Nice graph. How do we temperature variation from ice core samples? (I hope it's not inferred from CO2 concentration).
Oxygen-18 isotope ratios with O-16. The fractionation in the atmosphere into water vapor is directly related to the temperature of the atmosphere. In fact, in some of the presentations of the ice core data, CO2 concentration in the bubbles is plotted against "del"O18, which is the actual measured parameter that acts as a paleo-thermometer.