How do scientists know what the temperature was beyond 150 years ago? To reconstruct climate history, scientists use proxy data – records used to infer atmospheric properties such as temperature and precipitation. This subfield of climate science is referred to as paleoclimatology. Just like a crime scene investigator solves crimes by examining many different kinds of evidence, a paleoclimatologist can uncover clues about our current and future climate using many sources of proxy data. These sources include historical documents, tree and coral growth rings, deep-sea sediment cores, ice cores, and fossils.

Historical documents, such as personal diaries, mariner's logs, records of harvests and quality of wines, can provide indirect indications of past climate. These written documents, however, are not as reliable as the other proxy data sources described below.