Renewable energy is energy that is generated from natural processes that are continuously replenished, such as sunlight, wind, moving water, geothermal heat and organic material. Unlike fossil fuels, these sources do not become depleted over time and are considered virtually inexhaustible. Major forms of renewable energy include solar, wind, hydropower, biomass and geothermal.
Carbon-free energy refers to energy sources that emit no carbon dioxide during operation. While all renewable energy sources are low-carbon during operation, they may still produce some emissions over their full lifecycle, including manufacturing, installation and decommissioning. For example, solar energy systems produce 40 to 50 grams of carbon dioxide per kilowatt-hour, mainly due to their manufacturing processes, although this is far lower than for fossil fuels. Biomass can release substantial carbon dioxide depending on how it is produced and managed.
Not all carbon-free energy is renewable. For example, nuclear power is largely carbon-free (major exceptions including production of the concrete and steel for plant construction) but not renewable because it relies on uranium, which is a finite resource and not naturally replenished on a human timescale. However, this could change in the future if proposed technologies such as the extraction of uranium from seawater or nuclear fusion become practical.
While renewable energy sources generate minimal carbon dioxide emissions during operation and offer clear environmental benefits over fossil fuels, other adverse environmental effects can arise depending on site selection, technology, and management practices. These include habitat alteration, resource use in manufacturing, and wildlife interactions. However, with proper planning, mitigation strategies, and technological advancements, these effects can be greatly reduced, preserving the very substantial environmental advantages of renewable energy.
In 2025, renewable energy reached a major milestone by surpassing coal as the leading source of electricity generation worldwide, producing 34.3 percent of global electricity compared to coal's 33.1 percent share. This marked the first time in history that renewables, led by dramatic growth in solar and wind capacity, overtook a major fossil fuel source on a global scale. While overall fossil fuel emissions apparently reached new highs in 2025, renewable power sources are continuing their rapid expansion and may soon exceed the total for fossil fuels as a whole, including oil and natural gas.