Palaeoclimatology, from the Greek word "palaios", meaning "ancient", is the study of past climates and past climate change, prior the period of instrumental records. The study of palaeoclimatology can encompass much of Earth History, or at least that part of it for which reliable palaeoclimatic records are available to reconstruct palaeoclimates.
Palaeoclimatology may be distinguished from climatology and contemporary climate change, which studies present day climate and climate changes restricted to the most recent period (the last 150 years) since instrumental records of daily weather observations have become available.
To reconstruct palaeoclimates, palaeoclimatologists cannot use direct observations of temperature, rainfall and other climatic variables. Instead they use proxy records of natural phenomena which are climate-dependent. These include analyses of tree rings, ice cores, sea sediments and even rock strata exposed at the Earth's surface which may hold clues to the state of the climate millions of years ago. Climate models run on computers may also be used to test theories about possible mechanisms of palaeoclimate change.