A lower limit to atmospheric CO2 concentrations over the past 800,000 years

Global temperatures and atmospheric CO2 concentrations varied widely over the glacial cycles of the past 800,000 years. But despite this variability, Antarctic ice cores have shown that CO2 concentrations were very similar during all the coldest points of these cycles. Remarkably, the recurring minimum CO2 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 CO2 concentrations have both a cold mode and a central mode. Although multiple explanations are possible, the recurring CO2 minima and pronounced cold modes are consistent with a strong negative feedback to decreasing CO2 that resisted further cooling on timescales shorter than 10,000 years. We suggest that one possible negative feedback is CO2-limitation of photosynthesis, either directly or via CO2-limitation of N-2 fixation, which could have inhibited further lowering of CO2 by reducing carbon storage.