Ice cores from centuries-old ice deposits in Antarctica and Iceland provide scientists with records of both global temperatures and CO2 concentrations dating back as far as 160,000 years. The analysis of these ice deposits represents a clever piece of scientific detective work. These analyses show that past temperatures and CO2 levels have risen and fallen together.
To determine the historical CO2 levels in the atmosphere, the bubbles trapped in the ice are analyzed for the CO2 concentration - the air is trapped as light density snow is gradually compressed by the weight of new layers to form ice.
Isotopes of oxygen atoms (18 O and 16 O) in the ice molecules give estimates of the Earth's temperature during these ancient times. Scientists measure the ratio of the ice formed with heavy water,containing the 18 O isotope, to the normal water consisting of 16 O atoms. When the earth is warm, more of the heavier water containing 18 O evaporates to become snowfall, resulting in a higher ratio of 18 O in ice from that period. ***