Green energy is energy that is produced in such a way as to minimize its negative impact on the environment. Traditional energy sources, most notably fossil fuels, produce greenhouse gases that are believed to be the primary cause of an effect known as global warming or climate change. Sources of green energy, such as solar, wind, geothermal, and hydro energy, are developed and promoted as alternative sources that make little or no contribution to climate change. Even nuclear energy is sometimes considered a green energy source, because some types of nuclear technology produce much less waste than oil and coal.