Comment More FUD. It was much higher 450 million years ago (Score 3, Informative) 696
CO2 levels of more than 4000 parts per million (ppm) occurred during the Ordovician-Silurian (450 million years ago). There is also evidence of a glacial event occurring during this period.
from: http://www.climatechange.gov.au/climate-change/understanding-climate-change/understand-cc-long-term.aspx