Ice Cores and the Climate Record

Ice sheets accumulate snow year after year, compressing successive annual layers into firn and then solid ice much as tree rings record successive growing seasons, trapping tiny bubbles of ancient atmosphere as the ice forms around them. Drilling a core down through this layered ice therefore recovers a chronological archive of past climate: the ratio of heavy to light water isotopes, such as oxygen-18 to oxygen-16 (δ18O), preserved in the ice serves as a proxy for the temperature at which the original snow condensed, while the composition of the trapped air bubbles gives a direct sample of the actual atmospheric concentrations of carbon dioxide, methane, and nitrous oxide at the time each layer was sealed off from the surface.