The environmental impact of geothermal energy is minimal, especially compared to fossil fuel power plants. When sited and constructed carefully, geothermal power plants can be a reliable source of renewable and environmentally-friendly electricity.
First and foremost, geothermal power is a source of clean and renewable electricity. No fossil fuels need to be burned to generate geothermal power, and as long as the earth exists (likely for another 4 billion years), we won’t run out of geothermal energy. Geothermal energy generation isn’t limitless, however, as there are a finite number of suitable locations on Earth for geothermal power plants.
One way to compare the environmental impact of various electrical generation technologies is to analyze their life-cycle greenhouse gas emissions, which is the total amount of greenhouse gas output (measured in grams carbon dioxide equivalent, or gCO2eq) that can be expected from deploying a generator. Life-cycle analyses take into account the full life of the system, from obtaining materials through the construction process to operation to end-of-life waste management.
The Intergovernmental Panel on Climate Change, or IPCC, released a report in 2014 covering the mitigation of climate change. Their chapter on energy systems examines, among other things, the lifecycle emissions of various types of electricity generation plants, including both renewables and fossil fuels. The IPCC measures gCO2eq emissions per kilowatt-hour (kWh) of electricity produced from these various types of power plants:
Lifecycle emissions by generating source (IPCC)
According to the report, geothermal power plants have lifecycle emissions of 38 gCOeq/kWh. That's 95 percent less than coal (820 gCOeq/kWh) and 92 percent less than gas (490 gCOeq/kWh).
Not only do geothermal power plants lead to exponentially less pollution than fossil fuel power plants, but they also perform similarly to other renewable technologies. In the same climate change report, the IPCC reports the following lifecycle emissions for renewable technologies:
Lifecycle Emissions (g CO2eq/k Wh)
*Hydropower has several environmental consequences that are still being studied, and the actual lifecycle emissions for a hydropower plant have the potential to be significantly higher than 24 gCO2eq/kWh.
As with any generating technology, there are environmental concerns to take into account when it comes to building new geothermal power capacity.
The main environmental concern that comes with geothermal power plants is the potential for surface instability. Because geothermal plants remove water and steam from reservoirs within the earth, the land above those reservoirs can sometimes sink slowly over time. However, most geothermal plants re-inject used water into the earth via an injection well to reduce the risk of land subsidence.
An additional problem that can arise during the operation of geothermal power plants is increased earthquakes. Geothermal power plants are usually located near fault zones or geological "hot spots" that are especially prone to instability and earthquakes, and drilling deep into the earth and removing water and steam can sometimes trigger small earthquakes.