Improved high-resolution paleo records of atmospheric carbon dioxide (CO2) concentrations and reconstructions of Earth’s surface temperature are available. We analyse one authoritative Pleistocene dataset to explore how the climate sensitivity parameter S varies under different system states, using linear regression of mean annual surface temperature changes against CO2 forcing changes. Data are partitioned by path (deglaciation or glaciation). On the whole data set, S = 2.04 K/Wm−2 and CO2 forcing explains 64 % of the variance in temperature. During deglaciation periods, S = 2.34 K/Wm−2, explaining 75 % of the temperature variance; during glaciations, S = 1.59 K/Wm−2 and explains 48 % of the temperature variance. Possible process-related explanations are conjectured.