claim: Consequently, CO2 levels at around 1,000 to 2,300 ppm were actually low enough to promote glaciation in the southern continent of Gondwana.
evidence: Concentrations of CO 2 in the atmosphere were as high as 4,000 parts per million (ppm, on a molar basis) during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years.