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: Carbon dioxide content in fresh air ( averaged between sea-level and 10 kPa level, i.e., about 30 km ( 19 mi ) altitude ) varies between 0.036% ( 360 ppm ) and 0.041% ( 410 ppm ), depending on the location.