Geothermal energy is one of the cleanest power sources available, but it’s not impact-free. The best geothermal plants emit roughly 0.02 kg of CO2 equivalent per kilowatt-hour, a fraction of what natural gas or coal produces. The worst, however, can approach fossil fuel levels depending on the geology of the site and the technology used. Whether geothermal is “bad” for the environment depends heavily on where the plant is built and how it operates.
Carbon Emissions Vary Wildly by Plant Type
Not all geothermal plants have the same carbon footprint. Binary cycle plants, which use a closed loop to transfer underground heat to a separate working fluid, produce around 0.02 kg of CO2 equivalent per kilowatt-hour. That’s comparable to wind and solar. Flash steam plants, which pull hot water or steam directly from underground, release dissolved gases trapped in the earth’s crust. A lifecycle assessment of the Los Humeros geothermal station in Mexico found total emissions between 442 and 568 grams of CO2 equivalent per kilowatt-hour, putting it in the same range as natural gas.
The reason for that gap is geology. Some underground reservoirs contain large amounts of naturally dissolved carbon dioxide. When steam is brought to the surface and released, those gases come with it. At Los Humeros, 88 to 91% of the plant’s greenhouse gas emissions came directly from the CO2 naturally present in the steam, not from burning fuel. Only 9 to 12% came from building and maintaining the facility. A redesign of that plant using binary cycle technology achieved an 82% reduction in CO2 emissions, though electricity output dropped by about 16% due to lower conversion efficiency.
So the technology choice matters enormously. A binary plant in one location can be nearly carbon-neutral, while a flash steam plant tapping a CO2-rich reservoir can rival a gas-fired power station.
Hydrogen Sulfide and Air Quality
The gas most people associate with geothermal energy is hydrogen sulfide, the compound responsible for a rotten-egg smell. Flash steam and dry steam plants can release it because it’s naturally present in geothermal fluids. The concentration of noncondensable gases (which include hydrogen sulfide) in the steam feeding a turbine can range from 1% to 20%, depending on the reservoir.
At high concentrations, hydrogen sulfide is toxic and can irritate the eyes and respiratory system. Modern plants use chemical scrubbing systems to capture it before it reaches the atmosphere. One common approach uses an iron-based catalyst that converts hydrogen sulfide into plain elemental sulfur, a solid that can be safely handled and even sold. Another method injects a chelated iron solution into cooling water to neutralize dissolved hydrogen sulfide on contact. These systems are effective, but they add cost, and older or poorly maintained facilities in some parts of the world still release more than ideal levels.
Binary cycle plants sidestep this problem almost entirely. Because the geothermal fluid never contacts the atmosphere, hydrogen sulfide and other dissolved gases stay underground.
Earthquake Risk From Fluid Injection
Conventional geothermal plants tap naturally occurring hot water or steam reservoirs and pose minimal seismic risk. The concern arises with Enhanced Geothermal Systems (EGS), which inject high-pressure fluid into hot, dry rock to fracture it and create an artificial reservoir. This process can trigger small earthquakes, and in some cases, not-so-small ones.
EGS projects have recorded their largest earthquakes at magnitudes between 1.6 and 5.5. A magnitude 5.5 earthquake is strong enough to crack walls and knock items off shelves. What makes this especially tricky is that the biggest quakes sometimes happen after injection stops, not during it. Pressurized fluid continues to spread through rock fractures, and stress can build gradually on nearby faults through a combination of pressure diffusion, temperature changes in the rock, and the slow transfer of stress from one fault segment to another. Prolonged injection can also cause gradual, silent slippage on faults that eventually releases as a larger event.
This risk has stalled or shut down EGS projects in several countries. A notable example: an EGS project in Basel, Switzerland was abandoned after triggering a magnitude 3.4 earthquake in 2006. Newer “closed-loop” designs, sometimes called Advanced Geothermal Systems (AGS), aim to eliminate this risk. These systems circulate fluid through sealed underground pipes without injecting it into rock fractures. Because the working fluid never enters the rock itself, the seismic risk drops to near zero.
Water Use and Contamination
Geothermal plants need water, but how much depends on the system. Conventional hydrothermal plants that tap existing underground reservoirs are remarkably efficient: binary hydrothermal plants consume about 40 to 42 gallons per megawatt-hour, and flash hydrothermal plants use roughly the same. For context, coal plants typically consume 500 to 1,100 gallons per megawatt-hour.
Enhanced Geothermal Systems are a different story. Binary EGS plants consume between 230 and 4,200 gallons per megawatt-hour over their lifecycle, and flash EGS plants use 1,600 to 2,800 gallons per megawatt-hour. The wide range reflects differences in how much water is lost underground during the fracturing and circulation process. In arid regions where geothermal resources are often located, this level of consumption can strain local water supplies.
Contamination is a separate concern. Geothermal fluids pulled from deep underground can carry heavy metals, including mercury and arsenic, dissolved at high temperatures and pressures. If these fluids leak or are discharged at the surface, they can pollute soil and waterways. Reinjecting spent fluid back into the reservoir is the standard solution, and it also helps maintain reservoir pressure. However, reinjection is not universally practiced at every installation worldwide. In the United States, the EPA regulates geothermal injection wells under the Underground Injection Control program, which sets construction, operation, and closure standards designed to protect underground drinking water sources.
Land Use and Surface Disturbance
Geothermal plants have a relatively small physical footprint compared to solar or wind farms generating the same amount of electricity. A typical plant occupies a few acres, and because it runs 24 hours a day with capacity factors above 90%, it produces far more energy per acre than intermittent sources. Drilling pads, pipelines, and access roads do disturb the surface, but the total area affected is modest.
The bigger land-related concern is the potential degradation of natural geothermal features. In areas like Yellowstone or New Zealand’s Taupo Volcanic Zone, drawing fluid from the same underground system that feeds hot springs and geysers can reduce flow to those features. This has happened historically in New Zealand, where geothermal development altered or destroyed some natural geysers and hot springs.
How Technology Is Closing the Gap
The environmental profile of geothermal energy has improved significantly as the industry has shifted toward closed-loop designs. Binary cycle plants prevent the release of hydrogen sulfide, carbon dioxide, and heavy metals by keeping geothermal fluids entirely sealed from the surface environment. Advanced Geothermal Systems take this further by eliminating rock fracturing, which removes the earthquake risk that has been EGS’s most public liability.
AGS technology also opens up geothermal energy to locations far beyond traditional volcanic hotspots. Because the system simply extracts heat from deep rock through sealed pipes, it can theoretically work almost anywhere on Earth. If these systems scale successfully, geothermal’s environmental downsides shrink to little more than the construction impacts of drilling and building the surface facility, comparable to any small industrial installation.
The bottom line is that geothermal energy at its best is among the cleanest sources of electricity available. At its worst, in CO2-rich reservoirs using older open-loop technology, it can produce emissions that approach fossil fuel levels and carry real risks of water contamination and induced earthquakes. The difference comes down to site selection, technology choice, and how well the operation is managed.