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.