Renewable energy refers to energy sources that are replenished naturally and can be used repeatedly. These include solar, wind, hydro, geothermal, and biomass energy. Unlike non-renewable energy sources, such as fossil fuels (coal, oil, natural gas), which are finite and will eventually be depleted, renewable energy sources are sustainable and produce little to no greenhouse gas emissions, which contributes to the reduction of global warming. The increased use of renewable energy is considered to be a key solution to the problem of climate change and a way to reduce dependence on non-renewable resources.