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But if we go back before 400,000 years ago, there were much higher levels of CO2 in the atmosphere. Here's a quote from an article at Climate Depot:

Princeton’s Dr. Happer, who has authored 200 peer-reviewed scientific papers, explained in Senate testimony in 2009 that the Earth is currently in a ‘CO2 ‘famine.’ Happer explained to Congress: ”Warming and increased CO2 will be good for mankind…’CO2 is not a pollutant and it is not a poison and we should not corrupt the English language by depriving ‘pollutant’ and ‘poison’ of their original meaning,” Happer added. “Many people don’t realize that over geological time, we’re really in a CO2 famine now. Almost never has CO2 levels been as low as it has been in the Holocene (geologic epoch) – 280 (parts per million – ppm) – that’s unheard of. Most of the time [CO2 levels] have been at least 1000 (ppm) and it’s been quite higher than that,” Happer told the Senate Committee. “Earth was just fine in those times,” Happer added. “The oceans were fine, plants grew, animals grew fine. So it’s baffling to me that we’re so frightened of getting nowhere close to where we started,” Happer explained.
http://www.climatedepot.com/2013/05/14/co2-nears-400-ppm-relax-its-not-global-warming-end-times-but-only-a-big-yawn-climate-depot-special-report/