Paleoclimatology (in British spelling, palaeoclimatology) is the study of changes in climate taken on the scale of the entire history of Earth. It uses a variety of proxy methods from the Earth and life sciences to obtain data previously preserved within (e.g.) rocks, sediments, ice sheets, tree rings, corals, shells and microfossils; it then uses these records to determine the past states of the Earth's various climate regions and its atmospheric system. Paleoclimatology has wider implications for climate change today. Scientists often consider past changes in environment and biodiversity to reflect on the current situation, and specifically the impact of climate on mass extinctions and biotic recovery.

Paleoclimatologists employ a wide variety of techniques to deduce ancient climates.

On a longer time scale, geologists must refer to the sedimentary record for data.

A consortium called the European Project for Ice Coring in Antarctica (EPICA) has drilled at Dome C in the East Antarctic ice sheet, and retrieved an ice core which dates to roughly 740,000 years old. The project has been working in the same location to core a final section that could reach back to 900,000 years or even further. The international ice core community has, under the auspices of International Partnerships in Ice Core Sciences (IPICS), defined a priority project to obtain the oldest possible ice core record from Antarctica, an ice core record reaching back to or towards 1.5 million years ago. The deep marine record, the source of most isotopic data, only exists on oceanic plates, which are eventually subducted — the oldest remaining material is 200 million years old. Older sediments are also more prone to corruption by diagenesis. Resolution and confidence in the data decrease over time.