To trigger glacial inception, the summer maximum insolation at high latitudes in the Northern Hemisphere must be lower than a critical value. This value is not constant but depends on the atmospheric CO₂ concentration. Paleoclimatic data do not give enough information to derive the relationship between the critical threshold and CO₂. However, knowledge of such a relation is important for predicting future glaciations and the impact anthropogenic CO₂ emissions might have on them.