Figure 21.4 Ice cores. (a) Researchers from the Catlin Arctic Survey 2010 drill far down into ancient ice to collect ice cores that have been created in layers over the past 500,000 years. (b) Ice cores contain air bubbles that were trapped at the time the ice was formed. By melting each layer, researchers can determine the concentration of CO2 that was in the atmosphere over the past 500,000 years.
Photos by (a) Martin Hartley/eyevine/Redux; and (b) George Steinmetz/Corbis.