Understanding how atmospheric CO₂ has affected the climate of the past is an important way of furthering our understanding of how CO₂ may affect our climate in the future. There are several ways of determining CO₂ in the past; in this paper, we ground-truth one method (based on preserved organic matter from alga) against the record of CO₂ preserved as bubbles in ice cores over a glacial–interglacial cycle. We find that there is a discrepancy between the two.