The study of how global climate has changed through time.

is the reconstruction of ancient climates by using evidence such as tree rings and air trapped in ice cores. Researchers use such evidence to understand natural climatic shifts which can help us in understanding and eventually predicting future climate trends.

The study of past climates and climate change.

The study of past climates, throughout geological history, and the causes of the variations among climates. (source: NASA Earth Observatory)

Scientific study of the Earth's climate during the past.

The study of past climates throughout geologic and historic time (paleoclimates), and the causes of their variations.

Paleoclimatology is the study of climate change taken on the scale of the entire history of the Earth.