paleoclimates

Paleoclimatology is the study of past climates throughout Earth's history, aimed at reconstructing variations in temperature, precipitation, and atmospheric composition before reliable instrumental records. The field relies on indirect evidence, or proxies, derived from natural archives that preserve climate information over different time scales.

Proxies include ice cores, marine and lacustrine sediments, speleothems, pollen records, and fossil remains. Each proxy provides partial climate information and is interpreted with calibration studies and climate models. Common indicators are isotopic compositions, such as oxygen and hydrogen isotopes in ice and carbonate minerals, which reflect temperature and hydrological changes; trace elements and the magnesium-to-calcium ratio in foraminifera; greenhouse gas concentrations inferred from gas inclusions and stomatal density on plants; and biological indicators like pollen assemblages that track vegetation shifts.

Paleoclimate records cover a wide temporal range, from millions of years in the geological past to tens of thousands of years in the late Quaternary. Well-known events include long-term transitions such as the Cretaceous and Eocene greenhouse climates, the Eocene-Oligocene transition to cooler conditions, glacial-interglacial cycles driven in part by Milankovitch cycles, the Last Glacial Maximum about 26,500 to 19,000 years ago, and Holocene fluctuations including the Medieval Warm Period and the Little Ice Age. More contentious topics include abrupt climate events and issues surrounding the extent and causes of Snowball Earth episodes.

Understanding paleoclimates helps place current climate change in a long-term context and informs models that project future climate behavior, while also highlighting uncertainties due to proxy resolution, dating, and regional limitations.