claim: The killer proof that CO2 does not drive climate is to be found during the Ordovician- Silurian and the Jurassic-Cretaceous periods when CO2 levels were greater than 4000 ppmv ( parts per million by volume ) and about 2000 ppmv respectively.
evidence: The Silurian (  / sɪˈljʊər.i.ən, saɪ- /  sih-LYOOR-ee-ən, sy- ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago ( Mya ), to the beginning of the Devonian Period, 419.2 Mya.