In order to study in detail the pre‐industrial CO2 level (back to about 900 AD) and its temporal variations, several ice cores from Greenland and Antarctica were analysed in two laboratories, and compared with previous records. The agreement between the two laboratories and between the different cores of the same hemisphere is good. However, the comparison of the northern hemisphere (Greenland) and southern hemisphere (Antarctica) records shows values systematically higher in the north than in the south, ranging from 20 ppmv at the turn of this millennium to nearly zero around the 18th century. Based on our present knowledge of the carbon cycle, an inter‐hemispheric gradient of 20 ppmv is unrealistic. Thus, in the oldest part of the record, at least one profile should not represent the true atmospheric CO2 concentrations. A companion paper by Anklin et al. (submitted), discusses the possible processes which can alter the atmospheric CO2 once trapped in the ice. Due to the fact that the impurity content is one order of magnitude lower in the Antarctic than in the Greenland ice, we are much more confident in the Antarctic record. The new results from D47 and D57 (Adélie Land) presented in this paper, confirm the CO2 fluctuation of about 10 ppmv at the end of the 13th century, previously observed by Siegenthaler et al. (1988) on an ice core drilled at South Pole. This fluctuation corresponds to a small imbalance of the carbon cycle (∼ 0.3 GT C/ yr), but its duration led to a significant cumulative input into the atmosphere. The changes observed in the pre‐industrial level are discussed in terms of climatic noise and variability.