Why have atmospheric CO2 levels fluctuated over Earth’s history?
A. KAREN BICE: In the absence of human influences, the amount of CO2 in Earth’s atmosphere is controlled by the balance between sources and sinks for CO2, much like the amount of money in your bank account is controlled by how much you put in and how much you take out. The primary source for CO2 on geologic time scales is volcanism and outgassing from the mantle, deep within the Earth. Erosion of oil-rich, or petroleum-rich, rocks has also contributed to the amount of CO2 in the atmosphere in the past. The sink for CO2, on geologic time scales, is the erosion of silicate rocks. Silicates are the type of rock that make up mountains. So the erosion of mountains reduces CO2 in the atmosphere; volcanism increases CO2 in the atmosphere. If we go back to about 90 million years, to the mid-Cretaceous Period when the dinosaurs roamed the Earth, there is evidence from Cretaceous-age rocks that there was much more volcanic activity than there is today. We therefore would predict higher atmospher
