When snow falls in the antarctic, it builds up over time, thus providing a temporal record of not just different chemical properties of that snow (for example, its possible estimate the temperature of the overlying air in which the snow fell), but also of atmospheric gases that are trapped in microscopic air bubbles in the ice.  Analytical chemists can then extract this gas, and measure the concentration of  carbon dioxide other gases as well. 

Still fantastic, just not unbelievable.