The charts of CO2 levels in geologic history show you clearly that today’s carbon dioxide levels are some of the lowest, with 400 ppm (parts per million), especially if you compare it with those of the Paleozoic and Mesozoic Era. Today’s levels might be higher than the 1980s but they are still very low. You can see that during the Cambrian period of the Paleozoic Era, it was at the highest, with almost 7,000 ppm. When the CO2 levels were between 4,500 and 4,000, there was a massive proliferation of terrestrial plants and animal lives, with the appearance of the amphibians and primitive reptiles during the Devonian and Carboniferous period, respectively.
Not only did the quantity of carbon dioxide was much higher during the Paleozoic and Mesozoic than today’s levels but also the amount of nitrogen in the atmosphere was much, much higher than it is today, yet animals could still breathe, live, and thrive as there was enough oxygen for every living creature and enough CO2 for every plant. In the geological history of the Earth, CO2, oxygen, and nitrogen levels rose and fell without the existence of human beings and human industry.
During the Permian, the last period of the Paleozoic, the CO2 levels dropped sharply to about 700 ppm. However, the amount of carbon dioxide rose up steeply to about 2,100 and 2,500 ppm as animal and tropical plants flourished again, with the emergence and abundance of the dinosaurs during the Triassic and Jurassic respectively; also the first mammals appeared during the Cretaceous period. During the first part of the Cenozoic Era, they still remain high between the Paleocene and Eocene epoch at 2,200 and 1,700 ppm. Yet, at these high levels of carbon dioxide, mammals thrived and scattered to every corner of the planet. Then CO2 dropped sharply at the end of the Eocene and the beginning of the Oligocene. This coincided with a general cooling of the Earth temperatures and the disappearance of large tracts of rain forests, giving way to the emergence of the huge grassy plains, such as the African savanna, and the Caucasian, Russian, and Mongolian steppes. Abundant grass (Gramineae) boosted the thrive of ruminants (cattle, deer, sheep, buffalo, etc.) and equines; and with them carnivores flourished.
Below, the CO2 levels during the Cenozoic.
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