The problem with the use of ice cores as a proxy for CO2 measurements is that the ice is unavoidably contaminated by liquid water. Dr. Zbigniew Jaworowski of the Central Laboratory for Radiological Protection (CLOR) in Warsaw, Poland, in written testimony submitted to a U.S. Senate committee in March 2004 said:
Determinations of CO2 in polar ice cores are commonly used for estimations of the pre-industrial CO2 atmospheric levels. Perusal of these determinations convinced me that glaciological studies are not able to provide a reliable reconstruction of CO2 concentrations in the ancient atmosphere. This is because the ice cores do not fulfill the essential closed system criteria. One of them is a lack of liquid water in ice, which could dramatically change the chemical composition the air bubbles trapped between the ice crystals. This criterion, is not met, as even the coldest Antarctic ice (down to -73°C) contains liquid water. More than 20 physico-chemical processes, mostly related to the presence of liquid water, contribute to the alteration of the original chemical composition of the air inclusions in polar ice.