Alternative energy is carbon-free energy that creates little to no greenhouse gas emissions. This is in contrast to fossil fuels, which produce a significant amount of greenhouse gas emissions, including carbon dioxide and methane. Alternative energy is also not infinite in supply and can include resources like natural gas cogeneration, nuclear energy, and use of waste energy. Renewable energy is energy that comes from resources that are naturally replenished by nature such as sunlight, wind, water, and geothermal heat. Unlike fossil fuels, such as oil, natural gas, and coal, which cannot be replaced, renewable energy regenerates naturally in a short period of time.