Here are links to a few older posts about why belief in anthropogenic climate change is a form of secular apocalypticism.
But how much more CO2? Well.. it was almost 20 times higher than today in the Cambrian era and about 10 times more during the Ordovician. And it remained in the 8-12 x range for the Silurian and Devonian. Notably, these were the four geological ages when most animals and plants lived in or near oceans and other water bodies. It was also the heyday for Mollusks and shelled Cephalopods. Evidently, all that atmospheric CO2 had no negative effect on oceanic pH. More curiously, the Ordovician-Silurian (O-S) extinction was largely due to a short spell of global glaciation. And this global ice age occurred when CO2 levels were over 12 times higher than today.
As many of you might have guessed, our knowledge about the geology and paleontology of Antarctica is much sparser than for other continents. Nevertheless, a number of surveys for plant and animal fossils have revealed evidence which simply does not fit with the popular version of Antarctica being a frigid wasteland for the past 35 million years. For example, there is decent evidence that parts of Antarctica had extensive tundra like vegetation throughout the Oligocene to the mid Miocene (24-14 million years ago). Even conifers existed on that continent as late as 15 million years ago. But the most striking evidence concerns the last forests in Antarctica, which existed as late as 2.3-2.6 million years ago.
Here is one recent and accessible paper which goes into some detail about various methods used for reconstructing temperature conditions at the Oliver Bluffs site. As you can see, these plant fossils have been dated to the Pliocene (5.3-2.6 million years ago) for past thirty years. Also scientists have been talking about their implication on the climate of that region for almost that long. While some have tried to dispute the dating of these fossils, it is increasingly clear that they do indeed come from somewhere between 4 and 2.6 million years. For example, analysis of pliocene marine sediments from an offshore drill core dated to between 5 to 2.2 million years and over a thousand km from the site with those plant fossils has revealed the presence of fossil nothofagous pollen including from the species found at Oliver Bluffs.