Sawzaw's link above is to a piece by well-known climate change skeptic Zbigniew Jaworowski (actually, he is not a climate change skeptic - more of a global warming skeptic since he has predicted a new ice age on the way).

His complaint about the CO2 record is that inclusions in the Siple (Antarctica) ice cores (and ice cores in general) are not valid indicators of past atmospheric chemistry because depth-of-burial effects the chemistry of the air in the inclusions.

However, there are many, many ice cores from far-flung locations that are in very good agreement. This is important because the cores have different lengths (depths) representing the same (or significantly overlapping) time periods. For instance, the EPICA core goes back 800,000 years, but is relatively short, while the Vostok core represents only about half that time, but is much deeper. For the period of overlap, they show nearly identical variations in atmospheric chemistry across four glacial-interglacial cycles - so depth cannot be introducing a very strong signature.