In the field of paleoclimatology, the study of past climates, proxies, imprints created during past climates, are used to determine the characteristics of those past climates. Ice cores have layers of trapped gasses, isotopes, and pollen from previous climates. In tree cores, the number of rings can be counted to determine age, and the thickness of each ring can be used to determine changes in temperature and precipitation for that year. Ocean sediment can be analyzed for remains of ancient organisms and pollen to determine changes in past climate, since many organisms can only live in certain temperature ranges (Bruckner, n.d.). Corals contain isotopes of oxygen and trace metals that can be used to determine the temperature of the water in which that coral grew. Pollen in sediments can be analyzed to determine what plant they came from, which can show scientists the approximate climate in which they grew ("What Are "Proxy" Data?," n.d.).