Graph of Phanerozoic Carbon Dioxide
Here is a redrawn graph for easier viewing:
My interpretation of this graph is that the oceans removed most of the carbon dioxide from the air about 325 million years ago. In the nick of time, tectonic activity created volcanoes, which added more CO2. But oceans kept removing CO2, while volcanoes died down. Now biology is on the verge of extinction due to a shortage of CO2 for photosynthesis.
Tectonic plates were very thin at first resulting in them sticking together when they collided rather than going under and over each other as they do now. As the planet cools, the plates get thicker causing them to create volcanoes when they collide. But now, the plates are getting so thick that volcanic magma can not easily get to the surface, and the oceans are absorbing more CO2 than volcanoes produce.
The oceans constantly absorb CO2, as it combines with calcium to form calcium carbonate and limestone.