Over the long run, however, CO2 levels are driven by rocks. What happened to all the CO2 when it was, say, 20% of the atmosphere—that’s 200,000 parts per million? It’s now the carbonate part of limestone-calcium carbonate—and other rocks.
But it’s foolish to get hung up on CO2. Many, many things drive the temperature of the earth. Part of it is cosmic rays. They have a huge influence on the amount of cloud cover, which can change temperature immensely. Part of it is the changing tilt in the earth’s axis. Part is the gradual, and cyclical, change in the earth’s orbit around the sun. Part of it may well be the solar system’s orbit around the galaxy, believe it or not.