climate change, the researchers say.

Using climate projections for the end of the century, the team probed the feasibility of extracting underground heat in a warmer world. In the most optimistic warming scenario considered, which assumes greenhouse gas emissions peak about the year 2040, the researchers found that climate change would warm the ground enough by the end of the century that underground heat recycling at 81 percent of the studied locations could meet more than a quarter of locals’ heating demands. If there are no efforts to curb emissions, that number rises to 99 percent of locations.

Though the researchers focused mostly on Europe, Benz says that other continents probably also possess abundant underground heat that could be harnessed. In Europe and elsewhere, heat recycling might be most feasible in suburban areas, she says, where there is sufficient accumulated underground heat to help meet local heating demands, and space to install heat recycling systems.

Looking ahead, Benz plans to investigate whether cooling the subsurface can help reduce aboveground temperatures in urban environments. “This might actually be a little additional tool to control [aboveground] urban heat.”