© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Global temperatures and atmospheric CO 2 concentrations varied widely over the glacial cycles of the past 800,000 years. But despite this variability, Antarctic ice cores have shown that CO 2 concentrations were very similar during all the coldest points of these cycles. Remarkably, the recurring minimum CO 2 concentrations (190±7 ppm) fall on the lower bound of any known in Earth history. Here we show that although the volume of terrestrial ice sheets was normally distributed over the past 800,000 years, as might be expected from the approximately normal distribution of the orbital forcing that drove the glacial cycles, Antarctic temperatures have a strong cold mode, whereas CO 2 concentrations have both a cold mode and a central mode. Although multiple explanations are possible, the recurring CO 2 minima and pronounced cold modes are consistent with a strong negative feedback to decreasing CO 2 that resisted further cooling on timescales shorter than 10,000 years. We suggest that one possible negative feedback is CO 2 -limitation of photosynthesis, either directly or via CO 2 -limitation of N 2 fixation, which could have inhibited further lowering of CO 2 by reducing carbon storage.