INCREASING anthropogenic emissions of greenhouse gases are expected to influence the Earth's climate, but the mechanisms for this are not yet fully understood. One way to determine the effect of such gases on climate is to study their atmospheric concentrations during periods of past climate change, such as glacial to interglacial transitions. Previous studies on polar ice cores showed that the concentrations of the greenhouse gases CO2 and CH4 were significantly reduced during the last glacial period relative to Holocene values1–5. But no comparable studies have been reported for nitrous oxide (N2O), which is the next most important greenhouse gas and also affects stratospheric ozone6,7 and, potentially, the oxidative capacity of the troposphere8. Here we report results from Antarctic ice cores, showing that the atmospheric N2O concentration was about 30% lower during the Last Glacial Maximum than during the Holocene epoch. Our data also show that present-day N2O concentrations are unprecedented in the past 45 kyr, and hence provide evidence that recent increases in atmospheric N2O are of anthropogenic origin.