Greenhouse gas levels have varied throughout Earth history. For example, CO2 has been present at concentrations less than 200 parts per million (ppm) and more than 5,000 ppm. But for at least 650,000 years, CO2 has never risen above 300 ppm, during either glacial or interglacial periods. Natural processes add (volcanic eruptions and the decay or burning of organic matter) and remove absorption by plants, animal tissue, and the ocean) CO2 from the atmosphere. When plants are turned into fossil fuels the CO2 in their tissue is stored with them. So CO2 is removed from the atmosphere.