A major reason to study Earth’s past climate is to improve predictions of future climate change. A crucial measure for this is apparent Earth system sensitivity (AESS): how much global mean surface temperature increases with a doubling of atmospheric carbon dioxide (CO2). We examine AESS over the past 800,000 years. We find an unexpected trend: AESS appears to more than double over this period. Furthermore, this trend is robust to several sources of error and estimated uncertainty. However, site-specific temperature records do not become less variable relative to CO2 going further back in the past. If global surface temperature variations increased relative to CO2 variations, but local surface temperature variations did not, this suggests that local temperature variations became more synchronized over the past several glacial-interglacial cycles. What mechanism could cause such increasing synchronization is not clear. An alternative explanation is that the growth rate of age errors of site-specific temperature records with time are substantially underestimated. Either way, our results indicate that GMST appears to fluctuate more relative to CO2 in the recent past than was typical over the past ~50 or ~500 million years, while also indicating a paradox about global climate dynamics over the past 800,000 years.