CO2 levels have been much, much higher throughout Earth's history although I wouldn't really count the pre-Carbiniferous levels since it was a vastly different ecosystem(ie. not much land vegetation). But throughout the Mesozoic, it was much higher than today.
Just as a side note. There seems to be a correlation between points where the temperature rebounds after a lowpoint and mass extinctions. for Example Ordivician/Silurian extinction, and the end Permian. There's no real good parallel to our current period btw. The Permian was in terms of Temperature levels, but there was pretty much one giant continent and one giant ocean, so it was vastly diferent other than that.
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