Improved high-resolution paleo records of atmospheric carbon dioxide (CO<sub>2</sub>) concentrations and reconstructions of Earth’s surface temperature are available. We analyse one authoritative Pleistocene dataset to explore how the climate sensitivity parameter <i>S</i> varies under different system states, using linear regression of mean annual surface temperature changes against CO<sub>2</sub> forcing changes. Data are partitioned by path (deglaciation or glaciation). On the whole data set, <i>S</i> = 2.04 K/Wm<sup>−2</sup> and CO<sub>2</sub> forcing explains 64 % of the variance in temperature. During deglaciation periods, <i>S</i> = 2.34 K/Wm<sup>−2</sup>, explaining 75 % of the temperature variance; during glaciations, <i>S</i> = 1.59 K/Wm<sup>−2</sup> and explains 48 % of the temperature variance. Possible process-related explanations are conjectured.