Climate history and temperature data are gleaned from the study of proxies such as fossils, ice cores, tree rings, and sediment cores, which are key to the field of paleoclimatology.
Some of the best information on climate history, and especially temperature data, is based on the study of proxies like fossils, ice cores, tree rings, and sediment cores. These serve as indirect measurements to glean past climate conditions since we cannot directly measure atmospheric phenomena from ancient times.
For instance, scientists analyze foraminifera shells from sediment layers, which coil differently depending on the temperature, to infer relative seawater temperatures of the past. Similarly, Antarctic ice cores contain air bubbles and biological evidence that can reveal past temperature and carbon dioxide levels, offering insights into Earth's climatic history over hundreds of thousands of years.
An integral part of the field of paleoclimatology, these proxies allow reconstruction of the Earth's climate record for periods such as the past 65 million years. By examining such climate proxies, patterns of temperature fluctuation and carbon dioxide levels are identifiable, helping us understand climate trends and even predict future climatic changes by comparing current data with the historical record.