Tree ring patterns provide information about precipitation and other conditions during the time the tree was alive. Scientists can learn even more about precipitation and temperature patterns by studying certain chemicals in the wood. Modern trees can be interesting to compare with local measurements (for example, temperature and precipitation measurements from the nearest weather station). Very old trees can be even more interesting because they offer clues about what the climate was like before measurements were recorded. In most places, daily weather records have only been kept for the last 100 to 150 years. Thus, to learn about the climate hundreds to thousands of years ago, scientists need to use other sources such as trees, corals, and ice cores (layers of ice drilled out of a glacier or ice sheet—mostly in Greenland and Antarctica).