Geothermal energy comes from the hot rock deep down that makes geysers and warm pools for spas and mud baths. It mostly occurs at the junctions of tectonic plates. Worldwide, 10.7 GW of electricity is generated geothermally in 24 coun­tries, and another 28 GW is used for heating. The USA produces the most geother­mal power, 3 GW, in 77 plants mostly in California. The Philippines is second with 1.9 GW and gets more than a quarter of its energy from geothermal, as does Iceland. These numbers are very small on a world scale, and we need not say much about this energy source.

The capital expense of geothermal plants, used for exploration and drilling, is comparatively large. There is no fuel cost, but electricity is used to run the pumps. Once a bed of hot rock is found, a production well is drilled to extract the steam. If this is hot, above 180°C (360°F), it can be used directly to drive steam turbines to generate electricity. If it is cooler [below 150°C (300°F)], it is used for space or water heating. The used, cooled water is injected back into the rock in an injection well. With the steam, GHGs also came: CO2, methane, ammonia, and hydrogen sulfide, which smell. Whether these emissions are lower than from a comparable fossil-fuel plant depends on the location. The water also contains undesirable chemicals: mercury, arsenic, antimony, boron, and salt. All in all, geothermal energy is not going to be a solution to the world’s problems.