CO2 has varied a lot through Earth's history. It was 1800ppm in the Cretaceous causing high sea levels and warm polar regions.
India colliding with Asia built mountains which eroded, absorbing CO2 and gradually dropping the level over millions of years, cooling the Earth. This incidentally changed the vegetation of east Africa to grassland and facilitated the evolution of a bipedal naked ape that we all know and love.
Very low levels hundreds of millions of years ago led to a snowball Earth effect.
™ Version 4.2
