The Glacial-Interglacial CO$_2$-Temperature Relationship

The slope $S$ of the global mean surface temperature (GMST)–atmospheric carbon dioxide (CO$_2$) relationship, sometimes termed Earth system sensitivity, is a holistic measure of Earth’s response to radiative forcings. We identify a surprising trend in $S$ over the past 800kyr, when it reflects the net carbon cycle feedback’s response to GMST changes. We estimate $S$ roughly doubled over this period. This trend is robust to record completeness and age error, and across multiple GMST reconstructions and proxy types. However, site-specific records do not show increasing temperature variability relative to CO$_2$. This implies either age errors are substantially underestimated, or local variations in temperature became more synchronized over the more recent glacial-interglacial cycles, possibly via a strengthening ice-albedo feedback. Either way, our findings indicate GMST fluctuated more per unit CO$_2$ change over the past several glacial-interglacial cycles than the Cenozoic or Phanerozoic average.