Cities are polluting the air, ground, and water around them with heat. Roads and rooftops absorb sunlight, and swapping trees for pavement removes shade. Add all these factors together, and you get something called an "urban heat island," an air temperature increase of up to 22 degrees Fahrenheit as you get closer to a large city. While the average human is more likely to feel this in the air, these changes also create a well of heat below the ground — and that heat, ironically, can be used as renewable energy even as it changes the ecosystem. Now, thanks to a study from Germany’s Karlsruhe Institute of Technology, we know just where most of it’s coming from, and how it could be better harnessed to replace traditional heaters and air conditioners.

The EPA warns that urban heat islands can exacerbate heat waves in big cities and make ozone form faster, while higher underground water temperatures can encourage dangerous bacteria to grow. On top of all this, citizens spend more time with the AC on in summers, which usually means burning more fossil fuels. But it’s possible to recapture some of that energy. In general, the ground makes for a great insulation system, and there’s a long history of using it for climate control. The technology in question is called a ground source heat pump, a renewable energy system that began to gain traction in the 1960s and 1970s. In summer, the relatively cool ground can act as a heat sink; in the winter, water pumps can draw heat from underground.

While the pumps can be used in most places to some effect, they’re particularly helpful where humans can use a limited amount of resources to recapture some of the heat they’ve released just by living. "You still need some energy," says co-author Philipp Blum, since the machine itself uses power. "You put in one part of energy, and you get four parts of heat out of it." In 2004, the Oregon Institute of Technology’s Geo-Heat Center estimated that roughly 1.1 million ground source heat pump units were installed worldwide, primarily in the US and Sweden. They’re largely semi-isolated endeavors, helping heat apartments, hospitals, or private residences like George W. Bush’s Texas ranch. But while the technology has been in use for decades, the exact sources of urban underground heat have gone relatively unexplored.

"Before this study, it was not known where the energy really comes from," says Blum. "What are the real numbers on that?" There are plenty of factors that could play into the urban heat island, from heat-absorbing roads to leaking sewage; there’s even the chance that some heat changes aren’t due to urban growth at all but to climate change. Blum’s group started by drawing from groundwater temperature measurements around Karlsruhe, a German city of about 300,000. Using this data and a series of models for heat flux from various factors, the team ran simulations to determine the likely causes, comparing data from 1977 and 2011.

In 2011, it turned out, elevated ground surface temperatures — from things like roads and lack of shade — contributed significantly to groundwater warming, as did heat leaking from basements. Certain kinds of pollution, by contrast, contributed less heat than they did in the 1970s, when temperatures rose partly because of sewage leaks and the release of heated wastewater from, for instance, power plants. And whatever it’s doing to the world at large, global warming appeared to be a relatively small contributor to the underground heat island.

Overall, Blum estimates that Karlsruhe’s aquifers could provide heating to 18,000 households in the city, and it’s not the only place that’s sitting on a well of thermal energy or a viable heat sink. In early 2013, researchers in Glasgow kicked off a plan to use the city’s flooded mines to provide 40 percent of its heat, and Blum says that Munich is seriously considering adding heat and cooling pumps to its energy plan; other cities temporarily store heat generated from power plants underground. This research, he hopes, will make these options more viable.

"What we would like to see or promote is that you should actually consider this source seriously in your urban planning," he says. "It’s not sufficient just to do it building-wise." On a large scale, the hot or cold water that pumps carry back to aquifers could be its own kind of thermal pollution, but Blum says this work could help establish a blueprint for responsibly drawing and depositing heat. "Storage can really be managed on an urban scale, if you know the underlying processes," he says. "If you know the fluxes of heat in and out of the system, then you really can look at sustainable management."