Proxy Data refers to data that Paleoclimatologists gather from natural recorders, i.e., tree rings, ice cores, fossil pollen, ocean sediments, coral and historical data. By analyzing records taken form these and other proxy sources, scientists can extend our understanding of climate far beyond the 150 year instrumental record which is based on thermometers, rain gauges and other modern instruments used to record climate.
Historical documents contain a wealth of information about past climates.
Observations of weather and climatic conditions can be found in farmers' logs, travellers' diaries, newspaper accounts, and other written records. When properly evaluated, historical data can yield both qualitative and quantitative information about past climate. Corals build their hard skeletons from calcium carbonate, a mineral extracted from sea water. The carbonate contains oxygen and the isotopes of oxygen, as well as trace metals, that can be used to determine the temperature of the water in which the coral grew. These temperature recordings can then be used to reconstruct climate during that period of time that the coral lived.