Analyses of air extracted from polar ice cores are the most straightforward method of reconstructing the atmospheric concentrations of greenhouse gases and their variations for past climatic epochs. These measurements show that the concentration of the three most important greenhouse gases (other than water vapour) CO2, CH4 and N2O have steadily increased during the past 250 years due to anthropogenic activities (Prather and others, 2001; Prentice and others, 2001). Ice-core results also provided the first evidence of a substantial increase in the concentration of the three gases during the transition from the last glacial epoch to the Holocene (Raynaud and others, 1993). However, results from different cores are not always in agreement concerning details and small, short-term variations. the composition of the air enclosed in bubbles can be slightly changed by fractionation during the enclosure process, by chemical reactions and/or biological activity in the ice and by fractionation during the air extraction. We compile here several records with short-term variations or anomalies and discuss possible causes, taking into account improved analytical techniques and new results.