http://en.wikipedia.org/wiki/Carbon_dioxide
The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO2 levels were about 260 � 280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years.
The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years before the present.[21] During this time, the atmospheric carbon dioxide concentration has varied between 180 � 210 �L/L during ice ages, increasing to 280 � 300 �L/L during warmer interglacials.[22] The data can be accessed here.
Some studies have disputed the claim of stable CO2 levels during the present interglacial (the last 10 kyr). Based on an analysis of fossil leaves, Wagner et al.[23] argued that CO2 levels during the period 7 � 10 kyr ago were significantly higher (~300 �L/L) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO2.[24] Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO2 values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g., Smith et al.[25]) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO2 measurements.