This CO2 graph is one of the most monumental facts of science. There were several measurements showing similar results, and much evidence indicates the graph is correct. This version of the graph is redrawn from a combination graph for better visibility.
This graph tells the story of geology, biology and evolution over the span of terrestrial life. Much of the evidence lies elsewhere, while the graph shows the timing of events and quantitative significance of many events.
A good starting point for this subject is to understand that the earth's crust (and top mantle layer) continually gets thicker as the planet cools. It's not clear what happened during the first billion years on earth, but by the time terrestrial life began, about 500 million years ago (mya), the patterns were clearly established.
As terrestrial life began, about 500 mya, the land mass consisted of a supercontinent called Gondwana. It reformed into Pangaea about 335 mya. It then began to break up forming modern continents during the early dinosaur years, 175 mya.
Supercontinents formed as a result of tectonic plates sticking together as they bumped into each other. Tectonic plates no longer do that, because they are so thick now that they slide over and under each other as they move around. As the plates slide over and under each other, two major events occur. One is that mountains substantially increase due to the buckling the the crust. The other is that volcanoes form where the plates overlap.
There would not have been highly developed mountains on the early supercontinents. Mountains became significant with the formation Pangaea. As Pangaea formed mountains, conifer trees evolved, which was 300 mya.
The evolution of conifers was dependent upon mountains, because nonwoody brush was suppressing evolution of both plants and animals on the lowlands. Water washing off the sides of the mountains would have created inhospitable conditions for the brush and left space for the evolution of new species. Conifers filled that space having characteristics compatible with growth on slopes.
The CO2 graph shows an almost complete disappearance of CO2 in the atmosphere about when conifers began to evolve. The needly leaves of conifers would have been a result of a shortage of CO2, which required maximum surface area for absorption. Only later did broad leaf trees form, and then light would have been the shaping influence, as CO2 was abundant.
The reason why CO2 almost disappeared from the atmosphere 315 mya is because oceans continually absorb CO2 and tie it up as calcium carbonate and limestone. Calcium in the oceans attracts CO2, as do all alkalis, since CO2 is an acid. The oceans never run out of calcium, as it keeps entering through the rivers and gets recycled as land masses move out of the oceans over time. Calcium carbonate is a buffer in the oceans which holds the pH at 8.1, which is significantly alkaline. There is no reason to assume the oceans have ever been any other pH in the history of terrestrial life.
The amount of CO2 in the atmosphere began to increase just as the mountains were forming 300 mya and for the same reason. The tectonic plates were getting so thick that they began to move over and under each other as they pushed against each other. This effect causes volcanoes to increase, which adds CO2 to the atmosphere.
The reason why the CO2 level dropped back down in recent times is because volcanic activity decreased, as the tectonic plates got so thick that magma could not easily get to the surface. All the time, the oceans keep absorbing CO2 from the atmosphere due to the perpetual presence of calcium.
The large size of dinosaurs would have been caused by the nonwoody brush that they had to walk through and used for food. Walking through brush takes a lot of power. As a result, the size of dinosaurs increased as the nonwoody brush got more dense.
Dinosaurs could not eat the conifers which evolved on the slopes of mountains. There would have been evolutionary pressure causing conifers to resist being eaten by dinosaurs. The highly inedible chemistry that conifers acquired would have been largely a response to dinosaurs trying to eat them.
The conifers would have left some space around them for complex evolution which included grass, flowering plants and the puffball and bolete mushrooms. But there was so little space for these diverse species that there numbers were miniscule and almost invisible in the fossil record. It was only after the extinction event 66 million years ago that the nonwoody brush died down to a low enough level for modern species to prevail in large numbers.
It was grass that reshaped biology around flowering plants, broadleaf trees and the diversity of mammals. Grass prevented the nonwoody brush from getting reestablished, while flowering plants and mammals thrived with the grass. But grass was gradually taking over the nonwoody brush before the extinction event as indicated by Anzu wyliei, which was one of the last dinosaurs to evolve. It had long legs and looked like a large chicken. The long legs would have evolved from the need to walk through grass. It means grass was becoming significantly established by the time the extinction event occurred.