Paleoclimatology is the study of climate change taken on the scale of the entire history of the earth. Glaciers are a widely employed instrument in paleoclimatology. The ice in glaciers has hardened into an identifiable pattern, with each year leaving a distinct layer in an ice core. It is estimated that the polar ice caps have 100,000 of these layers or more. Inside of these layers scientists have found pollen, allowing them to estimate the total amount of plant growth of that year by the pollen count. The thickness of the layer can help to determine the amount of rainfall that year. Sediment layers have been studied, particularly those in the bottom of lakes and oceans. Characteristics of preserved vegetation, animals, pollen, and isotope ratios provide information. Sedimentary rock layers provide a more compressed view of climate, as each layer of sediment is made over a period of hundreds of thousands to millions of years. Scientists can get a grasp of long term climate by studying sedimentary rock.