Climate scientists use "proxy data", such as the information stored in coral, to study climates of the past.
Climate scientists use "proxy data" to study climates of the past, before humans with thermometers began keeping temperature records. These "proxies" include tree rings, layers within ice cores pulled from glaciers and ice sheets, growth layers in coral, and layers of sediments from the bottoms of lakes and oceans.
Each year, coral colonies add a new layer of growth onto exiting coral "skeletons". Climate scientists can deduce data about past climates from these annual growth rings in much the same way they look at tree rings. The proportions of oxygen isotopes in the coral tell us about ocean temperatures when that coral was formed. Climate data from coral is the only major source of paleoclimate information from Earth's tropical regions. Oceanographers use special tools to extract cores from coral skeletons which they study in their labs.
The video clips in this video are from the "To What Degree? What Science is Telling Us about Climate Change" web site developed by the National Science Foundation.