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. 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. 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.