Geothermal energy utilizes the heat produced by the Earth’s core to create electricity and to heat homes. Only certain locations have the appropriate mix of resource availability and high population densities to make this resource substitutable for fossil fuels.

Geothermal energy is naturally vented at the Earth’s surface in the form of volcanoes, geysers, and hot springs. Large amounts of geothermal energy are vented at the intersections of tectonic plates as well, such as along the Pacific Rim. Geothermal energy can be used to produce electricity by either harnessing steam directly from geothermal resources or by using hot geothermal water to produce steam to run a turbine. It is also used to heat and cool buildings.

With 15 gigawatt-hours of electricity generated in 2010 from more than 70 power plants, the United States is the world’s leading producer of electricity from geothermal sources—but this amounts to less than 1 percent of total nationwide electricity consumption. Few countries produce a significant share of electricity from geothermal sources; only Iceland, El Salvador, and the Philippines use it to generate more than 15 percent of their electricity. Since the footprint of a geothermal plant is fairly small, most of the environmental damage that comes from geothermal energy production is associated with the construction of the facility and its related transmission infrastructure.

Unlike wind and solar energy, which are intermittent sources of power, geothermal energy is consistent, and thus is one of the only renewable energy technologies that substitutes well for coal generation. (Coal power plants take hours to become hot enough to produce electricity efficiently, and then hours again to cool down—so plant operators try to use coal plants as always-on baseload power to decrease wasted energy during the start-up and shutdown periods.)

Geothermal energy may also be used to regulate temperature in buildings, providing an alternative to conventional heating and air conditioning. Hot water from a geothermal source can be pumped directly into buildings for heat. Alternatively, geothermal pumps can utilize the constant temperatures (between 50 and 60 degrees Fahrenheit) found only a few feet underground to cool buildings in the summer and heat them in the winter.

The next generation of geothermal energy, “enhanced” geothermal, aims to harness underground heat sources that otherwise lack water or permeability but are broadly distributed geographically. There is considerable interest in and hope for enhanced geothermal, but the technology is still in development and, like other emerging energy resources, would take time and investment capital to grow to any significant scale.