The heat generated by urban areas and their buildings, factories, sewers, and transportation systems could be used to power those cities, according to a new study by German and Swiss researchers. Thermal energy produced by the so-called “urban heat island effect” warms shallow aquifers lying below cities, and geothermal and groundwater heat pumps could tap into those warm reservoirs to heat and cool buildings, the scientists say. In the southwest German city of Karlsruhe, the researchers found that the city of 300,000 generated 1 petajoule of heat per year — enough to heat 18,000 households. Karlsruhe’s underground heat production increased by about 10 percent over the past three decades, the team reported in Environmental Science and Technology. The biggest contributors to the city’s underground heat flux were its densely populated residential areas and surface temperature increases associated with paving. Sewage pipes, underground district heating networks, and thermal waste water discharges also contribute to warming shallow aquifers, the study found.