Paleoclimatology is the study of climates that have not been directly measured. Because instrumental records only cover a small portion of Earth's history, understanding natural variation and the evolution of the current climate necessitates reconstruction of ancient climate. Paleoclimatology collects data from rocks, sediments, boreholes, ice sheets, tree rings, corals, shells, and microfossils using a range of proxy approaches from Earth and life sciences. Paleoclimate records are used to establish past states of Earth's atmosphere when combined with procedures for dating proxies. Paleoclimatology as a scientific topic reached maturity in the twentieth century. Paleoclimate research has been particularly useful in demonstrating how the Earth's climate system may transition radically between different climate states in a short period of years or decades. Past climate change research can also assist us understand how humans affect the Earth's climate system. The climatic record over the last thousand years clearly demonstrates that global temperatures rose dramatically in the twentieth century, and that this warming was likely unparalleled in the previous 1,200 years.