For as far back as geological evidence shows – at least 650,000 years – the Earth's natural carbon cycle has maintained a steady equilibrium of carbon dioxide in the atmosphere - around 275 parts per million (ppm). We discovered this by examining the contents of Antarctic ice cores. As a result of the natural carbon cycle: People and animals (source) use respiration to turn oxygen into carbon dioxide. Plants (sinks) absorb CO2 and release it back into the atmosphere. Over the seas, oceans both produce (source) and absorb (sink) carbon dioxide. Dead organic matter traps carbon underground in various forms such as fossil fuels (sink), while volcanic eruptions (source) can release CO2 from carbonate rocks deep inside the Earth.